Determination of salt in foods is an analytical experiment in which the amount of salt contained in foods is chemically measured.
Sodium chloride, the main component of table salt, dissociates into Na+ and Cl- in water.
Therefore, in experiments that examine the salt content of foods, a method in which chloride ions are determined and then converted to an equivalent amount of salt is commonly used.
In the determination of salt in foods, precipitation titration using silver nitrate, particularly the Mohr method, is a representative technique.
The amount of chloride ions is determined from the titration volume of silver nitrate by utilizing the reaction in which Ag+ and Cl- form a white precipitate of AgCl.
Furthermore, by converting the amount of Cl- to an amount of NaCl, the salt content of the food can be evaluated.
This article clearly explains, as examples of discussions that can be used in laboratory reports on salt determination in foods, the principle of the Mohr method, the relationship between chloride ions and salt content, pretreatment of food samples, extraction, endpoint determination, coexisting components, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of salt-determination results in foods obtained in food chemistry experiments, analytical chemistry experiments, and basic chemistry experiments at universities and similar institutions.
For the actual sample treatment, concentration of the silver nitrate standard solution, indicator, titration conditions, calculation formula, comparison with food-label values, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is the Determination of Salt in Foods?
- Main Items to Include in the Results
- Reference Experimental Values and Calculation Examples for Salt Determination by the Mohr Method
- Reference Experimental Conditions
- Concept of the Reactions in the Mohr Method
- Confirmation Titration of a Standard Salt Solution
- Example of Pretreatment of a Food Sample
- Titration Results for Different Foods
- Example Calculation of the Amount of Chloride Ions
- Example Calculation of NaCl Equivalent
- Example Calculation of Salt Concentration
- Simplified Calculation Including the Dilution Factor
- Comparison of Endpoint Judgments
- Effect of pH Conditions
- Comparison of Colored and Turbid Food Samples
- Effects of Coexisting Ions
- Example Comparison With Nutrition-Label Values
- Example Calculation of Relative Error
- Example of How to Write the Results
- Points for Connecting the Results to the Discussion
- Example Discussion
- Summary
- What Is the Mohr Method?
- Relationship Between Chloride Ions and Salt Content
- Relationship Between Silver Nitrate Titration Volume and Salt Content
- Importance of Pretreatment of Food Samples
- Errors Caused by Insufficient Extraction
- Effects of Filtration and Turbidity
- Discussion of Endpoint Determination
- Effect of pH Conditions
- Interference From Food Components
- Comparison With Food-Label Values
- Differences in Salt Content Among Foods
- Errors Caused by Nonuniformity of the Sample
- Discussion of the Dilution Factor and Calculations
- Concentration Error of the Silver Nitrate Standard Solution
- Importance of Blank Correction
- Causes of Error in Salt Determination in Foods
- When the Results Can Be Considered Good
- Example Discussion When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Salt Determination in Foods
- Summary
What Is the Determination of Salt in Foods?
Determination of salt in foods is an analysis in which the amount of salt contained in food is measured.
Salt is an important food component related to seasoning, preservability, fermentation, and processing properties.
However, excessive intake may lead to health problems, so understanding the amount of salt in foods is important from the perspectives of food chemistry and nutritional management.
Salt in foods mainly exists as NaCl.
Because NaCl dissociates into Na+ and Cl- when dissolved in water, determining Cl- makes it possible to calculate the amount of salt by converting it to NaCl.
However, not all Cl- in foods necessarily originates from NaCl, so in discussions the result is often treated as a “salt equivalent.”
Example Discussion:
In the determination of salt in foods, chloride ions extracted from the food are measured and converted to the amount of NaCl to determine the salt equivalent.
In this experiment, Cl- was determined by precipitation titration using silver nitrate.
Because the salt content of foods is related to taste, preservability, and nutritional labeling, it is an important analytical item for evaluating the properties of foods.
Main Items to Include in the Results
In the results of salt determination in foods, organize the type of food sample, sample mass, extraction method, dilution factor, concentration of the silver nitrate standard solution, titration volume, indicator, endpoint color change, amount of chloride ions, salt equivalent, and other information.
For food samples, pretreatment and extraction conditions greatly affect the results, so it is important to clearly describe the operating conditions.
- Type of food sample
- Mass of the sample
- Condition of the sample
- Whether grinding or homogenization was performed
- Amount of water used for extraction
- Whether heated extraction was performed
- Whether filtration was performed
- Dilution factor
- Concentration of the silver nitrate standard solution
- Silver nitrate titration volume
- Type of indicator
- Color change at the endpoint
- Blank value
- Amount of chloride ions
- Salt equivalent
- Comparison with the food-label value
- Causes of error and points for improvement
Example of How to Write the Results:
The food sample was extracted with water, and the filtered extract was titrated with a silver nitrate standard solution.
The amount of chloride ions was determined from the silver nitrate titration volume required to reach the endpoint and converted to NaCl to calculate the salt equivalent.
The obtained value was compared with the food-label value and with other food samples, and differences in salt content were discussed.
Reference Experimental Values and Calculation Examples for Salt Determination by the Mohr Method
Here, chloride ions in foods are determined by the Mohr method, and the process of calculating the amount of chloride ions, salt equivalent, and salt concentration in the food is organized using reference experimental values.
The Mohr method is a precipitation-titration method in which chloride ions are titrated with a silver nitrate standard solution and potassium chromate is used as the indicator to determine the endpoint.
After chloride ions react with Ag+ and precipitate as AgCl, excess Ag+ reacts with CrO42- to form reddish-brown Ag2CrO4, and the point at which this occurs is taken as the endpoint.
Reference Experimental Conditions
| Item | Details |
|---|---|
| Measurement target | Salt content in foods |
| Measurement method | Precipitation titration by the Mohr method |
| Titrant | 0.0500 mol/L silver nitrate standard solution |
| Indicator | Potassium chromate aqueous solution |
| Endpoint | The point at which a slight reddish-brown color remains in the white precipitate |
| Reaction ratio | Cl- : Ag+ = 1 : 1 |
| Molar mass of NaCl | 58.5 g/mol |
| Evaluation items | AgNO3 titration volume, amount of Cl-, salt equivalent, sources of error |
Concept of the Reactions in the Mohr Method
During titration, chloride ions first react with silver ions to produce a white precipitate of silver chloride.
Ag+ + Cl- → AgCl
After almost all chloride ions have precipitated, excess Ag+ reacts with chromate ions to produce reddish-brown silver chromate.
2Ag+ + CrO42- → Ag2CrO4
The point at which this reddish-brown color begins to remain without disappearing is taken as the endpoint.
Confirmation Titration of a Standard Salt Solution
First, an example is shown in which a salt solution of known concentration is titrated to confirm the concentration of the silver nitrate standard solution and the endpoint determination.
The standard salt solution is assumed to contain 0.0293 g of NaCl in 10.00 mL.
| Trial | Initial Burette Reading | Endpoint Burette Reading | AgNO3 Titration Volume | Calculated NaCl Amount |
|---|---|---|---|---|
| 1st | 0.10 mL | 10.08 mL | 9.98 mL | 0.0292 g |
| 2nd | 0.05 mL | 10.06 mL | 10.01 mL | 0.0293 g |
| 3rd | 0.20 mL | 10.19 mL | 9.99 mL | 0.0292 g |
Because the titration volumes for the standard salt solution were approximately 10.00 mL and agreed closely, the concentration of the silver nitrate standard solution and the endpoint determination were considered generally appropriate under these conditions.
Example of Pretreatment of a Food Sample
For food samples, the salt is extracted into water and insoluble substances are filtered off before titration.
Here, an example is shown in which 5.00 g of food is collected, extracted with water, and diluted to 100.0 mL.
| Step | Operation | Purpose |
|---|---|---|
| 1 | Accurately weigh 5.00 g of food | Provide the basis for calculating salt concentration |
| 2 | Add warm water and stir thoroughly | Dissolve and extract the salt from the food |
| 3 | Filter | Remove insoluble substances and turbidity |
| 4 | Dilute the filtrate to 100.0 mL | Prepare an extract whose concentration can be calculated easily |
| 5 | Aliquot 10.00 mL of the extract | Take a fixed amount for titration |
Titration Results for Different Foods
Reference examples are shown in which 5.00 g of food was extracted and diluted to 100.0 mL, and 10.00 mL of the extract was aliquoted and titrated.
| Sample | Food | Sample Mass | Extract Volume | Aliquot Volume | AgNO3 Titration Volume | Salt Equivalent | Salt Concentration |
|---|---|---|---|---|---|---|---|
| A | Soy sauce | 5.00 g | 100.0 mL | 10.00 mL | 8.95 mL | 0.262 g | 5.24% |
| B | Miso | 5.00 g | 100.0 mL | 10.00 mL | 7.40 mL | 0.216 g | 4.33% |
| C | Pickles | 5.00 g | 100.0 mL | 10.00 mL | 3.85 mL | 0.113 g | 2.25% |
| D | Soup | 5.00 g | 100.0 mL | 10.00 mL | 2.15 mL | 0.0629 g | 1.26% |
| E | White bread | 5.00 g | 100.0 mL | 10.00 mL | 1.10 mL | 0.0322 g | 0.64% |
Example Calculation of the Amount of Chloride Ions
Silver nitrate and chloride ions react in a 1:1 ratio.
Therefore, the amount of AgNO3 consumed corresponds to the amount of Cl- in the aliquoted extract.
Amount of Cl- = AgNO3 concentration × AgNO3 titration volume
For the soy-sauce sample, the AgNO3 concentration is 0.0500 mol/L and the titration volume is 8.95 mL = 0.00895 L.
Amount of Cl- = 0.0500 mol/L × 0.00895 L = 4.48 × 10−4 mol
Therefore, the 10.00 mL aliquot of extract was considered to contain 4.48 × 10−4 mol of chloride ions.
Example Calculation of NaCl Equivalent
Chloride ions in foods are assumed to originate from salt, NaCl, and are converted to a salt equivalent.
Because NaCl and Cl- correspond in a 1:1 ratio, the amount of Cl- is multiplied by the molar mass of NaCl.
NaCl equivalent in the aliquot = Amount of Cl- × Molar mass of NaCl
For the soy-sauce sample, the amount of Cl- is 4.48 × 10−4 mol and the molar mass of NaCl is 58.5 g/mol.
NaCl equivalent in the aliquot = 4.48 × 10−4 mol × 58.5 g/mol = 0.0262 g
This is the salt equivalent in 10.00 mL of the extract.
Because the total extract volume is 100.0 mL, the salt equivalent in the entire extract is 10 times this amount.
NaCl equivalent in the entire extract = 0.0262 g × 10 = 0.262 g
Therefore, the collected 5.00 g of soy sauce was calculated to contain 0.262 g of salt.
Example Calculation of Salt Concentration
The salt concentration in the food is determined as the proportion of the salt equivalent relative to the mass of the food sample.
Salt concentration (%) = Salt equivalent ÷ Food sample mass × 100
For the soy-sauce sample, the salt equivalent is 0.262 g and the food sample mass is 5.00 g.
Salt concentration = 0.262 ÷ 5.00 × 100 = 5.24%
Therefore, in this reference example, the salt concentration of the soy-sauce sample is determined to be 5.24%.
Simplified Calculation Including the Dilution Factor
When 5.00 g of food is diluted to 100.0 mL and 10.00 mL is aliquoted for titration, the calculation can be performed in the following sequence.
| Calculation Stage | Equation | Meaning |
|---|---|---|
| NaCl in the aliquot | AgNO3 concentration × Titration volume × 58.5 | Salt equivalent in the titrated 10.00 mL |
| NaCl in the entire extract | NaCl in the aliquot × 10 | Converted to the total 100.0 mL extract |
| Salt concentration | NaCl in the entire extract ÷ Food sample mass × 100 | Salt concentration in the food |
Comparison of Endpoint Judgments
In the Mohr method, missing the reddish-brown endpoint or continuing the titration until the color becomes dark causes errors in the measured value.
Using soy-sauce extract as an example, differences in endpoint judgment are compared below.
| Endpoint Judgment | AgNO3 Titration Volume | Salt Concentration | Effect on the Result |
|---|---|---|---|
| A slight reddish-brown color remains | 8.95 mL | 5.24% | Appropriate endpoint |
| Titration stopped before the color appears | 8.65 mL | 5.06% | Underestimation |
| Titrated until a dark reddish-brown color appears | 9.35 mL | 5.47% | Overestimation due to overtitration |
The endpoint is judged as the point at which a slight reddish-brown color begins to remain within the white precipitate.
If titration continues until a dark reddish-brown color appears, excess silver nitrate is added and the amount of salt is overestimated.
Effect of pH Conditions
In the Mohr method, the endpoint may become unclear under strongly acidic or strongly alkaline conditions.
If the pH is too low, the state of chromate ions changes and the endpoint tends to be delayed.
If the pH is too high, silver ions may react to form hydroxides or other compounds.
| Condition | pH | Titration Volume | Ease of Endpoint Observation | Effect |
|---|---|---|---|---|
| Appropriate conditions | Approximately 7 | 8.95 mL | Easy to observe | Standard measurement |
| Strongly acidic | Approximately 3 | 9.40 mL | Endpoint is delayed | May lead to overestimation |
| Strongly alkaline | Approximately 11 | 9.20 mL | Precipitate becomes cloudy | Effects of side reactions |
Comparison of Colored and Turbid Food Samples
In strongly colored or turbid samples such as soy sauce and miso, the reddish-brown endpoint may become difficult to observe.
In such cases, sufficient dilution, filtration, and confirmation with a blank become important.
| Sample Condition | Treatment | Titration Volume | Ease of Endpoint Observation | Interpretation of the Result |
|---|---|---|---|---|
| Dark, nearly undiluted soy sauce | Insufficient dilution | 9.20 mL | Difficult to observe | Overtitration is likely |
| Sufficiently diluted | Diluted to 100.0 mL | 8.95 mL | Relatively easy to observe | Standard measurement |
| Turbid | No filtration | 9.10 mL | Color of the precipitate is difficult to observe | Endpoint judgment may shift |
| After filtration | Insoluble substances removed | 8.95 mL | Easy to observe | Good reproducibility |
Effects of Coexisting Ions
In the Mohr method, components that react with Ag+ to form precipitates may cause the amount of chloride ions to be overestimated.
| Coexisting Component | Effect | How to Treat It in the Discussion |
|---|---|---|
| Br-, I- | Form precipitates with Ag+ | May be overestimated as Cl- |
| Phosphate ions | May form silver salts depending on conditions | The effect may need to be considered depending on food composition |
| Proteins and turbid components | Make the endpoint difficult to observe | Filtration and dilution are effective |
| Strongly acidic components | Affect the endpoint of the chromate indicator | Check the pH conditions |
In general evaluation of salt content in foods, chloride ions are treated as a salt equivalent.
However, if chloride sources other than salt or components that react with silver ions are present, caution is necessary when interpreting the result.
Example Comparison With Nutrition-Label Values
Comparing the measured salt equivalent with the value on the food label makes it easier to assess the validity of the experimental result.
The following reference example compares labeled and measured values.
| Food | Labeled Value | Measured Value | Difference | Relative Error |
|---|---|---|---|---|
| Soy sauce | 5.40% | 5.24% | -0.16% | -3.0% |
| Miso | 4.50% | 4.33% | -0.17% | -3.8% |
| Pickles | 2.30% | 2.25% | -0.05% | -2.2% |
| Soup | 1.20% | 1.26% | +0.06% | +5.0% |
Example Calculation of Relative Error
When comparing the difference between a labeled value and measured value, relative error makes it easier to evaluate the magnitude of the deviation.
Relative error (%) = (Measured value − Labeled value) ÷ Labeled value × 100
For soy sauce, the labeled value is 5.40% and the measured value is 5.24%.
Relative error = (5.24 − 5.40) ÷ 5.40 × 100 = -3.0%
In this reference example, the measured value is approximately 3.0% lower than the labeled value.
Insufficient extraction and differences in endpoint determination can be considered possible causes.
Example of How to Write the Results
A 5.00 g portion of food was extracted with water and diluted to 100.0 mL.
Then, 10.00 mL of the extract was aliquoted and titrated with 0.0500 mol/L AgNO3 standard solution.
For the soy-sauce sample, the AgNO3 titration volume was 8.95 mL, and the amount of Cl- in the aliquot was 4.48 × 10−4 mol.
When the amount of Cl- was converted to NaCl, the NaCl equivalent in 10.00 mL of the aliquot was 0.0262 g.
The amount in the total 100.0 mL extract was 0.262 g, and the salt concentration relative to the 5.00 g food sample was determined to be 5.24%.
Comparing the foods, the salt concentration was 5.24% for soy sauce, 4.33% for miso, 2.25% for pickles, 1.26% for soup, and 0.64% for white bread.
Soy sauce and miso had high salt concentrations, while white bread and soup had relatively low values.
Points for Connecting the Results to the Discussion
In salt determination by the Mohr method, it is important not only to calculate the salt equivalent from the titration volume but also to discuss endpoint judgment, pH, sample color and turbidity, and the effects of coexisting components.
- Has the 1:1 reaction between Ag+ and Cl- been correctly reflected in the calculation?
- Can the NaCl amount in the aliquot be converted to the amount in the entire extract and then to the amount in the entire food sample?
- Can the result be expressed as salt concentration relative to the food sample mass?
- Can it be explained that titrating until a dark reddish-brown endpoint causes the salt amount to be overestimated?
- Can it be discussed that the endpoint becomes unclear under strongly acidic or strongly alkaline conditions?
- Can it be explained that endpoint judgment becomes difficult in colored or turbid samples such as soy sauce and miso?
- Have the effects of chloride ions not originating from salt and other coexisting components that react with silver ions been considered?
- Can differences from labeled values be related to insufficient extraction, filtration, dilution, and titration errors?
Example Discussion
In this experiment, chloride ions in foods were determined by the Mohr method and expressed as a salt equivalent.
For the soy-sauce sample, the titration volume of 0.0500 mol/L AgNO3 standard solution was 8.95 mL.
Because Ag+ and Cl- react in a 1:1 ratio, the amount of Cl- in 10.00 mL of the aliquoted extract was determined to be 4.48 × 10−4 mol.
When this amount of chloride ions was converted to NaCl, the salt equivalent in the total 100.0 mL extract was 0.262 g.
Because the amount of soy sauce collected was 5.00 g, the salt concentration was 5.24%.
Compared with the labeled value of 5.40%, the measured value was approximately 3.0% lower and was generally close to the labeled value.
One possible reason the measured value was lower than the labeled value is that the salt in the food may not have been completely extracted.
Particularly in miso and solid foods, salt may remain inside the food, and insufficient stirring or extraction time may cause the result to be underestimated.
On the other hand, if titration is continued until a dark reddish-brown color appears, excess silver nitrate is added and the salt amount may be overestimated.
In the Mohr method, chloride ions react with Ag+ to form a white precipitate of AgCl.
After almost all chloride ions have been consumed, excess Ag+ reacts with chromate ions to form reddish-brown Ag2CrO4.
Therefore, the endpoint must be judged as the point at which a slight reddish-brown color remains in the white precipitate.
In strongly colored samples such as soy sauce and turbid samples such as miso, the reddish-brown endpoint may be difficult to observe.
Sufficient dilution and filtration are considered useful for reducing endpoint-determination error.
In addition, if ions such as Br- and I- that form precipitates with Ag+ are present, the amount of chloride ions may be overestimated, so attention must also be paid to the components of the sample.
Summary
In the Mohr method, chloride ions in foods are determined using the 1:1 reaction between Ag+ and Cl-.
By converting the obtained amount of chloride ions to NaCl, the salt equivalent in the food can be determined.
In this reference example, the salt concentration was 5.24% for soy sauce, 4.33% for miso, and 2.25% for pickles.
In a report, it is useful to discuss the titration volume, conversion to salt, dilution factor, endpoint judgment, pH, coloration and turbidity, and effects of coexisting components in relation to one another.
What Is the Mohr Method?
The Mohr method is a precipitation-titration method for determining chloride ions using a silver nitrate standard solution.
Cl- in a food extract reacts with Ag+ to form a sparingly soluble AgCl precipitate.
After almost all Cl- has precipitated, excess Ag+ reacts with chromate ions in the indicator to produce reddish-brown Ag2CrO4 precipitate.
The point at which this reddish-brown color appears persistently is taken as the endpoint, and the silver nitrate titration volume is read.
In the Mohr method, it is important to understand the sequence in which AgCl precipitate and Ag2CrO4 precipitate are formed.
Because endpoint determination greatly affects the result, the color change must be observed carefully.
Ag+ + Cl- → AgCl↓
2Ag+ + CrO42- → Ag2CrO4↓
Example Discussion:
In the Mohr method, Cl- in the food extract reacts with Ag+ to form a white precipitate of AgCl.
After Cl- has been consumed, excess Ag+ reacts with chromate ions to produce reddish-brown Ag2CrO4, so this color change can be used to determine the endpoint.
Because the silver nitrate titration volume corresponds to the amount of Cl-, it can be used to calculate the salt equivalent.
Relationship Between Chloride Ions and Salt Content
Sodium chloride, NaCl, dissociates into Na+ and Cl- when dissolved in water.
Because the Mohr method determines Cl-, the amount of NaCl is calculated from the determined amount of Cl-.
In NaCl, Na+ and Cl- are present in a 1:1 ratio, so the amount of Cl- corresponds to the amount of NaCl.
However, foods may contain chlorides other than NaCl, such as potassium chloride or chlorides derived from food additives.
In this case, if all Cl- is assumed to originate from NaCl, strictly speaking, the result represents a “salt equivalent” rather than the amount of “salt itself.”
Example Discussion:
Because NaCl dissociates into Na+ and Cl- in water, the amount of NaCl can be determined from the amount of Cl-.
In this experiment, the amount of Cl- determined by silver nitrate titration was converted to NaCl to calculate the salt equivalent in the food.
However, because not all Cl- in foods necessarily originates from NaCl, the result must be interpreted as a salt equivalent.
Relationship Between Silver Nitrate Titration Volume and Salt Content
A larger silver nitrate titration volume indicates that more Cl- is contained in the food extract.
Because Ag+ and Cl- react in a 1:1 ratio, the amount of Cl- can be determined from the concentration and titration volume of the AgNO3 standard solution.
The amount can then be converted to a salt equivalent using the molar mass of NaCl.
In other words, foods requiring larger titration volumes may contain more salt.
Pickles, miso, soy sauce, soup, ham, cheese, processed foods, and similar products may contain large amounts of salt.
However, if sample dilution or extraction conditions differ, direct comparison is not possible, so values must be converted to an amount per unit mass of sample before comparison.
Example Discussion:
Food samples requiring larger silver nitrate titration volumes are considered to contain larger amounts of Cl- in the extract and therefore larger salt equivalents.
Because Ag+ and Cl- react in a 1:1 ratio, AgNO3 consumption directly corresponds to the amount of Cl-.
When comparing salt contents among foods, the sample mass and dilution factor must be standardized and the results converted to units such as per 100 g.
Importance of Pretreatment of Food Samples
Foods are not simple aqueous solutions and may be solid, semisolid, liquid, or contain oils and other components with various properties.
Therefore, to accurately measure salt, chloride ions in the food must be sufficiently extracted into water.
Pretreatments such as finely crushing the sample, mixing it thoroughly, and extracting it with a fixed amount of water are important.
If pretreatment is insufficient, salt in the food may not completely transfer to the extract, causing the salt amount to be underestimated.
In nonuniform foods, the salt content may also differ depending on the part collected.
Preparing a representative sample is important.
Example Discussion:
In salt determination in foods, it is important to sufficiently extract Cl- from the sample.
If crushing or mixing of the sample is insufficient, salt in the food may not completely transfer to the extract and the salt equivalent may be underestimated.
Particularly for solid or nonuniform foods, a representative sample must be collected and sufficiently homogenized before measurement.
Errors Caused by Insufficient Extraction
Salt in foods often exists in water-soluble forms, but it may be trapped inside the food or distributed within proteins and tissues.
If the extraction time is short, stirring is insufficient, the sample pieces are large, or the amount of water is small, the salt may not be sufficiently extracted.
When extraction is insufficient, the amount of Cl- used for titration is smaller than the actual amount and the salt equivalent is determined as too low.
Heated extraction, sufficient stirring, and fine grinding may improve extraction efficiency.
However, it is important to follow the conditions specified in the laboratory manual.
Example Discussion:
One possible reason the salt equivalent was lower than expected is that Cl- in the food was not sufficiently extracted.
If the sample pieces are large or the stirring time is short, salt inside the food may not completely transfer to the extraction solution.
Therefore, finely dividing the sample and extracting it with a sufficient amount of water for an adequate period are important for accurate salt determination.
Effects of Filtration and Turbidity
Food extracts may contain solids, oils, proteins, starch, pigments, and other components.
If large amounts of these substances are present, the precipitate and endpoint color change during titration may become difficult to observe.
Therefore, filtration or centrifugation may be performed when necessary to obtain as transparent an extract as possible.
However, if part of the extract is lost during filtration or liquid containing chloride ions remains in the solids, the salt content may be underestimated.
Adhesion to filter paper or equipment and insufficient washing must also be considered.
Filtration makes endpoint determination easier but can also become a source of operational error.
Example Discussion:
If the food extract is turbid, the reddish-brown precipitate that indicates the endpoint of the Mohr method becomes difficult to distinguish and an error may occur in the titration volume.
Removing turbidity by filtration makes the endpoint easier to judge, but loss of extract during filtration may cause the amount of Cl- to be underestimated.
Therefore, equipment and residues should be appropriately washed after filtration to minimize loss of the extract.
Discussion of Endpoint Determination
In the Mohr method, reddish-brown Ag2CrO4 precipitate appears at the endpoint.
However, if the food extract is colored or turbid, the endpoint color may become difficult to observe.
In addition, when a large amount of white AgCl precipitate is formed, the solution becomes cloudy and the reddish-brown change may be difficult to judge.
If the endpoint is missed and too much silver nitrate is added, the amount of chloride ions is overestimated.
Conversely, stopping before the endpoint results in underestimation.
Near the endpoint, it is important to add the titrant carefully one drop at a time, mix thoroughly, and confirm the color change.
Example Discussion:
In the Mohr method, accurately determining the reddish-brown precipitate at the endpoint is important.
If titration continues beyond the endpoint, the AgNO3 titration volume becomes excessive and the amounts of Cl- and salt equivalent may be overestimated.
Because the endpoint becomes difficult to observe in colored or turbid food extracts, filtration and thorough mixing should be performed and titration near the endpoint should be carried out carefully.
Effect of pH Conditions
In the Mohr method, pH conditions affect endpoint determination.
Under strongly acidic conditions, the state of chromate ions changes and reddish-brown silver chromate precipitate may become difficult to form.
On the other hand, under excessively alkaline conditions, silver ions may form other precipitates.
Food extracts may be acidic, neutral, or alkaline depending on the type of food.
Pickled foods, fermented foods, foods containing fruit juice, and similar products may be acidic.
Therefore, when using the Mohr method, it is necessary to check whether the pH has been adjusted to the range specified in the laboratory manual.
Example Discussion:
Because chromate ions are used as the indicator in the Mohr method, pH conditions affect endpoint determination.
If the food extract is acidic, formation of the reddish-brown endpoint precipitate may become difficult and an error may occur in the titration volume.
Therefore, the pH conditions must be checked according to the properties of the food sample and titration performed within the specified range.
Interference From Food Components
Food extracts contain many components other than chloride ions.
Proteins, amino acids, organic acids, pigments, sugars, oils, phosphates, food additives, and other components may affect precipitate formation or endpoint determination.
Particularly in strongly colored foods, the reddish-brown endpoint may become difficult to observe.
In addition, if halide ions other than Cl- are present, they also form precipitates with Ag+ and may cause the chloride-ion amount to be overestimated.
Although Cl- is the primary target in ordinary foods, the effects of coexisting components must be considered depending on the type of sample.
Example Discussion:
Because food extracts contain many components such as proteins, organic acids, pigments, and sugars, they may affect endpoint determination and precipitate formation.
Particularly in strongly colored foods, the reddish-brown color of Ag2CrO4 may be difficult to distinguish and may cause an error in the titration volume.
In addition, if ions other than Cl- that react with Ag+ are present, the salt equivalent may be overestimated.
Comparison With Food-Label Values
In salt determination in foods, the obtained salt equivalent may be compared with the food-label value.
Labeled values are generally based on specified analytical methods or calculations, but they may not completely agree with experimental values because of product variation, measurement method, sample amount, and extraction conditions.
Particularly in homemade or nonuniform foods, the salt content may differ depending on the part sampled.
If the experimental value is higher than the labeled value, possible causes include titration beyond the endpoint, silver nitrate consumption by coexisting components, and errors in the dilution factor.
If it is lower, possible causes include insufficient extraction, errors in sample mass, losses during filtration, and stopping titration before the endpoint.
Differences in measurement methods should also be considered when comparing with labeled values.
Example Discussion:
Possible reasons the experimentally determined salt equivalent differed from the food-label value include nonuniformity of the sample, insufficient extraction, endpoint-determination error, and differences in measurement methods.
Whereas the labeled value represents an average value for the entire product, only part of the sample is used in the experiment, so differences caused by the sampled region may occur.
Therefore, when comparing with food-label values, the representativeness of the sample and differences in measurement methods must be considered.
Differences in Salt Content Among Foods
The salt content of foods varies greatly depending on the type of food and processing method.
Pickles, miso, soy sauce, soups, noodle dipping sauces, ham, cheese, fish-paste products, instant foods, and similar products tend to contain large amounts of salt.
In contrast, fresh foods and nearly unsalted foods contain lower amounts of salt.
In processed foods, salt may be used not only for seasoning but also to maintain preservability and texture.
Therefore, when discussing differences in salt content, attention should also be paid to the purpose of processing and the storage method.
Even the same type of food may have different salt contents depending on the product and manufacturing method.
Example Discussion:
If a high salt equivalent was obtained for pickles or processed foods, this may be because salt was added not only for seasoning but also to improve preservability.
On the other hand, fresh or lightly seasoned foods contain smaller amounts of Cl- and therefore require smaller silver nitrate titration volumes.
The salt content of foods varies greatly depending on food type, processing method, preservation purpose, and degree of seasoning.
Errors Caused by Nonuniformity of the Sample
The salt content of food may differ depending on the part of the sample.
For example, salt distribution may differ between the surface and interior of pickles, between solid ingredients and broth in foods containing soup, and between the edges and center of ham or cheese.
If only one part is removed and measured, it may not represent the average value of the entire food.
To improve representativeness, the sample is thoroughly mixed and, when necessary, finely chopped, ground, or homogenized.
Collecting samples from multiple locations and averaging them is also effective.
Homogenization of the sample is extremely important in food analysis.
Example Discussion:
One possible reason for variation in the salt equivalent is that the food sample was nonuniform.
Salt content may differ depending on the part of the food, and measuring only one portion may not reflect the average value of the entire sample.
Therefore, the sample should be finely divided and thoroughly mixed so that a representative portion can be used for measurement.
Discussion of the Dilution Factor and Calculations
If the chloride-ion concentration of a food extract is high, the extract may be diluted to bring it into a range that is easier to measure.
When a diluted sample is titrated, the dilution factor must be correctly reflected in the calculation.
Forgetting to apply the dilution factor causes the salt amount in the original food to be greatly underestimated.
The mass of the food sample, total volume of the extract, aliquot volume used for titration, dilution factor, and conversion to NaCl must all be handled correctly.
The salt content of foods may be expressed per 100 g or per serving depending on the purpose.
Errors in handling units can greatly distort the entire discussion.
Example Discussion:
When a food extract is diluted before measurement, the dilution factor must be reflected in the obtained amount of Cl-.
Errors in the dilution factor or total extract volume cause the salt equivalent in the food to be underestimated or overestimated.
In addition, when comparing with food-label values, it is important to convert all values to the same unit, such as per 100 g or per serving.
Concentration Error of the Silver Nitrate Standard Solution
In salt determination in foods, the concentration of the silver nitrate standard solution serves as the basis for the calculation.
If the standard-solution concentration is inaccurate, the salt amounts for all samples are systematically shifted.
Because silver nitrate solution may change depending on storage conditions, it is standardized as necessary before use.
If the concentration of the silver nitrate standard solution is treated as higher than its actual value, the amount of Cl- may also be overestimated.
Conversely, if the concentration is underestimated, the salt equivalent also becomes lower.
Management of the standard-solution concentration is directly related to the reliability of titrimetric analysis.
Example Discussion:
Because the salt equivalent is calculated from the concentration and titration volume of the AgNO3 standard solution, an error in the standard-solution concentration affects the entire result.
If the concentration of the AgNO3 standard solution is inaccurate, the salt content of all food samples is systematically determined as either too high or too low.
Therefore, the standard solution must be accurately prepared and standardized when necessary before use.
Importance of Blank Correction
Blank correction is performed to correct for silver nitrate consumption originating from reagents, water, equipment, and operational procedures.
If the water or reagents used for extraction contain trace amounts of chloride ions, Cl- not originating from the food sample is also measured.
In foods with low salt content, the effect of the blank is relatively large.
If the blank value is large, the purity of the water, insufficient cleaning of equipment, contamination of reagents, and contamination during operation should be checked.
For accurate determination, it is important to isolate and evaluate the amount of chloride ions originating from the food sample.
Example Discussion:
Blank correction is important for removing the effects of Cl- originating from reagents, water, and equipment.
If blank correction is not performed, chloride ions not originating from the food are also included in the calculation and the salt equivalent may be overestimated.
Particular care is required for foods with low salt content because the effect of the blank value becomes relatively large.
Causes of Error in Salt Determination in Foods
Causes of error in salt determination in foods include nonuniformity of the sample, insufficient extraction, loss during filtration, errors in the dilution factor, endpoint-determination error, concentration error in the silver nitrate standard solution, burette-reading error, inappropriate pH conditions, interference from food components, and insufficient blank correction.
In food analysis, errors in pretreatment as well as in the titration procedure can become large.
In high-salt foods, errors in dilution procedures can have a large effect, while in low-salt foods the titration volume is small, so the effect of an error of one drop or of the blank becomes large.
In addition, endpoint determination becomes difficult in strongly colored foods and turbid extracts.
It is important to consider sources of error according to the type of food.
Example Discussion:
Possible causes of error in salt determination in foods include nonuniformity of the sample, insufficient extraction of Cl-, deviation in endpoint determination, and errors in the dilution factor.
If titration continues beyond the endpoint, the AgNO3 titration volume becomes excessive and the salt equivalent is overestimated.
On the other hand, insufficient extraction or loss during filtration may cause the salt content of the food to be underestimated.
When the Results Can Be Considered Good
Salt-determination results in foods can be considered good when the food sample has been sufficiently homogenized, the extraction conditions are clear, the endpoint is easy to determine, and multiple titration values agree closely.
In addition, if the obtained salt equivalent does not greatly contradict the type of food or the labeled value, the result is easier to consider valid.
For example, high values for pickles and processed foods and low values for fresh foods are consistent with the properties of the foods.
If a value close to the food-label value is obtained, the extraction and titration can be considered generally appropriate.
However, exact agreement with the labeled value is not necessary, and differences in measurement methods and samples must be considered.
Example Discussion:
In this experiment, there was no large variation among repeated titration volumes and the endpoint could also be observed relatively clearly.
In addition, the salt equivalent did not greatly contradict the type of food or the labeled value.
From these results, the salt determination by the Mohr method used in this experiment was considered to approximately reflect the amount of Cl- in the food.
Example Discussion When the Experiment Did Not Go Well
When salt determination in foods does not go well, possible causes can be considered from results such as difficulty observing the endpoint, variation in titration values, large differences from labeled values, unnatural trends among foods, turbid extracts, and unclear handling of the dilution factor.
Organizing the causes according to pretreatment, extraction, filtration, titration, calculation, and comparison with labeled values makes the discussion easier.
Example Discussion:
In this experiment, the obtained salt equivalent was lower than the food-label value.
Possible causes include insufficient transfer of Cl- from the food into the extract, loss of part of the extract during filtration, and stopping titration before the endpoint.
In addition, if the sample was nonuniform, the portion used for measurement may not have represented the average salt content of the entire food.
How to Write Points for Improvement
In a discussion of salt determination in foods, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into sample preparation, extraction, titration, and calculation and analysis.
Improvements to Sample Preparation
- Finely chop the food sample
- Mix the sample thoroughly
- Collect a representative portion
- Accurately weigh the sample mass
- Collect samples from multiple locations and average them
- Homogenize both solid and liquid portions
Improvements to Extraction and Pretreatment
- Accurately measure the amount of extraction water
- Stir for a sufficient period
- Perform heated extraction when necessary
- Avoid loss of liquid during filtration
- Wash the residue into the solution
- Record the dilution factor
Improvements to Titration and Calculation
- Confirm the concentration of the silver nitrate standard solution
- Remove air bubbles from the burette
- Add the titrant one drop at a time near the endpoint
- Check the pH conditions of the extract
- Perform blank correction
- Reflect the dilution factor in the calculation
- Use consistent units, such as per 100 g or per serving, for comparison
Example of How to Write Points for Improvement:
To improve the accuracy of salt determination in foods, the food sample must be sufficiently finely divided and thoroughly mixed so that a representative sample can be collected.
In addition, to completely extract Cl-, the extraction time and stirring conditions should be kept constant and losses during filtration minimized.
During titration, silver nitrate should be added one drop at a time near the endpoint, and the dilution factor and blank correction must be correctly reflected in the calculation.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of salt determination in foods, simply writing that “there was a lot of salt” or that “the sample was titrated with silver nitrate” results in a superficial discussion.
A good discussion relates the conversion between Cl- and NaCl, the endpoint of the Mohr method, pretreatment of food samples, differences from labeled values, and causes of error.
| Superficial Discussion | Good Discussion |
|---|---|
| There was a lot of salt. | Because the silver nitrate titration volume was large, the amount of Cl- in the extract was considered large, and the salt equivalent calculated as NaCl was also high. Processed foods may contain large amounts of salt for seasoning and preservation. |
| It was measured by the Mohr method. | In the Mohr method, Ag+ and Cl- form an AgCl precipitate, and after Cl- is consumed, reddish-brown Ag2CrO4 precipitate appears and is used as the endpoint to determine the amount of Cl-. |
| It differed from the labeled value. | The difference from the food-label value may have resulted from nonuniformity of the sample, insufficient extraction, loss during filtration, deviation in endpoint determination, or differences in measurement methods. |
| The values varied. | The variation in titration values may have resulted from insufficient homogenization of the sample, differences in the amount of Cl- extracted, individual differences in endpoint determination, burette-reading error, or difficulty observing the endpoint color because of food components. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of salt determination in foods.
Adjust the necessary parts according to your own experimental results.
- Because salt in foods dissociates into Na+ and Cl- in water, the salt equivalent can be determined from the amount of Cl-.
- In the Mohr method, Cl- is determined using the reaction in which Ag+ and Cl- form an AgCl precipitate.
- The larger the silver nitrate titration volume, the greater the amount of chloride ions considered to be present in the food extract.
- Titrating beyond the endpoint may cause the salt equivalent to be overestimated.
- If the food extract is colored or turbid, the reddish-brown endpoint may be difficult to distinguish.
- If extraction is insufficient, the amount of Cl- in the food may be underestimated.
- In nonuniform foods, the salt content may differ depending on the sampled region.
- Differences from food-label values may result from differences in measurement methods and sample representativeness.
- For diluted samples, the dilution factor must be correctly reflected in the calculation.
- The value calculated from the amount of Cl- should strictly be treated as a salt equivalent.
Points to Check When Discussing Salt Determination in Foods
Checking the following points before writing the report makes the discussion easier to write.
- Is the reason salt in foods is determined from Cl- explained?
- Is the precipitation reaction in the Mohr method explained?
- Is the 1:1 reaction between Ag+ and Cl- understood?
- Is the relationship between the amount of chloride ions and conversion to NaCl described?
- Are pretreatment and extraction conditions for the food sample clearly stated?
- Is the possibility of insufficient extraction considered?
- Is interference with endpoint determination by food components considered?
- Are the effects of pH conditions checked?
- Are the dilution factor and sample mass handled correctly?
- When comparing with food-label values, are the units consistent?
- Are blank correction and errors in the standard-solution concentration considered?
- Do the points for improvement correspond to the causes of error?
Summary
Determination of salt in foods is an analysis in which chloride ions extracted from food are determined by precipitation titration such as the Mohr method and converted to an amount of NaCl to obtain the salt equivalent.
In the Mohr method, Ag+ and Cl- form an AgCl precipitate, and after Cl- has been consumed, excess Ag+ reacts with chromate ions to produce reddish-brown Ag2CrO4, which is used as the endpoint.
The salt content of foods is related to seasoning, preservability, processing methods, and food labeling.
Pickles and processed foods tend to have high salt contents, while fresh and lightly seasoned foods tend to have lower values.
However, because foods are nonuniform samples, the sampled region, homogenization, extraction conditions, and filtration procedure greatly affect the results.
In a report, rather than simply writing that “the salt content was high or low,” organize and discuss the principle of the Mohr method, conversion between Cl- and NaCl, silver nitrate titration volume, pretreatment of food samples, insufficient extraction, endpoint determination, coexisting components, differences from food-label values, causes of error, and points for improvement.
In salt determination in foods, it is important to understand the connection between precipitation titration in analytical chemistry and the properties of foods.
