Blank Correction Discussion Examples | How to Handle Blank Values and Measured Values

What Is Blank Correction?

Blank correction is the operation of subtracting a measured value obtained under conditions without a sample from the measured value obtained with the sample.
A signal or consumption amount that occurs even when no sample is present is called a blank test value or blank value.
By subtracting this blank value, a value originating from the sample itself can be determined more accurately.

Blank values may arise from impurities in reagents, absorption by solvents, contamination of equipment, zero-point shifts in instruments, background light, consumption of titrant, adsorption to reaction vessels, and other factors.
If blank correction is not performed, values not originating from the sample may also be included in the measurement, causing the concentration or amount to be overestimated.

Corrected measured value = Sample measured value – Blank value

Main Items to Include in the Results

To discuss blank correction, it is necessary to clearly distinguish the sample measured value, blank test value, and corrected value.
In addition, checking whether the blank value is large or small, what proportion it represents relative to the measured value, and whether the corrected value is theoretically reasonable makes the discussion easier to write.

Main Items to Include in the Results

• Sample measured value
• Blank test value or blank value
• Corrected measured value
• Solution or conditions used for the blank
• Operations performed in common for the sample and blank
• Number of measurements
• Mean
• Standard deviation
• Magnitude of the blank value
• Proportion of the blank value relative to the measured value
• Difference before and after correction
• Concentration or amount after correction
• Whether the value became negative or close to zero
• Causes of error
• Points for improvement

What Is a Blank Test Value?

A blank test value is the value obtained when the same operations as in the sample measurement are performed without adding the sample.
For example, pure water or solvent is used in place of the sample, and reagent addition, heating, reaction, titration, and measurement are performed in the same way.
The blank test value represents a measured value caused by factors other than the sample.

If the blank test value is close to 0, the influence of factors other than the sample can be considered small.
However, if the blank test value is large, the effects of reagent impurities, solvent absorption, contamination of equipment, or instrument drift may be large.
The blank test value is also important for evaluating the reliability of the measurement method.

Why Is Blank Correction Necessary?

Blank correction is necessary because measured values may include effects other than those of the sample component.
For example, in absorbance measurements, the solvent or reagents themselves may absorb light.
In titration, impurities or dissolved components in the reagents may consume titrant even when no sample is present.

If these background values are not corrected, the concentration or amount of the sample component will be estimated as larger than it actually is.
Particularly for low-concentration samples, the blank value may account for a large proportion of the measured value, and whether correction is performed can greatly affect the result.
Blank correction is important for improving the accuracy of measured values.

Reference Experimental Values and Calculation Example for Blank Correction

Here, the calculation process is examined using reference experimental values in order to understand the concept of blank correction more specifically.
Rather than simply ignoring the blank value, subtracting it from the measured value makes it possible to handle the signal or concentration originating from the sample more appropriately.

Reference Experimental Conditions

Example Measurement Results

First, suppose that the blank sample, standard solutions, and unknown sample are measured under the same conditions.
Absorbance may contain not only values originating from the target component but also slight effects from the solvent, reagents, cell, and instrument.
Therefore, the blank value is measured and subtracted from each measured value for correction.

How to Calculate Blank Correction

The following formula is used for blank correction.

Corrected value = Measured value − Blank value

In this example, the absorbance of the blank is 0.032, so 0.032 is subtracted from the measured absorbance of each sample.

How to Determine Concentration Using a Calibration Curve

The relationship between concentration and absorbance is examined using the values of the standard solutions after blank correction.
Here, suppose that the following calibration curve was obtained from the results of the standard solutions.

Corrected absorbance = 0.0762 × Concentration

Because the corrected absorbance of the unknown sample is 0.378, the concentration can be determined as follows.

0.378 = 0.0762 × Concentration
Concentration = 0.378 ÷ 0.0762 = 4.96 mg/L

Therefore, in this reference example, the concentration of the unknown sample is determined to be approximately 4.96 mg/L.

How the Result Changes Before and After Blank Correction

If the absorbance of the unknown sample, 0.410, is used directly without blank correction, the concentration is calculated as follows.

Concentration = 0.410 ÷ 0.0762 = 5.38 mg/L

On the other hand, when the corrected absorbance of 0.378 is used, the concentration is 4.96 mg/L.
In this way, whether the blank value is subtracted causes a difference in the final concentration.

Example of How to Write the Results

In a laboratory report, rather than simply writing that “blank correction was performed,” it is better to clearly state which value was subtracted and how the corrected value was used.

Because the absorbance of the blank sample was 0.032, correction was performed by subtracting 0.032 from the measured absorbance of each standard solution and the unknown sample.
The measured absorbance of the unknown sample was 0.410, and the absorbance after blank correction was 0.378.
When the concentration of the unknown sample was determined using the calibration curve prepared from the corrected standard solution values, it was 4.96 mg/L.

Points for Connecting the Results to the Discussion

In a discussion of blank correction, it is important to examine the magnitude of the correction value and its effect on the measurement result.
Even when the blank value is small, its relative effect may be large for low-concentration samples.

• How large was the blank value relative to the measured value?
• How much did the concentration of the unknown sample change before and after correction?
• Could the effects of the solvent, reagents, cell, or instrument be considered as causes of the blank value?
• Could the concentration be overestimated if blank correction were not performed?
• In low-concentration samples, could variation in the blank value have a large effect on the result?

Example Discussion

In this experiment, an absorbance of 0.032 was also observed for the blank sample.
This was considered to be because the absorbance included effects originating from the solvent, reagents, cell, instrument, and other factors in addition to the sample component.
Therefore, correction was performed by subtracting the blank value from each measured value.

For the unknown sample, the concentration calculated from the uncorrected absorbance of 0.410 was 5.38 mg/L, whereas the concentration calculated from the absorbance of 0.378 after blank correction was 4.96 mg/L.
This result shows that if blank correction is not performed, the concentration of the unknown sample may be overestimated.

Particularly for low-concentration samples, the blank value tends to account for a large proportion of the measured value, so the effect of whether blank correction is performed cannot be ignored.
Therefore, for accurate quantitative analysis, it is important to measure the blank under the same conditions as the sample and use that value for correction.

Summary

Blank correction is a basic process used to remove effects other than those of the sample from measured values.
By subtracting the blank value from the measured value, the value originating from the target component can be handled more appropriately.

In a report, organizing the blank value, uncorrected measured value, and corrected value in a table and explaining how the result changed as a result of the correction makes the discussion more persuasive.

Discussion of Reagent Blanks

A reagent blank is a background value originating from the reagents themselves.
Even without a sample, a measured value may arise if the reagents contain trace impurities, if the reagents absorb at the measurement wavelength, or if they contain components that consume titrant.
Correcting for this value makes it possible to determine the value originating from the sample.

If the reagent blank is large, possible causes include deterioration of the reagent, contamination with impurities, errors in concentration preparation, or poor storage conditions.
If the reagent blank is large relative to the measured value, the uncertainty of the corrected value also becomes large.
Quality control of reagents and the use of fresh reagents are possible improvements.

Discussion of Solvent Blanks

A solvent blank is the effect of the solvent itself on the measured value.
In absorbance measurements, the solvent may absorb a small amount of light at the measurement wavelength.
In addition, impurities in the solvent, dissolved oxygen, or turbidity of the solvent may affect the measured value.
To correct for these effects, the blank is measured using only the solvent.

If the solvent blank is inappropriate, all measured values will be shifted.
If the type or concentration of solvent, pH, or added reagents differ between the sample solution and blank solution, the correction will not be accurate.
The blank must be measured under conditions other than the presence of the sample that are as similar as possible to those of the sample.

Discussion of Instrument Blanks and Zero-Point Drift

An instrument blank is a background value originating from the measuring instrument itself.
In optical instruments, stray light, detector dark current, zero-point drift, contamination of the cell holder, and other factors may have an effect.
In electrochemical measurements, electrode offset or background current may affect the measured value.

If the zero point of the instrument is shifted, all measured values may shift in the same direction.
In such a case, the intercept may deviate greatly even if the correlation coefficient is high.
Zero adjustment before measurement, blank measurement, and stabilization of the instrument are important.

Blank Correction in Absorbance Measurements

In absorbance measurements, not only the sample component but also the solvent, reagents, cell, turbidity, and background signal of the instrument may affect the absorbance.
Therefore, zero adjustment is performed using a blank solution that does not contain the sample, and the absorbance originating from the sample component is determined.
As a basic rule, the blank solution should contain the same components as the sample solution except for the sample itself.

If the blank is inappropriate, the absorbance will be measured as too high or too low.
For example, if the color-developing reagent is omitted from the blank, absorption originating from the reagent cannot be corrected.
Contamination or bubbles in the cell also affect the blank and sample values, so it is important to measure them under the same cell conditions.

Discussion of Blank Test Values in Titration

In titration experiments, titrant may be consumed even under conditions without a sample.
This is because impurities in the reagents, dissolved components, indicators, solvents, or components originating from the container may react with the titrant.
By subtracting the blank test value, the amount of titrant actually consumed by the sample component can be determined.

If the blank test value is large, it greatly affects the concentration or content determined by titration.
Particularly when the titration volume of the sample is small, even a small error in the blank test value results in a large relative error.
It is important to perform the blank test using the same operations except for the sample.

Corrected titration volume = Sample titration volume – Blank titration volume

Blank Correction in COD Measurements

In COD measurements, organic matter and other substances in a sample are oxidized using an oxidizing agent, and the degree of pollution is evaluated from the amount of oxidizing agent consumed.
However, even under conditions without a sample, the oxidizing agent may be consumed as a result of reagents or operations.
Therefore, the blank test value must be measured and used to correct the sample measured value.

If blank correction is not performed, the amount of oxidizing agent consumed by organic matter in the sample may be overestimated.
In experiments such as COD measurements that deal with oxidizing-agent consumption, the stability of the blank test value has a large effect on the reliability of the results.
Deterioration of reagents or contamination of equipment may cause the blank test value to increase.

Effects of Not Performing Blank Correction

If blank correction is not performed, values that do not originate from the sample remain included in the measurement result.
In many cases, the measured value becomes larger than the actual value, resulting in overestimation of concentration or content.
Particularly when the amount of the component in the sample is small, the effect of the blank value becomes relatively large.

For example, if the sample value is 0.100 and the blank value is 0.010, the blank accounts for 10% of the measured value.
If this is not corrected, the result contains a large systematic error.
Blank correction becomes more important as the concentration being measured decreases.

Discussion When the Blank Value Is Large

If the blank value is large, factors other than the sample may be having a large effect on the measured value.
Possible causes include contamination of reagents, impurities in the solvent, insufficient cleaning of equipment, zero-point drift of the instrument, errors in preparing the blank solution, and contamination from the environment.
The larger the blank value, the more likely the reliability of the corrected value is to decrease.

If the blank value is close to the sample value, the corrected value becomes small, and even a small error in the blank has a large effect on the result.
In this case, the sensitivity of the measurement method may be insufficient, the sample concentration may be too low, or blank management may be inadequate.
It is necessary to review the reagents and equipment and take measures to reduce the blank value.

Discussion When the Blank Value Is Small

If the blank value is small, background signals or consumption amounts originating from factors other than the sample are small, and the measurement conditions can be considered relatively good.
The reagents and solvents may have been highly pure, the equipment may have been clean, and the zero point of the instrument may have been stable.
The change in the value caused by blank correction will also be small.

However, even if the blank value is small, its relative effect may not be negligible if the sample measured value is extremely small.
In addition, the blank value may have happened to be small by chance, so performing the blank test multiple times and confirming its stability increases reliability.

When the Corrected Value Becomes Smaller

When blank correction is performed, the blank value is subtracted from the sample measured value, so the corrected value becomes smaller than the uncorrected value.
This is the result of removing the background value included in the sample measured value.
It is natural for the corrected value to become smaller.

However, if the corrected value decreases much more than expected, the blank value may be too large.
If the blank value is close to the sample measured value, the reliability of the corrected value decreases.
For low-concentration samples, the corrected value may also be close to the detection limit or quantification limit.

When the Corrected Value Becomes Negative

The value after blank correction may become negative.
This occurs when the sample measured value is smaller than the blank value.
Chemically, a concentration or amount cannot be negative, so a negative corrected value may indicate that the measured value is approximately the same as the blank or within the range of measurement error.

If a negative value is obtained after correction, possible causes include an extremely low sample concentration, an unstable blank value, large measurement noise, or a mismatch between the blank and sample conditions.
In this case, the possibility that the result is below the detection limit and the need for remeasurement should be discussed.
It is important not to treat a negative value directly as a physical concentration.

Blank Correction in Low-Concentration Measurements

Blank correction is particularly important in low-concentration measurements.
Because the signal originating from the sample is small, background signals originating from reagents or instruments have a relatively large effect.
Even if the blank value is small, if it represents a large proportion of the sample value, considerable uncertainty arises in the corrected concentration.

For low-concentration samples, variation in blank test values, measurement noise, detection limits, and quantification limits must be considered.
If the standard deviation of the blank value is large, the reliability of the corrected value decreases.
It is desirable to perform multiple blank measurements and confirm the mean and variation.

Blank Correction and the Detection Limit

The detection limit is a guideline for the minimum concentration or amount at which the target component can be judged to be present.
If variation in the blank value is large, the signal of a low-concentration sample may be buried within the variation of the blank, making it difficult to determine that the target component has been detected.
Therefore, the detection limit is closely related to the stability of the blank value.

If the value after blank correction is small or negative, the sample signal may be near the detection limit.
In this case, instead of treating the corrected value as a definitive concentration, it may be discussed as being below the detection limit or difficult to quantify.
Handle the result according to the instructions in the laboratory manual or those given by the instructor.

Blank Correction and Calibration Curves

When preparing a calibration curve, blank correction affects the intercept and slope.
If blank correction is insufficient, a measurement signal may remain even at concentration 0, and the intercept of the calibration curve may deviate greatly.
A large intercept also affects the calculation of the concentration of an unknown sample.

In a calibration curve, it is important to measure the standard solutions and blank solution under the same conditions and correctly subtract the background signal.
However, a nonzero intercept should not automatically be attributed entirely to insufficient blank correction; zero-point drift of the instrument, absorption by reagents, and concentration preparation errors must also be considered.

Variation in Blank Values

Measuring the blank value only once is not always sufficient.
Because of instrument noise and variation in operations, the blank value may also vary from one measurement to another.
If variation in the blank value is large, uncertainty in the corrected value also becomes large.

Particularly in low-concentration measurements, variation in the blank value greatly affects the result.
Performing multiple blank measurements and using the mean can reduce the effect of random error.
In addition, checking the standard deviation of the blank value makes it possible to evaluate the stability of the measurement method.

Importance of Keeping Sample and Blank Conditions the Same

In blank correction, conditions other than the presence of the sample must be kept as similar as possible to those of the sample measurement.
If the amount of reagent, solvent, pH, reaction time, heating conditions, measurement wavelength, cell, container, or other conditions differ, the blank value may not correctly represent the background value of the sample measurement.
A blank with inconsistent conditions is inappropriate for correction.

For example, if the color-developing reagent is included in the sample measurement but not in the blank, absorption originating from the color-developing reagent cannot be corrected.
In experiments involving heating, the blank must also be heated in the same way.
It is important that the blank is not simply pure water but a solution under the same conditions with only the sample omitted.

Causes of Error in Blank Correction

Causes of error related to blank correction include errors in preparing the blank solution, differences in reagent amounts, differences in measurement conditions, variation in blank test values, contamination of equipment, zero-point drift of the instrument, an insufficient number of blank measurements, and differences in pH between the sample and blank.
If these occur, correct correction cannot be performed.

If the blank value is measured as larger than it actually is, the corrected value becomes too small.
Conversely, if the blank value is measured as smaller than it actually is, the corrected value becomes too large.
Because blank correction directly affects the entire result, it is important to confirm the validity of the blank test value.

When the Results Can Be Considered Good

From the perspective of blank correction, results can be considered good when the blank value is small and stable and is not excessively large relative to the sample measured value.
In addition, if the corrected value agrees well with the theoretical value or the value of a standard sample and the reproducibility of the measurement is also good, the blank correction can be considered appropriate.

However, even if the blank value is small, caution is necessary when the sample value is extremely small.
Checking the proportion of the blank relative to the measured value and considering whether the corrected value is sufficiently larger than the detection or quantification limit leads to a more reliable discussion.

Example Discussions When the Experiment Did Not Go Well

If blank correction did not work well, consider the cause based on results such as a large blank test value, large variation in blank test values, a negative corrected value, a greatly shifted calibration curve intercept, or a corrected concentration that does not agree with the theoretical value.
Organizing the causes into reagents, solvents, equipment, instruments, operating conditions, and the effects of low-concentration measurements makes the discussion easier.

How to Write Points for Improvement

In a discussion of blank correction, writing not only about the cause of the blank test value and the validity of the corrected value but also about how errors originating from the blank can be reduced makes the report easier to organize.
Points for improvement can be organized into reagents, equipment, measurement conditions, the number of blank measurements, and measures for low-concentration measurements.

Improvements to Reduce the Blank Value

• Use high-purity reagents and solvents
• Prepare fresh reagents
• Thoroughly clean the equipment
• Avoid contamination of reagents and solvents
• Stabilize the instrument before measurement
• Check cells and containers for contamination
• Prepare the blank solution under the same conditions as the sample
• Repeat the measurement if the blank value is abnormally large

Improvements to Increase the Reliability of Correction

• Measure the blank multiple times
• Use the mean of the blank test values
• Check the standard deviation of the blank values
• Check the proportion of the blank value relative to the sample measured value
• Consider the detection limit for low-concentration samples
• Confirm the validity of the correction method using a standard sample
• Check whether the intercept of the calibration curve is reasonable
• If the corrected value becomes negative, consider remeasurement or concentration of the sample

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of blank correction, simply writing “the blank value was subtracted” or “correction was performed” results in a superficial discussion.
A good discussion explains what the blank test value originates from, how the result would shift without correction, and how reliable the corrected value is.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of blank correction and blank test values.
Adjust the necessary parts according to your own experimental results.

• Blank correction is performed to subtract background signals that arise under conditions without a sample.
• The blank test value represents measured values originating from reagents, solvents, equipment, and instruments.
• If blank correction is not performed, the amount of the sample component may be overestimated.
• By subtracting the blank test value from the sample measured value, the net measured value originating from the sample was obtained.
• If the blank test value is large, contamination of reagents or zero-point drift of the instrument may be possible causes.
• If the blank value is large relative to the sample measured value, the uncertainty of the corrected value increases.
• If the corrected value becomes negative, the sample signal may have been near the detection limit.
• The blank solution must have the same conditions as the sample solution except for the presence of the sample.
• In low-concentration measurements, variation in the blank value greatly affects the result.
• To increase the reliability of blank correction, it is important to perform the blank test multiple times and confirm the mean and variation.

Points to Check When Discussing Blank Correction

Checking the following points before writing the report makes the discussion easier to write.

• Are the sample measured value and blank test value shown separately?
• Has the corrected value been calculated?
• Is the origin of the blank test value explained?
• Is the effect of not performing blank correction described?
• Has it been evaluated whether the blank value is large or small?
• Has the proportion of the blank value relative to the sample measured value been checked?
• Has it been confirmed that the sample and blank conditions are consistent?
• Has it been checked whether the corrected value is negative or close to zero?
• Is the detection limit considered for low-concentration measurements?
• Has variation in the blank test value been considered?
• Has the relationship with the intercept of the calibration curve been considered?
• Do the points for improvement correspond to the causes of error?

Summary

Blank correction is the operation of subtracting the blank test value obtained under conditions without a sample from the sample measured value to obtain the net value originating from the sample.
Blank test values originate from reagents, solvents, equipment, instruments, background signals, and other factors.
If correction is not performed, the amount or concentration of the sample component may be overestimated.

In blank correction, it is important to keep the conditions of the sample and blank consistent.
If the amount of reagent, solvent, pH, reaction time, heating conditions, measurement wavelength, cell, or other conditions differ, the correct background value cannot be subtracted.
In addition, if the blank value is large or shows large variation, the uncertainty of the corrected value also increases.
Particularly in low-concentration measurements, the effect of the blank value becomes very large.

In a report, rather than simply writing that “the blank test value was subtracted,” organize and discuss the meaning of the blank test value, the values before and after correction, causes of background signals, magnitude of the blank value, effects on low-concentration measurements, negative corrected values, the intercept of the calibration curve, causes of error, and points for improvement.
Discussion of blank correction is important for determining whether the measured value truly represents a value originating from the sample.