Discussion Examples for Electrolytic Refining of Copper | Anode Slime, Deposition Amount, and Current Efficiency

What Is Electrolytic Refining of Copper?

Electrolytic refining of copper is an electrochemical refining method used to obtain high-purity copper from impure blister copper.
The blister copper is used as the anode and pure copper as the cathode, and they are placed in an aqueous solution such as CuSO4 and supplied with direct current.
Copper at the anode then dissolves as Cu2+, while Cu2+ at the cathode accepts electrons and is deposited as metallic copper.

In this method, metals that are less easily ionized than copper are difficult to dissolve at the anode and precipitate as anode slime.
On the other hand, metals that are more easily ionized than copper may dissolve from the anode and remain in the solution, but under certain conditions they are less likely to deposit at the cathode than copper.
Therefore, relatively high-purity copper is deposited at the cathode.

Main Items to Include in the Results

In the results of an electrolytic refining of copper experiment, organize the type of electrolyte, electrode materials, current, voltage, energization time, changes in anode mass, changes in cathode mass, condition of the deposited copper, presence or absence of anode slime, changes in the color of the electrolyte, and other information.
When calculating deposition amount and current efficiency, the current and energization time must be recorded accurately.

Main Items to Include in the Results

• Type of electrolyte
• Electrolyte concentration
• Anode material
• Cathode material
• Anode mass before energization
• Anode mass after energization
• Cathode mass before energization
• Cathode mass after energization
• Decrease in anode mass
• Increase in cathode mass
• Color and condition of deposited copper
• Presence or absence of anode slime
• Current value
• Voltage
• Energization time
• Theoretical deposition amount
• Current efficiency
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Electrolytic Refining of Copper

Here, electrolysis is carried out using blister copper as the anode and a pure copper plate as the cathode, and dissolution of the anode, deposition of copper on the cathode, formation of anode slime, and current efficiency are organized.

In electrolytic refining of copper, the blister copper at the anode dissolves to form Cu2+, while at the cathode, Cu2+ accepts electrons and is deposited as high-purity copper.
Difficult-to-dissolve components such as gold and silver contained in blister copper may remain at the bottom as anode slime.

Reference Experimental Conditions

Confirmation of Electrode Reactions

In electrolytic refining of copper, copper is oxidized at the anode and copper ions are reduced at the cathode.
Overall, copper in the blister copper moves to the cathode side and is deposited as higher-purity copper.

Mass Measurement Results Before and After Electrolysis

The following shows an example in which the masses of the anode and cathode were measured before and after electrolysis.
The mass of the anode decreases because copper dissolves, while the mass of the cathode increases because copper is deposited.

Example Calculation of Electric Charge

The electric charge passed through the system is determined from the product of current and time.

Electric charge Q = Current I × Time t

In this reference example, the current is 0.30 A and the energization time is 1800 s, so the electric charge is calculated as follows.

Q = 0.30 A × 1800 s = 540 C

Therefore, 540 C of electric charge was passed through the system in this experiment.

Example Calculation of Theoretical Deposition Amount

The copper-deposition reaction at the cathode is a reaction in which Cu2+ accepts two electrons and becomes Cu.

Cu2+ + 2e− → Cu

According to Faraday’s law, the theoretical deposition amount of copper is determined using the following equation.

Theoretical deposition amount m = Q × M ÷ (n × F)

Here, Q = 540 C, M = 63.5 g/mol, n = 2, and F = 96500 C/mol.

m = 540 × 63.5 ÷ (2 × 96500)

m = 34290 ÷ 193000 = 0.1777 g

Therefore, if all of the electric charge passed through the system were used for copper deposition, approximately 0.178 g of copper would theoretically be deposited on the cathode.

Example Calculation of Current Efficiency at the Cathode

Current efficiency expresses as a percentage how much copper was actually deposited relative to the theoretical deposition amount.

Current efficiency (%) = Measured deposition amount ÷ Theoretical deposition amount × 100

In this reference example, the measured cathode deposition amount is 0.168 g and the theoretical deposition amount is 0.1777 g.

Current efficiency = 0.168 ÷ 0.1777 × 100 = 94.5%

From this result, approximately 94.5% of the electric charge passed through the system was considered to have been used for copper deposition at the cathode.

Comparison of Anode Mass Decrease and Cathode Deposition Amount

In electrolytic refining, ideally, the amount of copper dissolved at the anode and the amount of copper deposited at the cathode approximately correspond.
However, because blister copper contains impurities, the decrease in anode mass and increase in cathode mass do not always completely agree.

In this reference example, the difference between the decrease in anode mass, 0.174 g, and the increase in cathode mass, 0.168 g, is 0.006 g, which is approximately equal to the mass of the anode slime, 0.006 g.
Therefore, some impurities in the blister copper were considered to have remained undissolved as anode slime.

Example Observation of Anode Slime

Anode slime is a collection of components that are more difficult to dissolve than copper and components that precipitate during electrolysis.
In an experiment, it may be observed as a black or gray precipitate at the bottom of the electrolytic cell or near the anode.

Comparison When the Current Is Changed

Increasing the current increases the electric charge passed during the same period, so the theoretical deposition amount also increases.
However, if the current is too large, the deposition on the cathode surface may become rough and side reactions may occur more readily.

In this reference example, the deposition amount increases as the current becomes larger, but the current efficiency decreases slightly.
Particularly at 0.50 A, the cathode surface became rough, and some of the deposited copper may have detached.

Behavior of Impurities in Blister Copper

Impurities in blister copper do not all behave in the same way during electrolysis.
Components that are more easily ionized than copper tend to dissolve at the anode, while components that are more difficult to dissolve than copper tend to remain as anode slime.

Example of How to Write the Results

Electrolytic refining was performed using a blister copper plate as the anode and a pure copper plate as the cathode.
Because a current of 0.30 A was passed for 1800 s, the electric charge passed through the system was 540 C.
The theoretical deposition amount of copper determined from Faraday’s law was 0.1777 g, while the measured increase in cathode mass was 0.168 g.
Therefore, the current efficiency at the cathode was calculated to be 94.5%.

In addition, the mass of the anode decreased from 15.842 g to 15.668 g, giving a mass decrease of 0.174 g.
Meanwhile, the increase in cathode mass was 0.168 g and the recovered anode slime was 0.006 g.
The difference between the decrease in anode mass and the increase in cathode mass was 0.006 g, which was approximately equal to the mass of the anode slime.
From this, some impurities in the blister copper were considered to have remained undissolved as anode slime.

Points for Connecting the Results to the Discussion

In a discussion of electrolytic refining of copper, explaining not only the increase in cathode mass but also the decrease in anode mass, anode slime, and current efficiency together makes the refining mechanism easier to organize.

• Can it be explained that copper was oxidized to Cu2+ at the anode?
• Can it be explained that Cu2+ was reduced and deposited as copper at the cathode?
• Are the decrease in anode mass and increase in cathode mass close in value?
• If there is a difference, can it be explained by anode slime, side reactions, or detachment of deposits?
• Can the anode slime be considered to originate from impurities that are more difficult to dissolve than copper?
• Can the reason why the current efficiency was below 100% be related to side reactions and the deposition condition?
• Under conditions with excessively high current, did the cathode surface become rough or did deposits detach?

Example Discussion

In this experiment, electrolytic refining was performed using blister copper as the anode and a pure copper plate as the cathode.
At the anode, the oxidation reaction Cu → Cu2+ + 2e− occurred, while at the cathode, copper was considered to have been deposited through the reduction reaction Cu2+ + 2e− → Cu.
As a result, the mass of the anode decreased and the mass of the cathode increased.

Under the condition where a current of 0.30 A was passed for 1800 s, the theoretical deposition amount was 0.1777 g, whereas the measured deposition amount was 0.168 g.
The current efficiency at this time was 94.5%, indicating that most of the electric charge passed through the system was used for copper deposition.
However, possible reasons why the current efficiency did not reach 100% include side reactions such as hydrogen evolution, detachment of deposits from the cathode surface, and errors during washing, drying, and weighing.

In addition, the decrease in anode mass was 0.174 g and the increase in cathode mass was 0.168 g.
The difference between the two, 0.006 g, was approximately equal to the recovered anode-slime mass of 0.006 g.
From this, insoluble components other than copper and difficult-to-dissolve components such as silver and gold contained in the blister copper were considered to have remained as anode slime.
In electrolytic refining, copper can therefore be deposited on the cathode while impurities are separated into the anode slime or electrolyte.

Under conditions with larger current, the amount of copper deposited on the cathode increased, but the current efficiency decreased and roughness was observed on the surface.
This was considered to result from the higher current density causing the supply of copper ions to fail to keep up and making nonuniform deposition and side reactions more likely.
Therefore, to obtain high-purity copper with good adhesion, the current, electrolyte concentration, stirring, distance between electrodes, and other conditions must be appropriately controlled.

Summary

In electrolytic refining of copper, the blister copper at the anode dissolves and high-purity copper is deposited at the cathode.
By comparing the decrease in anode mass, increase in cathode mass, and formation of anode slime, the transfer of copper and separation of impurities can be confirmed.

In this reference example, the current efficiency at the cathode was 94.5%, and the difference between the decrease in anode mass and cathode deposition amount approximately corresponded to the amount of anode slime.
In a report, it is useful to relate the electrode reactions, Faraday’s law, anode slime, behavior of impurities, and current efficiency to one another.

Reaction Occurring at the Anode

The anode in electrolytic refining is made of blister copper.
Because the anode is connected to the positive terminal of the external power supply, an oxidation reaction in which electrons are removed occurs.
Copper atoms in the blister copper lose electrons and dissolve into the electrolyte as Cu2+.

Because copper dissolves at the anode, the mass of the anode decreases after energization.
This decrease in mass is related to the amount of copper and impurities that dissolved from the anode.
However, impurities that are less easily ionized than copper remain undissolved and may form anode slime.

Anode: Cu → Cu2+ + 2e-

Reaction Occurring at the Cathode

The cathode is connected to the negative terminal of the external power supply and is supplied with electrons.
Cu2+ in the electrolyte accepts electrons at the cathode surface and is deposited as metallic copper, Cu.
Therefore, the mass of the cathode increases after energization.

The copper deposited at the cathode is Cu2+ dissolved from the anode that has been reduced.
Under appropriate conditions, relatively high-purity copper is deposited at the cathode.
However, if the current density is too high or the electrode surface is dirty, the deposit may become rough or more likely to peel off.

Cathode: Cu2+ + 2e- → Cu

How to Consider the Overall Reaction

In electrolytic refining of copper, copper dissolves at the anode and copper is deposited at the cathode.
Combining the anodic and cathodic reactions, copper appears to move from the anode to the cathode.
Cu2+ in the electrolyte is supplied at the anode and consumed at the cathode.

Ideally, the amount of copper dissolved at the anode and the amount of copper deposited at the cathode are equal.
However, in practice, they may not completely agree because of dissolution of impurities, formation of anode slime, detachment of deposited copper, side reactions, measurement errors, and other factors.

What Is Anode Slime?

Anode slime is a precipitate formed near the anode from impurities in blister copper that are difficult to dissolve during electrolysis.
Metals that are less easily ionized than copper, such as silver Ag, gold Au, and platinum Pt, are less likely than Cu to dissolve at the anode and may remain as solids.
These materials accumulate beneath the anode as anode slime.

Anode slime is not simply dirt and may contain precious metals in blister copper.
In industrial electrolytic refining of copper, recovering precious metals from anode slime is also important.
If a precipitate or blackish substance is observed near the anode in an experiment, it can be discussed as formation of anode slime.

Behavior of Impurities More Easily Ionized Than Copper

If blister copper contains metals such as iron Fe and zinc Zn that are more easily ionized than copper, they may be oxidized at the anode and dissolve into the solution.
These metals remain in the electrolyte as ions such as Fe2+ and Zn2+.
However, because Cu2+ is more readily reduced at the cathode, copper is usually preferentially deposited.

Therefore, metals that are more easily ionized than copper may dissolve at the anode but are less likely to deposit at the cathode and may accumulate in the electrolyte.
This property increases the purity of the copper deposited at the cathode.
However, co-deposition and side reactions may occur depending on the conditions.

Behavior of Impurities Less Easily Ionized Than Copper

Impurities that are less easily ionized than copper are difficult to oxidize at the anode and therefore do not readily enter the solution like Cu.
As a result, after copper in the blister copper dissolves, these impurities remain as solids and form anode slime.
Representative examples include silver, gold, and platinum.

The fact that these impurities do not readily move to the cathode and deposit is one reason high-purity copper can be obtained by electrolytic refining.
Formation of anode slime indicates that impurities in the blister copper were separated from the copper.
In a report, anode slime should be treated not as “waste” but as an important separation result of electrolytic refining.

Role of the Electrolyte

In electrolytic refining of copper, an electrolyte consisting of CuSO4 aqueous solution with added sulfuric acid may be used.
CuSO4 aqueous solution supplies Cu2+ and creates an environment in which copper can be deposited at the cathode.
The electrolyte also has the role of allowing ions to move and current to flow.

If the Cu2+ concentration in the electrolyte is too low, side reactions such as hydrogen evolution may become more likely at the cathode.
Adding sulfuric acid may increase conductivity and stabilize the condition of the solution.
The concentration and composition of the electrolyte affect the copper-deposition condition and current efficiency.

Changes in Cu2+ Concentration

In electrolytic refining, Cu at the anode dissolves as Cu2+, while Cu2+ at the cathode is deposited as Cu.
Ideally, the amount of Cu2+ generated at the anode and the amount consumed at the cathode are equal, so the Cu2+ concentration in the electrolyte does not change greatly.

However, in practice, the Cu2+ concentration may change because of dissolution of impurities, side reactions, differences in current conditions, differences in electrode area, and other factors.
If the Cu2+ concentration decreases, hydrogen evolution may become more likely than copper deposition at the cathode.
If the intensity of the blue color of the electrolyte changes, this can be discussed as a change in Cu2+ concentration.

Condition of Copper Deposited at the Cathode

Under appropriate conditions, copper deposited at the cathode forms a dense, adherent, reddish-brown metal layer.
However, if the current density is too high, the supply of Cu2+ cannot keep up, or the electrode surface is dirty, the deposited copper may become coarse, powdery, or dendritic.
Such deposits peel off easily and affect mass measurements.

The condition of the deposited copper is also related to current efficiency and purity.
If the surface is nonuniform, current may concentrate locally and make the deposition even more nonuniform.
In a report, it is useful to treat not only the increase in cathode mass but also the color, luster, adhesion, and uniformity of the deposit as observation results.

Decrease in Anode Mass and Increase in Cathode Mass

In electrolytic refining of copper, the anode mass decreases because copper at the anode dissolves, while the cathode mass increases because copper is deposited at the cathode.
Ideally, the amount of copper dissolved at the anode and the amount of copper deposited at the cathode are approximately equal.
However, if blister copper contains impurities or side reactions occur, the two values do not completely agree.

The decrease in anode mass may include dissolution of not only copper but also impurities that are more easily ionized than copper.
On the other hand, impurities remaining as anode slime do not enter the solution and therefore affect the way the anode collapses and the formation of precipitates.
The increase in cathode mass mainly reflects the amount of copper deposited, but if the deposit detaches, the measured value becomes smaller.

Faraday’s Law and Deposition Amount

According to Faraday’s law, the amount of substance reacting during electrolysis is proportional to the electric charge passed through the system.
The electric charge Q is determined from the product of current I and energization time t.
The electric charge carried by 1 mol of electrons is the Faraday constant F, approximately 96500 C/mol.

Because the copper-deposition reaction is Cu2+ + 2e- → Cu, 2 mol of electrons are required to deposit 1 mol of copper.
Therefore, the amount of electrons can be determined from the electric charge passed through the system and divided by 2 to obtain the theoretical amount of copper deposited.

Q = I × t

Amount of electrons = Q ÷ F

Cu2+ + 2e- → Cu

Amount of Cu deposited = Amount of electrons ÷ 2

What Is Current Efficiency?

Current efficiency is a value representing the proportion of the electric charge passed through the system that was used for the intended copper-deposition reaction.
It is determined by comparing the amount of copper actually deposited with the theoretical deposition amount.
The closer the current efficiency is to 100%, the more effectively the electric charge is considered to have been used for copper deposition.

Causes of low current efficiency include side reactions such as hydrogen evolution, detachment of deposited copper, contamination of electrode surfaces, fluctuations in current, and decreases in copper-ion concentration.
If the experimental value is smaller than the theoretical value, these factors are considered.
Conversely, if the current efficiency greatly exceeds 100%, insufficient drying, residual moisture, and mass-measurement errors should be suspected.

Current efficiency = Measured deposition amount ÷ Theoretical deposition amount × 100

Reasons Why the Theoretical and Measured Values Differ

The theoretical deposition amount calculated from Faraday’s law and the measured value do not always completely agree in experiments.
Possible causes include side reactions, fluctuations in current, detachment of deposited copper, insufficient drying of electrodes, weighing errors, contamination of electrode surfaces, and changes in electrolyte concentration.
In particular, if the deposited copper is powdery and easily detached, the measured value becomes smaller than the theoretical value.

On the other hand, if the cathode is weighed before it is sufficiently dried, residual moisture or electrolyte causes the mass to be measured as larger than it actually is.
In this case, an unnatural result such as current efficiency exceeding 100% may be obtained.
The difference between the theoretical and measured values must be discussed from both experimental operation and reaction conditions.

Side Reaction of Hydrogen Evolution

In electrolytic refining of copper, the intended reaction is the reduction of Cu2+ at the cathode and deposition of copper.
However, depending on the conditions, side reactions may occur in which hydrogen ions or water are reduced and H2 is generated.
Hydrogen evolution may become more likely particularly when the Cu2+ concentration is low, the current density is too high, or the voltage is too high.

When hydrogen evolution occurs, part of the electric charge is used for gas generation instead of copper deposition, so the current efficiency decreases.
In addition, if bubbles adhere to the cathode surface, copper deposition may become nonuniform.
If many bubbles were generated at the cathode during the experiment, this must be discussed as a side reaction.

2H+ + 2e- → H2

2H2O + 2e- → H2 + 2OH-

Effect of Current Density

Current density is the magnitude of current flowing per unit electrode area.
When the current density is appropriate, copper is deposited in a relatively uniform and adherent form.
However, if the current density is too high, the supply of Cu2+ at the cathode surface cannot keep up and the deposit may become rough or dendritic.

If the current density is too high, side reactions such as hydrogen evolution also become more likely.
On the other hand, if the current density is too low, the deposition rate is slow and sufficient mass change may not be obtained within the experimental period.
Current density is an important condition affecting the quality of the deposited copper and current efficiency.

Effect of Electrode Spacing

The distance between the electrodes also affects the results of electrolysis.
If the electrode spacing is too large, the resistance of the solution increases and current may flow less readily even at the same voltage.
On the other hand, if the electrodes are too close, the current distribution may become uneven or the deposits may contact the anode side.

Keeping the electrode spacing constant improves experimental reproducibility.
In addition, if the electrodes are not arranged parallel to each other, the current density may vary depending on location and deposition may become nonuniform.
Electrode arrangement is an easily overlooked but important condition.

Effects of Electrode Washing and Drying

In experiments that measure electrode mass changes, washing and drying of the electrodes are extremely important.
If the electrode surface is dirty before energization, copper deposition or dissolution may become nonuniform.
In addition, if electrolyte or moisture remains when the mass is measured after energization, the recorded mass becomes larger than the actual value.

On the other hand, if deposited copper is rubbed off during washing or the deposit peels off during drying, the increase in cathode mass is measured as smaller.
Therefore, when handling the electrodes, careful operation is required so that excess liquid is removed without losing the deposits.

Causes of Error in Electrolytic Refining of Copper

Causes of error in electrolytic refining of copper include fluctuations in current, errors in energization time, detachment of deposited copper, insufficient drying of electrodes, contamination of electrode surfaces, adhesion of anode slime, changes in electrolyte concentration, excessive current density, bubble adhesion, side reactions such as hydrogen evolution, and weighing errors.
In experiments involving mass changes, even small differences in operation affect the results.

Causes that make the measured deposition amount smaller include peeling of deposited copper, losses during washing, decreases in current efficiency due to hydrogen evolution, and decreases in current.
Causes that make the measured deposition amount larger include insufficient drying, adhesion of electrolyte, adhesion of anode slime or impurities, and weighing errors.
Organizing them into overestimation and underestimation makes the discussion easier to write.

When the Results Can Be Considered Good

Results can be considered good in electrolytic refining of copper when the anode mass decreases, the cathode mass increases, reddish-brown copper is deposited uniformly on the cathode, and anode slime is observed near the anode.
In addition, if the measured deposition amount is close to the theoretical deposition amount calculated from Faraday’s law and the current efficiency is reasonable, the intended reaction can be considered to have proceeded mainly.

If the color of the electrolyte does not change greatly and the current is stable, the supply of Cu2+ at the anode and consumption of Cu2+ at the cathode can be considered relatively balanced.
However, even if the values do not completely agree, the discussion is sufficient if the causes of error can be explained.

Example Discussion When the Experiment Did Not Go Well

When electrolytic refining of copper does not go well, possible causes are considered from results such as a small increase in cathode mass, peeling of copper, blackish or powdery deposition, unstable current, adhesion of anode slime to the cathode, or a large difference between the theoretical and measured values.
Organizing the causes according to electrode condition, current density, electrolyte concentration, washing and drying, weighing, and energization conditions makes the discussion easier.

How to Write Points for Improvement

In a discussion of electrolytic refining of copper, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to electrode treatment, current conditions, electrolyte, mass measurement, and handling of products.

Improvements to the Electrodes and Electrolyte

• Clean the electrode surfaces before use
• Lightly polish the electrode surfaces when necessary
• Keep the distance between the electrodes constant
• Arrange the electrodes parallel to each other
• Keep the immersed electrode areas consistent
• Prepare the electrolyte concentration accurately
• Prevent anode slime from adhering to the cathode

Improvements to Energization Conditions

• Keep the current constant
• Measure the energization time accurately
• Record the current during the experiment
• Avoid excessively high current density
• Review the conditions if a large amount of hydrogen is generated
• Suppress temperature rise

Improvements to Mass Measurement

• Dry the electrodes under the same conditions before and after energization
• Wash carefully so that the deposited copper does not peel off
• Thoroughly remove electrolyte and moisture
• Allow the electrodes to cool completely before measurement
• Weigh accurately using an electronic balance
• Perform multiple measurements and calculate the average value

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of electrolytic refining of copper, simply writing that “copper was deposited” or “anode slime formed” results in a superficial discussion.
A good discussion relates the reactions at the anode and cathode, the ionization tendencies of impurities, Faraday’s law, the difference between the measured and theoretical values, and current efficiency.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of electrolytic refining of copper.
Adjust the necessary parts according to your own experimental results.

• At the anode, Cu is oxidized and dissolves into the electrolyte as Cu2+.
• At the cathode, Cu2+ is reduced and deposited as metallic Cu.
• In electrolytic refining, copper appears to move from the anode to the cathode.
• Impurities that are less easily ionized than copper are difficult to dissolve at the anode and remain as anode slime.
• Impurities that are more easily ionized than copper dissolve into the solution but are less likely to deposit at the cathode.
• The amount of copper deposited at the cathode can be compared with the theoretical value calculated from Faraday’s law.
• Current efficiency indicates how much of the theoretical deposition amount was actually obtained.
• Possible causes of low current efficiency include side reactions and detachment of deposited copper.
• If the current efficiency exceeds 100%, insufficient drying or adhesion of electrolyte should be suspected.
• The deposition condition is affected by current density and the condition of the electrode surface.

Points to Check When Discussing Electrolytic Refining of Copper

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

• Are the reactions at the anode and cathode distinguished?
• Is it stated that oxidation occurs at the anode and reduction at the cathode?
• Is Cu → Cu2+ + 2e- explained?
• Is Cu2+ + 2e- → Cu explained?
• Is the reason anode slime forms explained?
• Are the ionization tendencies of impurities considered?
• Are the decrease in anode mass and increase in cathode mass compared?
• Is the theoretical deposition amount calculated using Faraday’s law?
• Is current efficiency considered?
• Are side reactions and detachment of deposited copper considered as causes of error?
• Are the effects of electrode washing and drying considered?
• Do the points for improvement correspond to the causes of error?

Summary

Electrolytic refining of copper is a method for obtaining high-purity copper by electrolysis using blister copper as the anode and pure copper as the cathode.
At the anode, Cu dissolves as Cu2+, while at the cathode, Cu2+ is reduced and deposited as metallic copper.
Therefore, the anode mass decreases and the cathode mass increases.

Impurities in blister copper behave differently depending on their ionization tendencies.
Metals that are less easily ionized than copper are difficult to dissolve and remain as anode slime.
Metals that are more easily ionized than copper may dissolve into the solution, but because copper is preferentially deposited at the cathode, relatively high-purity copper can be obtained there.

In a report, rather than simply writing that “the anode dissolved” or “copper was deposited at the cathode,” organize and discuss the electrode reactions at the anode and cathode, the meaning of anode slime, the behavior of impurities, Cu2+ concentration, Faraday’s law, theoretical deposition amount, current efficiency, causes of error, and points for improvement.
Electrolytic refining of copper is an important experiment for understanding the relationship among redox reactions, electric charge, and metal refining.