Voltage and its effect on the rate of Sodium Hydroxide production in Membrane Cells

Voltage and its effect on the rate of Sodium Hydroxide production in Membrane Cells

Isla Jones – St Ives High School

Membrane cells containing electrolytes powered by a potential difference are the primary technology used to produce sodium hydroxide. The optimal conditions of membrane cells have been the subject of multiple investigations. This report outlines an investigation into the effect of voltage on the rate of sodium hydroxide production in membrane cells. Sodium hydroxide was produced by a means of electrolysis in a membrane cell from a saturated solution of sodium chloride. Experiments were carried out at voltages of 6V, 8V and 10V for various periods of time. The sodium hydroxide solutions produced were titrated against a standard solution of hydrochloric acid and their concentrations were calculated. Limitations in this experiment meant that the data collected was unable to show a statistically significant difference in the rate of sodium hydroxide production at different voltages for a confidence interval of 95%. However, there is a positive correlation between increased voltage and rate of reaction to a confidence interval of 80%.

LITERATURE REVIEW

Sodium hydroxide (NaOH) is a highly basic, inorganic compound used in the manufacture of products such as detergents, pharmaceuticals, paper and the food industry (Chlor-alkali industry review 2019-2020, 2020). Sodium hydroxide is produced in conjunction with chlorine and hydrogen gas as a part of the chlor-alkali industry. This industry accounts for 55% of all chemical manufacture in EU-27 and EFTA countries (Brinkmann et al., 2014). As such, the electrochemical processes used in manufacture are designed to maximise the rate of production at the lowest possible cost to corporations (Brinkmann et al., 2014).

There are three primary types of electrolytic cell used in the chlor-alkali industry; mercury cells, diaphragm cells and membrane cells (Cardarelli, 2008). Of these technologies, both mercury and diaphragm cells pose serious environmental concerns due to their reliance on mercury and asbestos respectively (Crook and Mousavi, 2016). Concerns regarding mercury waste include the accumulation of mercury in aquatic ecosystems and the human health risks associated with consuming affected seafood (Sanborn and Brodberg, 2006). Similarly, the use of asbestos in diaphragm cells induces irreversible health conditions in workers (Crook and Mousavi, 2016). Due to these environmental concerns and recent environmental protection laws, membrane cells are the preferred technology accounting for 83% of the chlor-alkali industry’s production in Europe (Chloralkali industry review 2019-2020, 2020). The industrial shift towards membrane technology is the reason why this investigation was specifically conducted on membrane cells. However, it is understood that this investigation is merely a model of an industrial process to investigate relationships and that any results are not necessarily applicable on an industry scale.

Membrane cells consist of two inert electrodes separated by a semipermeable membrane which acts as an ion exchanger (Domga et al., 2017). In membrane cells, a potential difference is applied over the electrodes to initiate the electrochemical reaction (Cardarelli, 2008). Due to this potential difference and flow of electrons, the anode and cathode are oppositely charged and attract chlorine and sodium ions respectively. It is the negatively charged cathode that provides attractive force to the positively charged sodium ions, enabling them to pass through the membrane (Domga et al., 2017). The membrane is usually made from polytetrafluoroethylene (PTFE) (Crook and Mousavi, 2016). However, due to limitations in equipment, this investigation was conducted using gelatin as a substitute for the semipermeable membrane. Gelatin was chosen as it creates a seal with the sides of the test tube, restricting the flow of solutions and only allowing ions to migrate between compartments. However, in pilot studies it was observed that the membrane began to break or separate from the test tube at around 120 minutes. As such, gelatin is by no means a perfect substitute for an ion exchange membrane due to its lack of longevity.

This study aims to investigate the effect of different voltages on the rate of sodium hydroxide in membrane cells. To inform initial experimental design and the alternate hypothesis, other investigations in this field of study have been subject to review.

The minimum cell voltage for electrolysis is calculated using reduction potentials and experimentally determined to be 2.83V by Domga et al. in their 2017 study on electrolysis parameters in membrane cells. Their results indicate that when the distance between electrodes is increased, the cell voltage also increases proportionally to maintain a constant conductivity of the solution (Domga et al., 2017)(Rosales- Huamani et al., 2021.). As such, it was hypothesised that if voltage was increased and distance was controlled, the conductivity of the cell would increase. An increase in electrical conductivity of a cell is directly proportional to the flow of electrons through the circuit. As such, if an increase in voltage increases the flow of electrons, the rate of reaction will be increased for the production of NaOH.

SCIENTIFIC RESEARCH QUESTION

How does voltage affect the rate of sodium hydroxide production in membrane cells?

SCIENTIFIC HYPOTHESIS

An increase in the input voltage of a membrane cell will increase the rate of sodium hydroxide production.

METHODOLOGY

Preparation of Solutions and Membranes

A 12.5% w/v solution of gelatin was prepared by dissolving gelatin powder in room temperature water. The solution was brought to boiling point and 15mL of liquid was pipetted into each u-tube. The membranes were placed into a fridge left to set for a minimum of 12 hours.

To prepare the sodium chloride solution, NaCl was dissolved in distilled water until the solution was saturated. Blue food dye was also added to this solution to visually monitor defects in the membrane.

Preparation of Electrodes and Assembly

The electrodes used for this experiment were a graphite anode and a copper cathode. Before each trial, both electrodes were rinsed with distilled water and sanded down with steel wool. They were then weighed, attached to alligator clips and suspended at a controlled height above the membrane.

Figure 1 – the experimental set up

As seen in Figure 1 (above), 20mL of NaCl solution was added to the anode (left) side of the membrane and 20mL of distilled water (H2O) was added to the cathode (right) side of the membrane.

Production of Sodium Hydroxide

The membrane cell’s DC power supply was turned on and run at one of three different voltages (6V, 8V or 10V) for a given time period (30, 60 & 90 mins). Once the power was turned on, the initial current running through the set-up was recorded. At the end of the time period, the change in current was noted down and the power was turned off. Both electrodes were removed from the solution, dried and weighed. The NaOH sample was extracted from the cell and transferred into conical flasks for titrations.

Titrations

Immediately after collecting a NaOH sample, a drop of phenolphthalein was added to 5mL of the sample and titrated against a standard solution of hydrochloric acid (HCl). The volume of HCl required to neutralise the NaOH was recorded and the process was repeated twice for the sample. The volume of HCl required to neutralise the samples was t hen used to calculate the concentration of each sample.

RESULTS

Analysis was performed using a combination of linear regression and t-tests.

Figure 2 A concentration vs time graph depicting the rate of NaOH production at different voltages.

Each point is an average of three NaOH samples, all of which have been titrated three times. The R² values for each voltages’ trend is high (between 0.9 and 1), indicating a strong, positive correlation between time and the concentration of NaOH at the respective voltage. The difference in the coefficient of x between the equations of the trendlines indicates that there is a difference in the gradient, as visually depicted in the graph. This difference in gradient shows that there is a difference of the rate of sodium hydroxide production. To investigate the significance of this difference, a series of t-tests were required.

Figure 3: Table of p-values depicting the significance of a difference in the NaOH concentration.

Whilst the none of differences between voltages at each time have been shown to be significant for a p-value of 0.05, there are some significant differences for a p-value of 0.20. The difference between 6V and 10V has been shown to be

significant at all times with an 80% confidence interval. Similarly, the difference between 6V and 8V at times of 30 and 90 minutes are shown to be statistically significant for a p-value of 0.20, indicating a confidence interval of 80%. However, at a p value of 0.20, there is still no statistically significant difference in the rate of NaOH production between 6V and 8V at a time of 60 minutes and between 8V and 10V at all times.

The difference in initial and final weight of both electrodes was negligible for every trial. On the other hand, the change in current during each trial was unpredictable and there appears to be no real relationship.

DISCUSSION

The results of this experiment yielded several trends worthy of discussion. Both the R² values and the respective gradients of each voltage series provide insights into the relationships between voltage and NaOH production.

From the high R² values and positive correlation for each voltage trend, it can be determined that an increase in time results in an increased concentration of NaOH at all voltages.

A similar trend has been observed in comparable experiments, including an experiment on the factors influencing sodium hydroxide production conducted by Rosales-Huamani et al., 2021. As longer exposure to the power supply continued to increase the concentration of NaOH, further experimentation and analysis of the cell efficiency would be required to determine the optimum time frame of operation.

The difference in gradients of voltage series and thus the rate of reaction is not considered statistically significant at a 95% confidence interval, however, some differences are statistically significant at a confidence interval of 80%. The greatest difference in rate of reaction is between the 6V series and the 10V series. This supports results found by Domga et al., 2017 and Rosales-Huamani et al, 2021; a greater potential difference between the electrodes results in a solution with increased conductivity. This increased conductivity is indicated by the increased rate of production of NaOH.

Sources of Error

The greatest potential for random error in this experiment was the positioning of the electrodes above the membrane. As confirmed in a study on the relationship between resistance and conductivity of electrodes, an increased distance between electrodes reduces their conductivity (Cosoli et al., 2020). Despite efforts to control the distance between electrodes, there is potential for this factor to affect the results.

There was a complication with one of the alligator clips several trials into data collection. This clip was replaced within a new wire of the same length, however, there is a potential random error as result of a difference in conductivity of the two wires. Due to time constraints the trials conducted with the first wire were unable to be revised.

Another potential source of error for this experiment was the low concentrations and small volumes of the NaOH solutions. Due to limitations of equipment, the concentration of NaOH produced was relatively low. To titrate each sample multiple times, only small volumes of

solution were available. These small volumes led to high relative uncertainty for each titration and consequently reduced the reliability of the investigation.

Limitations and Further Research

This investigation and the conclusions that can be drawn from its data are fairly limited. The combined nature of the small data set and high variance between trials means that it was difficult to identify and remove outliers. In further research, the size of this study could be expanded to provide more data and reduce the margin of error, increasing reliability.

Another limitation of this investigation was the nature of the gelatin membrane. Whilst it held up well for the time frames proposed in this study, in initial tests defects were observed in the membrane from times onwards of 120 minutes. This indicates that the reliability of a gelatin membrane decreases over time and as such, to expand the scope of the investigation to longer time frames, the use of a different type of membrane would be necessary.

CONCLUSION

This report investigated the effects of voltage on the rate of sodium hydroxide production in membrane cells. It was found that an increase in time resulted in an increased concentration of sodium hydroxide at all voltages. However, the results indicate that there was not a statistically significant difference between the rate of sodium hydroxide production at these voltages for a confidence interval of 95%. While this does not provide a conclusive relationship between voltage and rate of sodium hydroxide production, the data does suggest a relationship between the variables at a confidence interval of 80%. The reduced confidence in the relationship is due to the potential for error in this investigation. Further study will include obtaining a larger data set to increase the reliability of the data, thus reducing the margin of error.

REFERENCES

2020. Chlor-alkali industry review 2019- 2020. [ebook] EuroChlor. Available at: <https://www.chlorineindustryreview.com/ wp- content/uploads/2020/09/Industry- Review-2019_2020.pdf> [Accessed 16 September 2021].

Brinkmann, T., Santonja, G., Schorcht, F., Roudier, S. and Delgado Sancho, L., 2014. Best Available Techniques (BAT) Reference Document for the Production of Chlor-alkali.

Cardarelli, F., 2008. Materials Handbook. A Concise Desktop Reference. 2nd ed. Springer.

Cosoli, G., Mobili, A., Tittarelli, F., Revel, G. and Chiariotti, P., 2020. Electrical Resistivity and Electrical Impedance Measurement in Mortar and Concrete Elements: A Systematic Review. Applied Sciences, 10(24), p.9152.

Crook, J. and Mousavi, A., 2016. The chlor- alkali process: A review of history and pollution. Environmental Forensics, 17(3), pp.211-217.

Domga, R., Noumi, G. and Tchatchueng, J., 2017. Study of Some Electrolysis Parameters for Chlorine and Hydrogen Production Using a New Membrane Electrolyzer. International Journal of Chemical Engineering and Analytical Science, 2(1), pp.1-8.

Rosales-Huamani, J., Medina-Collana, J., Diaz-Cordova, Z. and Montaño-Pisfil, J., 2021. Factors Influencing the Formation of Sodium Hydroxide by an Ion Exchange Membrane Cell. Batteries, 7(2), p.34.

Sanborn, J. and Brodberg, R., 2006. EVALUATION OF BIOACCUMULATION FACTORS AND TRANSLATORS FOR METHYLMERCURY. California: State of California.

APPENDIX

Figure 4: Concentrations of sodium hydroxide for each trial.

Figure 5: T-test calculations for 95% confidence

Figure 6: T-test calculations for 80% confidence.

Figure 7: Additional calculations