Question: We reduce chromium (VI) compounds with sodium dithionite. This is not the cheapest solution, but it is the best for us. In order to avoid unnecessary surpluses and save costs, we wanted to calculate the ideal addition. Unfortunately, we couldn't find a suitable tool in the Galvano calculator [1] and we couldn't find any reliable information in the specialist literature either. Do you know of an appropriate program or app that can help us?
Answer: Many additions in electroplating are made using ampere hours. These can be calculated in the electroplating calculator. This is different for wastewater treatment, as you can see in [2].
As you probably know, the reaction with sodium dithionite in the acidic range proceeds as follows:
Na2Cr2O7 + Na2S2O4 + 3 H2SO4 → Cr2(SO4)3 + 2 Na2SO4 + 3 H2O
This means that 261.97 g of sodium dichromate reacts with 174.10 g of sodium dithionite. Their mass ratio is therefore ~1.5:1. Or to put it another way: To reduce 1 kg Na2Cr2O7 in theory, 0.66 kg Na2S2O4 (100 %) is required. For 1 kg Cr6+, 1.69 kg Na2S2O4 is required stoichiometrically.
The problem with wastewater treatment and many other chemical reactions is that they do not proceed ~100 % in the direction of the products. In order to influence the equilibrium accordingly, surpluses are used. How high the excess must be depends, among other things, on the pH value, temperature [3] and other substances that can shift the reaction in one direction or the other. In addition, there are substances, in this specific case oxidizing agents, which may be present alongside the chromium(VI) and additionally consume the reducing agent. If, for example, a cyanide oxidation takes place first, sodium dithionite is consumed for the excess of sodium hypochlorite, hydrogen peroxide or caroate. It quickly becomes clear why there is no corresponding program that calculates an addition from a more or less arbitrary solution that leads, for example, to the limit value of 0.1 mg/L chromium(VI) in accordance with the Waste Water Ordinance, Annex 40, as the mixture from electroplating plant A may be different from that in electroplating plant B. Here is a general example from [3]: Silver chloride is considered an almost insoluble salt. However, the solubility improves by 20 % if 0.02 mol/L KNO3 is present. This shows how much solubilities and reactions can be influenced by the small presence of other substances. This property is utilized in precipitation reactions [2, 4].
Nevertheless, we can recommend two practices that will help you to solve the problem.
Empirical determination
If you do not wish to make any further investments (see below), we would first carry out a series of beaker experiments using the tried and tested empirical method. Determine the initial value of the chromium(VI) concentration and stoichiometrically add the corresponding amount of sodium dithionite, whereupon you measure the chromium(VI) concentration after several minutes of exposure. Then add 10 % of the initial value, measure again and repeat this until you have reached or fallen below the limit value. It is also important to document the pH value and temperature. Please note that products containing sodium dithionite, such as Plexolide, only contain 80 % active substance.
Then check the data determined in the batch. Do not add 100 % of the chemical, but only 75 to 80 %, wait for the reaction time and then measure the chromium(VI) concentration. The reason for this is inaccuracies in the laboratory, as well as slightly changed conditions such as temperature, which can have a greater effect on the batch.
If you do this over several batches, you should get helpful tables and graphs to make more accurate predictions. Depending on the concentration of the starting substances, deviations may still occur. If these occur more frequently, the tests must be carried out again.
To achieve greater accuracy, you can divide the recorded values into primary and secondary data. Primary data would be Cr6+ concentration, pH value and temperature, while secondary data would be interfering substances such as other oxidizing agents, which increase the consumption of the reducing agent. This gives you greater accuracy, but also increases the analysis effort to the same extent, which is why automation with measurement and control technology is probably worthwhile here.
Addition after potential measurement
As with cyanide detoxification, for example, a jump in potential can also be observed here. However, depending on the wastewater mixture and pH value, this may not be as significant, which can be compensated for by using the appropriate software. Together with the reducing agent Plexolide [5], whose main component is sodium dithionite, software tailored to this application exists. The requirement is 5.5 g plexolide per 1 g Cr6+ with 30 % excess for a complete reduction. Additional oxidizing agents in the waste water increase the requirement. Plexolide can also reduce chromates in an alkaline medium, although the reaction is faster in an acidic medium. Compared to sodium hydrogen sulphite, sodium dithionite can efficiently reduce chromium(VI) in an alkaline medium, e.g. regenerates of an anion exchanger. Sodium dithionite is also frequently used for the removal of oxidizing agents.
The potential jump is not very pronounced for all RedOx reactions in complex wastewaters. A special software tool has therefore been developed. This remembers the potential before the reduction (or oxidation reaction) and repeatedly adds reducing agent, waits a certain time and adds chemistry again if the change is too small. This is a type of titration controller, but with more steps and setting options. This software is also used for CN oxidation, NO2 reduction and organosulphide dosing.
References
[1] Galvano calculator: https://www.galvanotechnik-for-you.de/galvano-rechner/
[2] Course UT1: https://www.galvanotechnik-for-you.de/uebersicht-kurse/umwelttechnik-teil-1-abwasserbehandlung/
[3] Chemistry III course: https://www.galvanotechnik-for-you.de/uebersicht-kurse/chemie-iii-das-chemische-gleichgewicht/
[4] Handbook of wastewater and recycling technology, 3rd edition, Hanser Verlag 2017
[5] Plexolide: https://hauserwalz.ch/hauserwalz/chemische-produkte/alternative-verfahren
