Stability of sodium hypochlorite in solution after adding sodium hydroxide

Transcription

1 Stability of sodium hypochlorite in solution after adding sodium hydroxide Maxime Richard 1 Pierre-Gilles Duvernay 2 29 April 2012 Abstract The chlorination of water with hypochlorite shortly before consumption is an easy method for producing drinking water. A study of the degradation of sodium hypochlorite in solution was conducted. The hypochlorite used was produced from a NaCl solution using WATA electrolysers. The focus of the study was the extent to which the stability of a hypochlorite solution depended on the amount of NaOH added to that solution. Initially, the effect of adding NaOH for the WataTest assay was simulated by titration with a KI solution. While no difference was observed when 0.1 g/l NaOH was added, adding 0.71 g/l increased the mean volume required for titration by around 22%. Next, the impact of the presence of air inside the bottles during storage was studied. After 28 days there was no difference between the bottles stored with or without air inside. Various amounts of NaOH were added to hypochlorite solutions in order to determine the amount of NaOH required for adequate storage. For storage at 34 C and with the water used for these tests, 400 mg NaOH gave storage times of 150 to 200 days. It should be noted that solutions basified in this way cannot be used in nursing and are intended only for disinfecting drinking water at 4000x dilution. Using data from outside this study, we note a high degree of variability in the dose of sodium hydroxide required to achieve a ph of 11.9 in field trials. We therefore do not recommend a standard dose of NaOH but advise the use of a ph-meter to achieve a ph of 11.9 [1]. We also recommend using the purest possible water for electrolysis and storing the solutions produced at a temperature below 40 C. 1 2 MSc ETH Chemistry Technical adviser, Antenna Technologies Foundation, Geneva, Switzerland 1

2 Table of contents Stability of sodium hypochlorite in solution after adding sodium hydroxide...1 Table of contents Purpose Theory Mechanisms of decomposition of hypochlorite Method Experimental methods Results and discussion Efficacy of the WataTest for basic solutions Adding 250 and 400 mg/l NaOH Storage in less than full bottles Stability of hypochlorite based on NaOH concentration Conclusion Acknowledgements...10 Bibliography

3 1 Purpose One of the strategies recommended by the Pan American Health Organization and the U.S. Centers for Disease Control and Prevention to improve access to drinking water is to add hypochlorite to water just before consumption [1]. The Antenna Technologies Foundation sells devices with which hypochlorite can be produced from a solution of NaCl (cooking salt). A previous study has shown that the hypochlorite produced by these devices is not stable and degrades quickly, mainly because the ph of these solutions is not basic enough [2]. For this reason, we tested the stabilisation of hypochlorite by adding NaOH. The results of these tests are presented in this report. 2 Theory 3 Mechanisms of decomposition of hypochlorite The decomposition of hypochlorite in a sufficiently basic solution follows second-order kinetics by the mechanisms below: Stability of sodium hypochlorite in solution after adding sodium hydroxide...1 Table of contents Purpose Theory Mechanisms of decomposition of hypochlorite Method Experimental methods Results and discussion Efficacy of the WataTest for basic solutions Adding 250 and 400 mg/l NaOH Storage in less than full bottles Stability of hypochlorite based on NaOH concentration Conclusion...9 3

4 7 Acknowledgements...10 Bibliography...10 There is therefore a strong correlation between hypochlorite degradation and ph. 4 Method 4.1. Experimental methods Sodium hypochlorite (NaOCl) was produced using the WATA devices, consisting of electrodes made of titanium coated with ruthenium dioxide and iridium oxide. Production took place as recommended by Antenna Technologies using 50 g NaCl (pro analysis, Merck or JuraSel cooking salt) dissolved in 2 L water. For each operation, the solution was electrolysed for 2 hours inside the containers supplied with the device. The output of this production had a chlorine content of around 6 g/l. The water supply in Geneva has an average hardness of 14.0 fh, conductivity of 318 µs/cm and a ph (25 C) of 8.00, and it contains 2.71 mg/l nitrates on average [3]. The hypochlorite in the solutions was determined using the WataTest product from the Antenna Technologies Foundation. The solutions were stabilised by adding a 2M NaOH solution. One solution was buffered with 21.3 ml of 1M Na 2 C0 3 and 3.7 ml of 1M NaHCO 3. The hypochlorite solutions were stored at a temperature of 34 C. 4

5 The bottles used (Nalgene) had a capacity of 250 ml and were filled to capacity. To determine whether air contained in the bottles during storage had any influence on storage time, 500 ml of solution was stored in 1000 ml bottles. In order to observe the consequences on the serviceability of the WataTest of adding a base to the hypochlorite solutions, the following method was used: 10 ml of the NaOCl solution was diluted in deionised water, then a few millilitres of starch solution (Acros Organics) was added. Next, this solution was titrated with a potassium iodide solution (Sigma- Aldrich, >99.0%) at an arbitrary concentration. 5 Results and discussion 5.1. Efficacy of the WataTest for basic solutions NaOH added [g/l] KI solution [ml] TABLE 1 Quantity of a potassium iodide solution at an arbitrary concentration required for titration of a sodium hypochlorite solution based on the mass of NaOH added. The results of the WataTest solution tests are shown in Table 4.1. When no NaOH or 0.1 g/l were added, the volume of titrant required was nearly the same. The fact that the volume was smaller for 0.1 g/l than for no added NaOH is probably due to the inaccuracy of the assay method. However, the four titrations carried out with the solution to which 0.71 g/l NaOH had been added gave greater volumes than without NaOH. Furthermore, there was great divergence among these results (mean: 16.8; standard deviation: 1.53). For instance, while adding a small amount of NaOH did not seem to affect the WataTest, adding a larger amount changed the result and accuracy of the test. 5

6 5.2. Adding 250 and 400 mg/l NaOH days after production Figure 1 Data collected after adding 250 mg/l NaOH. The squares represent measurements of ph and the circles represent measurements of the amount of chlorine (g/l]. The various experiments were conducted under similar conditions, the only difference being the salt used for production. The salt was NaCl pro analysis from Merck in the case of the black line, and iodised and fluorinated salt for the others. The results for the addition of 250 and 400 mg/l NaOH are shown in Figures 1 and 2 respectively. These graphs demonstrate clearly that there is a strong correlation between the reduction in ph and the concentration of hypochlorite. The reason for accelerated degradation of hypochlorite beyond a certain ph is understandable. Indeed, we can see from equation (3) that when hypochlorous acid reacts with chlorate, protons are released, causing the ph to diminish and consequently increasing the proportion of hypochlorous acid in solution. This is supported by the case where 400 mg/l NaOH was added. Both the ph and the concentration of hypochlorite were kept more or less constant for a longer period. Thus the black lines and, to a lesser extent, the red lines are deflected almost simultaneously. Although, in Graph 2, the sample produced with cooking salt seems to have degraded faster than the others, this is not so in Graph 1. The impurities in the cooking salt tested probably did not accelerate the degradation process. 6

7 days after production FIGURE 2 Same graph as in Figure 1, but after adding 400 mg NaOH Storage in less than full bottles The results of the tests with half-filled bottles are shown in Figure 3. Over the first 28 days, the rate of degradation was very similar for both types of filled bottles.

8 days after production FIGURE 3 The black and red dots were obtained after adding 250 mg/l NaOH and the green and blue dots, after adding 400 mg/l NaOH. The data traced in green and red represent the samples with 500 ml stored in 1000-mL bottles and those traced in blue and black represent the samples with 250 ml stored in 250-mL bottles. The squares represent measurements of ph and the circles represent measurements of the amount of chlorine (g/l] Stability of hypochlorite based on NaOH concentration The results of the tests to determine the stability of the solutions depending on ph are shown in Figure 4. The green and blue lines overlap. The starting ph of the hypochlorite solutions after adding 0, 250, 300, 350 and 400 mg was 9.23, 10.97, 11.10, and respectively. If the ph is calculated by the following equation: ph=14+log(c b ), where c b is the concentration of the base, the solutions to which the above amounts of NaOH (250, 300, 350 et 400 mg) were added should have a ph of 11.80, 11.88, and 12, respectively. This graph shows a strong correlation between ph and chlorine content. However, after about 80 days, the four samples to which NaOH were added seem to lose hypochlorite whereas the ph of none of the samples diminished compared with the first day. A hypochlorite solution was buffered to a ph of slightly above 10. Its apparently lower concentration compared with the other samples was due to the addition of the buffer solution. Adding this solution stabilised the ph during the experiment. However, the degradation of the hypochlorite did not slow down significantly compared with the sample to which no NaOH was added. According to these tests, storage for a period of 165 days required 400 mg NaOH to be added. Other possible factors for degradation should also be considered, such as ionic strength [4] or the presence of metals [5] [6], for example.

9 days after production Figure 4 The black, red, blue, green and yellow dots represent samples to which 0, 250, 300, 350 and 400 mg/l NaOH were added respectively. The purple dots represent measurements made on the sample to which a buffer solution was added. The squares represent measurements of ph and the circles represent measurements of the amount of chlorine (g/l]. 6 Conclusion This study considered the degradation of solutions of sodium hypochlorite (NaOCl) to which NaOH had been added. The solutions in question contained around 6 g/l active chlorine and were produced by means of the Standard-WATA electrolyser. The first step was to investigate the consequences on the operation of titration with a potassium iodide solution of adding a base to the hypochlorite solutions. This method is similar to the WataTest method. Two different amounts of NaOH were added for these tests: 0.10 and 0.71 g/l. The smaller amount did not significantly change the result of iodometric titration compared with no addition of base. Adding 0.71 g/l NaOH increased the mean volume required for titration by around 22%. Different masses of NaOH were also added to hypochlorite solutions and then changes in the concentration of active chlorine were monitored over time. It is useful to find out the minimum ph required for satisfactory stabilisation of a hypochlorite solution. Too high a ph in this solution could actually lead to a substantial increase of the ph in chlorinated water for drinking purposes. In our tests we observed that it was necessary to add 400 mg NaOH in order to hold the concentration of chlorine above 80% of its

10 initial concentration [1]. However, we should bear in mind that these data are highly dependent on the quality of the water used for the operation. These results are borne out by a study conducted over a longer period with two batches of three samples: 250 mg NaOH was added to the first batch and 400 mg NaOH to the second one. With the smaller amount of base, none of the samples was usable (over 20% loss in chlorine concentration according to [1]) after 150 days. In contrast, adding 400 mg NaOH allowed two samples to remain usable for around 200 days and one sample for at least 165 days (no assay was performed on this one subsequently). A study of the impact of the presence of air inside the bottles during storage was conducted. After 28 days there was no difference between the bottles stored with or without air. 7 Acknowledgements We are grateful to M. Pfister for his contribution to this project. Bibliography [1] D. Lantagne, K. Preston, E. Blanton, N. Kotlarz, H. Gezagehn, E. van Dusen, J. Berens, K. Jellison, J. Environ. Eng., 2011, 131. [2] M. Richard, P.-G. Duvernay, J.-P. Bourgeois, Stabilité de l hypochlorite de sodium aqueux produit par électrolyse d une solution de NaCl, [3] [4] J. R. Lewis, J. Phy. Chem., 1928, [5] L. C. Adam, G. Gordon, Inorg. Chem., 1999, [6] E. T. Gray, R. W. Taylor, D. W. Margerum, Inorg. Chem., 1977, 3047.