The issue of free chlorine in hydrochloric acid
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This is our method for determining free chlorine in high-purity acids: Take about 50 ml of the laboratory sample and place it in an iodometric flask containing approximately 100 ml of distilled water. Add 10 ml of potassium iodide solution (100 g/l), seal the flask with a water stopper and let it stand in the dark for 5 minutes. Then add 1 ml of starch indicator, and titrate using sodium thiosulfate standard solution. The question is why 100 ml of water needs to be added. I forgot to add water again; I made it too concentrated, and after adding water it disappeared. Also, sometimes blue color can be seen when starch is added, but it disappears once shaken. Adding more starch makes this color visible. Does that count then?
The iodination reaction requires to be carried out in the presence of dilute acid; water is added to dilute the acid. Starch turns blue, and this blue color disappears upon shaking, which is due to the oxidation of KI by light and air
Since the titration reaction between the I2 produced by the reaction and the sodium thiosulfate standard solution must take place under weakly acidic or neutral conditions, a large amount of water is added for dilution in order to reduce the acidity and prevent the decomposition of the sodium thiosulfate standard solution. Forgetting to add water directly led to the decomposition of the sodium thiosulfate standard solution during titration, causing an increase in the volume of this solution used in the titration, and ultimately resulting in higher values.
Uncombined chlorine is called free chlorine. Its determination method is generally the indirect iodimetry. However, this method is affected by Fe~(3+), causing the measurement results to be often on the high side. A sulfuric acid solution of methyl orange and potassium bromide can be used as the absorption solution for hydrogen chloride, which helps to avoid interference from Fe~(3+). The principle of its determination is that in an acidic solution, chlorine displaces bromine from potassium bromide; bromine is capable of oxidizing methyl orange, and the excess methyl orange is titrated using a sodium hypochlorite standard solution, which also serves as a blank. Reagent (1): Methyl orange solution, 0.01%. Prepare according to the instructions. (II) Potassium bromide solution, 1% (W/V) (III) Sulfuric acid solution, 10 mol (IV) Sodium hypochlorite standard solution, 10 μl Cl₂·ml~(-1).
Hello, could you please explain your steps? Thank you. Are you measuring the free chlorine in hydrochloric acid?
The high result may be due to some of the sodium thiosulfate having reacted with the acid
In our process, after chlorination of the reactants, we measure the free chlorine in the reaction gas by first adding KI solution and then starch indicator; once the gas is introduced, if a blue color appears, it indicates the presence of free chlorine, with a deeper color corresponding to a higher concentration. This method is fast for measurement, but it is not suitable for accurate quantification.