Question

‎‏Question 1: From your previous study, provide how to: Determine the chloride ion in water sample (e.g swimming pool or tap water) by two different analytical methods:

write 200-300 word and give diagram

Answer 1

Determination of the chloride ion in water sample (e.g swimming pool or tap water) by two different analytical methods:

1) Voltammetry :
It is easy-to-operate and highly sensitive free chlorine determination method using differential pulse voltammetry(DPV) with an Au working electrode has been developed. This method can be used to monitor the residual free chlorine concentration in drinking water. The electrochemical measurements were carried out using a single compartment cell. An Au disk (1.6 mm diameter), Ag/AgCl (saturated KCl) and Pt wire is used as the working, reference and counter electrodes, respectively. For pretreatment of the Au working electrode, the surface of the electrode is polished with an aluminium oxide suspension, carefully sonicated in water, and then rinsed with Milli-Q water. A 0.1 mole/dm³ potassium chloride  solution us as the supporting electrolyte. The pH of solution for the DPV measurements was adjusted to 5 using acetate buffer solution. To a 20 cm3 water sample, potassium chloride is added until the final concentration is 0.01 mole/dm³. All measurements are carried out at room temperature thermostated at about 25 °C. A set of measurements of different concentration is done ti plot a calibration curve. The limit of detection (LOD) was estimated using the 3Ѕ_B/m_{​​​​​} IUPAC criteria, where m is the slope of the linear calibration plot and the Ѕ_B correspond to the standard deviation of the sensor response in the absence of chlorine (blank). The detection limit of this method is 0.04 mg Cl /dm³.

2) Iodometry :
The indirect iodometric titration method or iodometry deals with the titration of iodine liberated (as triiodide,i.e.,I₃―) in chemical reactions. This titration method is one of the oldest methods for determining chlorine, and is generally used for the total chlorine determination above 1 mg/L Cl₂. To determine chlorine in a sample solution, a small excess of potassium iodide (KI) is added to the sample solution to react with chlorine to form the triiodide (I₃-) as shown in equation below―

Cl₂ + 3I^―→ I₃^―+ 2Cl^―

The triiodide thus formed is then titrated with a sodium thiosulfate (Na₂S₂O₃) solution as:

I₃― + 2S₂O_3^― → 3I^―+ S₄O₆^​2―

In the titration of above equation starch is added at the beginning of the titration as an end-point indicator. The end-point of the titration is indicated by the disappearance of the blue coloured starch-iodide complex. The titration method is usually performed at a sample pH between 3 - 4. It has been shown that sample temperatures above 20°C can produce significant errors if starch is used as the titration end-point indicator. This indicated the release of triiodide from the starch helix is temperature-dependent. In order to get the maximum accuracy, iodometric titrations using the starch indicator should be performed at sample temperatures less than 20°C. A “back titration” should be performed for waters containing potential chemical interferences. In the back titration, a known amount of thiosulfate is added in excess of the chlorine in the sample solution to completely reduce the chlorine. The amount of unreacted thiosulfate is then titrated with a standard iodine solution. The total chlorine is calculated from the amount of the reacted thiosulfate. Since, sensitivity of the iodometric titration is not very high, it requires a large volume of sample to titrate the sample with a low chlorine concentration.