DETERMINATION OF IODIDE IN WATER


A rapid simple method for the measurement of iodide in water in the range of 0.01 ppm - 10,000 ppm

Equipment Required

1. Model DR359TX Ion Meter

2. 3281 Iodide Combination ISE

3. 21303 Iodide 1000ppm Standard Solution

4. 30303 Iodide Ionic Strength Adjustment Buffer

5. Glassware. Beakers 250ml, Volumetric Flask 100ml. 10ml pipette

6. Nickel sulphate 1M (dissolve 26.3g of NiSO 4 .H 2 O in distilled water and dilute to 100 mls)

7. Deionised water.

Standard Preparation

Prepare standards of 10 and 100ppm Iodide by serial dilution of the 1000ppm standard. This is

achieved by pipetting 10ml of the standard into a 100ml Volumetric flask and diluting to the mark with

deionized water. This is now a 100ppm Standard solution. Prepare further standards as necessary.

Sample Preparation

To any sample containing Sulphide or Cyanide add 1 Molar Nickel Sulphate. Addition of 0.1 ml of

Nickel Sulphate to 100 mls sample will react with 58 ppm Sulphide or 117 ppm Cyanide. No addition

required if Sulphide and Cyanide are not present.

Method

To 100ml of each of the standards and samples add 2ml of 30303 ISAB and mix the contents. Be sure the beakers are clean and make sure not to touch the inside of the beakers with bare hands as contamination from sweat etc. is common. Beakers that have been washed with softened water or tap water will be contaminated. In these cases rinse the beakers with deionised water.

Immerse the electrode in each of the standards in increasing concentration steps in the calibration mode of the Ion Meter, rinsing the electrodes with distilled water and dabbing off the excess water between standards.

Using the DR359TX Ion meter will allow the result to be read directly on the display. It also allows for up to a 5 point calibration if required.

If you do not have an ION meter you can read the mV values using a pH/mV meter such as an EDT model FE257. Record the mV response and plot a graph of mV vs log of Concentration.

Interferences

Sulphide and cyanide ion interference is removed by addition of NiSO 4 .6H 2 O.

Bromide does not interfere in concentrations up to 5000 times that of Iodide.

Chloride and Thiosulphate ions do not interfere when present in excess of iodide by up to 10 6 and 10 5 respectively. Mercury should be absent.

Concentration of free metal ions causing 10% error are

Bismuth 4 ppm, Cadmium 50 ppm ,Lead 1000 ppm.