The 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. pH/ION analyzer or pH meter with millivolt scale
2. Iodide combination ion selective electrode

Reagents

1. Iodide standard, 10,000 ppm
2. Ionic strength adjustment buffer (ISAB), 5M NaNO₃
3. Nickel sulfate 1M (dissolve 26.3g of NiSO₄. 6H₂O in distilled water and dilute to 100 ml.)

Ionic Strength Adjustment Buffer (ISAB) Preparation

Dissolve 42.5g A.R. grade NaNO₃ in distilled water and dilute to 100 ml.

Iodide Standard Preparation 

Dissolve 11.81g of dried NaI (sodium iodide) in distilled water and dilute to 1000 ml. Prepare 10, 100, 1000, ppm standards by serial dilution of the 10,000 ppm standard.

Sample Preparation

To any sample containing sulfide or cyanide add 1 molar nickel sulfate. Addition of 0.1 ml of nickel sulfate to 100 ml sample will react with 58 ppm sulfide or 117 ppm cyanide.

To each 100 ml of sample add 2 ml of ISAB and stir thoroughly.

Method 

To 100 ml of each standard add 2 ml of ISAB. If any sample requires treatment for interferents prepare standards with the same background.

Immerse the electrodes in each of the standards in increasing concentration steps, rinsing the electrodes between standards and recording the mV response of the electrode.

 

Plot in graph of mV response against each standard concentration used on semi-log paper. 

Immerse the electrodes in the sample solution, record the mV response of the electrodes and plot sample concentration from graph.

This determination may be carried out directly in concentration units by use of the “activity” modes on pH/ION analyzers.

 

Calculations

As both, standard and samples have been diluted by the same amount, the result obtained from the graph relates to the concentration of the original sample.

Interferences

Sulfide and cyanide ion interference is removed by addition of NiSO₄. 6H₂O.

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

Chloride and thiosulfate ions do not interfere when present in excess of iodide by up to 10⁶ and 10⁵ respectively. Mercury should be absent.

Concentration of free metal ions causing 10% error are:

                        Bismuth                                               4 ppm

                        Cadmium                                 50 ppm

                        Lead                                        1000 ppm