THE DETERMINATION OF CHLORIDE IN SOIL

The chloride is leached from the oven dried soil using deionised water and is then titrated with standard AgNO₃ using either the chloride ISE or the silver ISE as the end point detector.

Equipment Required:

1. Corning pH/Ion analyser OR Corning pH meter with millivolt scale
2. Doubel junction reference electrode
3. Reference electrode lead
4. 50 ml graduated beaker
5. Accurate balance
6. Magnetic stirrer

Reagents:

1. Silver nitrate (analar)
2. Potassium nitrate

Standard Preparation                

Dissolve 1.6987g AgNO₃ with deionised water in a litre volumetric flask, dilute to the mark with deionised water. This is 0.01 Molar AgNO₃ to be used as the titrant.

Reference Electrode Outer Filling Solution

Dissolve 1.O11g KNO₃ with deionised water in a 100ml volumetric flask and dilute to the mark with deionised water. This is 0.1M KNO₃.

Sample Preparation

Accurately weigh 4.00g of air dried and ground soil into a 100ml plastic screw cap bottle. Add 50ml of deionised water and shake for 1 hour.

Filter the solution into a 250ml beaker ensuring to wash the residue with deionised water and adding these washings to the filtrate.

Method:

1. Fill the clean burette with standard AgNO₃ solution. If the solution

appears cloudy in the burette, discard and was the burette thoroughly with deionised water. Rinse the burette with a small volume of standard and then refill.

2. Note the burette reading before any additions are made.
3. Place the sample solution on a magnetic stirrer and stir the solution constantly throughout analysis.
4. Immerse the electrodes in the sample solution and record the stable potential.
5. Add small amounts of AgNO₃ to the sample and note the stable potential after each addition.
6. Reduce the volume of the additions when the end point approaches, this can be detected by a gradual increase in potential when a silver electrode is used.
7. The end point is where the largest increase in potential occurs per unit volume of titrant.
8. Plot mV reading vs volume of titrant to attain the end point in mis of AgNO₃.

Calculations:

Concentration of C1^- in the sample solution =

End point volume x concentration of AgNO_{3    =  X}

1000                     

Grams of chloride present in the sample = X x 35.45 x volume of titrant    ₌    Y

1000

To express the result in mg chloride/lOOg soil

=Yx _____________100________

mass of soil sample taken

Note: Chloride content varies greatly with soil type, therefore for certain types the concentration of titrant or the mass of soil taken has to be varied.