DETERMINATION OF POTASSIUM IN SOILS

Equipment Required:

1. pH/Ion Analyzer or pH Meter with millivolt scale
2. Potassium Combination Ion Selective Electrode

Reagents:

Ionic strength adjustment buffer. I.S.A.B. (10M TEAC)

Standard potassium solution (1000 ppm)

O.lm TEAC reference electrode outer fill solution

Standards preparation:

I.S.A.B 10m Tetraethylammoinium Chloride (TEAC)

Dissolve 183.72g of TEAC in distilled water and dilute to 100mls

Potassium standard solution 1000 ppm

Dissolve 1.91g of potassium chloride in distilled water and dilute to 1000 mis

(1000 ppm) prepare 0.1, 10 and 100 ppm standards by serial dilution of the

1000 ppm standard.

Sample preparation:                 

1. Air dry soil samples, crush and pass through a 2mm sieve.
2. Add 40ml of distilled water to a 20.9g soil sample.
3. Shake for 30 sec, stand for 15 ml and shake again for 5 sec. Repeat this shaking procedure three times. Allow soil to settle.
4. To 25 ml of supernatant liquid add 0.25ml of ISAB.

Method:

To 100 ml of each standard add 1ml of I.S.A.B. Immerse electrodes into 0.1 ppm standard/I.S.A.B. solution and record mV reading. Repeat this operation for each standard (rinsing electrode with distilled water between measurements). In increasing concentration plot calibration curve, mV reading vs concentration on semi-log paper.

Immerse electrode into sample solution and record mV reading and read concentration of potassium directly.

Calculation:

Sample concentrations as read from the graph should be multiplied by 2 to allow for sample diltuion.