This method is for the determination of water-soluble potassium.
All the potassium fertilisers listed in Group 3(a) of Section A and Groups 1, 3 and 4 of Section B and Group 2 of Section C of the Table in Schedule 1 of the Fertilisers Regulations 1991.
The potassium is extracted with water and after the removal of interfering substances, the potassium is precipitated in a slightly alkaline medium in the form of potassium tetraphenylborate (KTPB).
Formaldehyde, 25 – 35% solution, filter if necessary before use.
Potassium chloride.
Sodium hydroxide, 10 M solution. Care should be taken to ensure that the sodium hydroxide is free from potassium.
Indicator solution: dissolve 0.5 g phenolphthalein in 100 ml 90% ethanol.
EDTA solution: 4 g of the dihydrated disodium salt of ethylenediaminetetra-acetic acid (EDTA) per 100 ml. Store this reagent in a plastic container.
STPB solution: dissolve 32.5 g sodium tetraphenylborate in 480 ml of water, add 2 ml sodium hydroxide solution (4.3) and 20 ml of a magnesium chloride solution (100 g of MgCl2.6H2O per litre). Stir for fifteen minutes and filter through a fine, ashless filter paper. Store this reagent in a plastic container.
Wash liquid: dilute 20 ml of the STPB solution (4.6) to 1 litre with water.
Bromine water: saturated bromine solution in water.
Filter crucibles with a porosity of 5 to 20 microns.
Oven regulated at 120° + 10° C.
See Method 1.
In the case of potassium salts the sample must be ground finely enough to ensure that a representative sample is obtained for analysis. For these products, Method 1, paragraph 6(a) must be used.
7.1 Weigh to the nearest 0.001 g, 10 g of the prepared sample (5 g for potassium salts containing more than 50% of potassium oxide or 20 g for fertilisers containing less than 5% of potassium oxide) and place in a 600 ml beaker with approximately 400 ml of water. Bring to the boil and maintain heat for 30 minutes. Cool, transfer quantitatively into a 1 litre graduated flask, make up the volume, mix and filter into a dry receiver. Discard the first 50 ml of the filtrate.
Note:If the filtrate is dark in colour, transfer by pipette, an aliquot portion containing at the most 100 mg of K20 and place in a 100 ml beaker, add bromine water and bring to the boil to eliminate any surplus bromine. After cooling transfer quantitatively to a 100 ml graduated flask, make up to the volume, filter and determine the potassium in an aliquot portion of the filtrate.
7.2 Transfer by pipette an aliquot portion of the filtrate containing 25-50 mg of potassium (see Table on page 50) into a 250 ml beaker; make up to 50 ml with water.
To remove interferences, add 10 ml of the EDTA solution (4.5), several drops of the phenolphthalein solution (4.4) and stir in, drop by drop, sodium hydroxide solution (4.3) until a red colour persists. Finally add a few more drops of sodium hydroxide to ensure an excess (usually 1 ml of sodium hydroxide is sufficient to neutralise the sample and ensure an excess).
Boil gently for 15 minutes to eliminate most of the ammonia. Add water to make the volume up to 60 ml.
Bring the solution to the boil, remove the beaker from the heat and add 10 ml formaldehyde (4.1). Add several drops of phenolphthalein solution (4.4) and if necessary, more sodium hydroxide solution until a distinct red colour appears. Cover the beaker with a watch glass and place it on a steam bath for fifteen minutes.
7.3 Dry the filter crucible (5.1) to constant weight in the oven at 120° C (5.2) (about 15 minutes). Allow the crucible to cool in a desiccator and weigh it.
7.4 Remove the beaker from the steam bath, stir in drop by drop 10 ml of the STPB solution (4.6). This addition should take about 2 minutes; allow to stand for a least 10 minutes before filtering.
7.5 Filter under vacuum into the weighed crucible, rinse the beaker with the wash liquid (4.7), wash the precipitate three times with the wash liquid (60 ml in all of the wash liquid) and twice with 5 to 10 ml of water.
7.6 Wipe the outside of the crucible with a filter paper and place in the oven (5.2) for one and half hours at a temperature of 120° C. Allow the crucible to cool in a desiccator to ambient temperature and weigh rapidly.
7.7 Carry out a blank test under the same conditions (omitting only the sample) and allow for this in the calculation of the final result.
7.8 Carry out the determination on an aliquot portion of an aqueous solution of potassium chloride, containing at the most 40 mg of K2O.
8.1 If the quantities and the dilutions shown in the Table are used, the following formulae apply:
% K2O in the fertiliser = (A – a) × f
or
% K in the fertiliser = (A – a) × f1
where:
A = mass in grams of the precipitate from the sample
a = mass in grams of the precipitate from the blank
f and f1 = factors — see Table.
With samples and dilutions which differ from those shown in the Table use the following formulae:
No math image to display
or
No math image to display
where:
F = conversion factor, KTPB into K2O = 0.1314
F1 = conversion factor, KTPB into K = 0.109
D = dilution factor
M = mass in grams of sample for analysis.
| % of K2O in the fertiliser | % of K in the fertiliser | Sample for analysis (g) | Aliquot portion to be taken as a sample for precipitation (ml) | Conversion factor f % K2O g KTPB | Conversion factor f1 % K g KTPB |
|---|---|---|---|---|---|
| 1 – 5 | 0.8 – 4.2 | 20 | 50 | 13.14 | 10.91 |
| 5 – 10 | 4.2 – 8.3 | 10 | 50 | 26.28 | 21.81 |
| 10 – 20 | 8.3 – 16.6 | 10 | 25 | 52.56 | 43.62 |
| 20 – 50 | 16.6 – 41.5 | 10 | 10 | 131.40 | 109.10 |
| more than 50 | more than 41.5 | 5 | 10 | 262.80 | 218.10 |