# Fertilisers (Sampling and Analysis) Regulations (Northern Ireland) 1996, Paragraph 5: PROCEDURE


Standardisation of the silver nitrate solution
5.1 Take 5.00 ml and 10.00 ml of the standard reference potassium chloride solution (3.6) and place in two low-form beakers of convenient capacity (for example 250 ml). Carry out the following titration of the contents of each beaker.
Add 5 ml of the nitric acid solution (3.2), 120 ml of acetone (3.1) and sufficient water to bring the total volume to about 150 ml. Place the rod of the magnetic stirrer (4.3) in the beaker and set the stirrer in motion. Immerse the silver electrode (4.1) and the free end of the bridge (4.2) in the solution. Connect the electrodes to the potentiometer (4.1) and, after verifying the zero of the apparatus, note the value of the starting potential.
Titrate, using the microburette (4.4), adding initially 4 or 9 ml respectively of the silver nitrate solution corresponding to the standard reference potassium chloride solution used. Continue the addition in 0.1 ml portions for the 0.004 M solutions and in 0.05 ml portions for the 0.1 M solutions. After each addition, await the stabilisation of the potential.
Record the volumes added and the corresponding value of the potential in the first two columns of a table.
In a third column of the table, record the successive increments (Δ1E) of the potential E. In a fourth column, record the differences (Δ2E) positive or negative, between the potential increments (Δ1E). The end of the titration corresponds to the addition of the 0.1 or 0.05 ml portion (V1) of the silver nitrate solution which gives the maximum value of Δ1E.
In order to calculate the exact volume (Veq) of the silver nitrate solution corresponding to the end of the reaction, use the formula:
Veq=Vo+(V1×bB) where:
Vo is the total volume, in ml, of the silver nitrate solution immediately lower than the volume which gives the maximum increment of Δ1E;
V1 is the volume, in ml, of the last portion of the silver nitrate solution added (0.1 or 0.05 ml);
b is the last positive value of Δ2E;
B is the sum of the absolute values of the last positive values of Δ2E and the first negative value of Δ2E (see example in Table 1).
Blank test
5.2 Calculate the blank value using the equation below and take account thereof when calculating the final result.
The result V4 of the blank test on the reagents is given, in ml, by the formula:
V4=2V3—V2 where:
V2 is the value, in ml, of the exact volume (Veq) of the silver nitrate solution corresponding to the titration of 10 ml of the potassium chloride standard reference solution used;
V3 is the value, in ml, of the exact volume (Veq) of the silver nitrate solution corresponding to the titration of 5 ml of the potassium chloride standard reference solution used.
Check test
5.3 The blank test can at the same time serve as a check that the apparatus is functioning satisfactorily and that the test procedure is being implemented correctly.
Determination
5.4 Take a portion of sample in the range of 10 to 20 grams and weigh to the nearest 0.01 gram. Transfer quantitatively to a 250 ml beaker. Add 20 ml of water, 5 ml of nitric acid solution (3.2), 120 ml of acetone (3.1) and sufficient water to bring the total volume to about 150 ml.
Place the rod of the magnetic stirrer (4.3) in the beaker, place the beaker on the stirrer and set the stirrer in motion. Immerse the silver electrode (4.1) and the free end of the bridge (4.2) in the solution, connect the electrodes to the potentiometer (4.1) and, after having verified the zero of the apparatus, note the value of the starting potential.
Titrate with the silver nitrate solution, by additions from the microburette (4.4) in increments of 0.1 ml. After each addition, await the stabilisation of the potential.
Continue the titration as specified in 5.1, starting from the fourth paragraph: ‘Record the volumes added and the corresponding values of the potential in the first two columns of a table …'

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Source: https://www.legislation.gov.uk/nisr/1996/513/schedule/2/part/I/chapter/14f./paragraph/5 (the official version)
Page: https://openlegislation.uk/fertilisers-sampling-and-analysis-regulations-northern-ireland-1996/schedule-2/part-I/chapter-14f./paragraph-5
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.