Standard Procedure SP 0007:2005

Method for preparing a titration reagent

1 Scope

This Standard Procedure is adapted from BS 5086:1985 Analysis of butter - Part 4: Method for determination of salt content. It can be used to prepare and standardise a solution of potassium thiocyanate for use in SP 0008.

2 Definitions

standardise
to determine the exact concentration of a solution

3 Principle

A known mass of potassium thiocyanate is dissolved to give a known volume of solution of approximately the required concentration. The exact concentration is determined by titration against a standardised solution of silver nitrate.

4 Apparatus and Reagents

balance capable of weighing to 0.01 g
250 cm³ beaker
250 cm³ volumetric flask
stirring rod
1 cm³, 10 cm³ and 50 cm³ measuring cylinders
25 cm³ pipette and pipette filler
50 cm³ burette
250 cm³ conical flask
eye protection
potassium thiocyanate
standardised 0.025 mol dm^-3 silver nitrate solution Caution: stains clothing and skin black
approximately 5 mol dm^-3 nitric acid Caution: corrosive
50% ammonium iron(III) sulfate solution (50g in 95 cm³ water + 5 cm³ 5 mol dm^-3 nitric acid)

5 Procedure

Use distilled water throughout.
Weigh out 0.60 ± 0.05 g potassium thiocyanate into a 250 cm³ beaker. Add about 50 cm³ water and stir gently to dissolve. Pour into a 250 cm³ volumetric flask.
Add a further 50 cm³ of water to the beaker to dissolve any remaining potassium thiocyanate. Transfer to the flask.
Rinse the beaker into the flask with a further portion of water.
Make up to the mark, stopper and mix well. This gives an approximately 0.025 mol dm^-3 solution.
Pipette 25.0 cm³ of 0.025 mol dm^-3 silver nitrate solution into a 250 cm³ conical flask. Add 25 cm³ distilled water, 5 cm³ of 5 mol dm^-3 nitric acid and 1 cm³ of 50% ammonium iron(III) sulfate solution.
Titrate with the potassium thiocyanate solution until an orange tint lasts for about 15 seconds.
Repeat the titration. Calculate the average of the two volumes of potassium thiocyanate needed.
Label the flask and keep the potassium thiocyanate solution for SP 0008.

6 Expression of Results

Calculate the concentration C of potassium thiocyanate using the formula:

C = 25 x 0.025 / V

where

C is the concentration in moles per cubic decimetre;

V is the average volume of potassium thiocyanate needed in the titration.

NOTE: If the concentration of the standardised silver nitrate is not exactly 0.025 mol dm^-3, replace 0.025 in the formula with the correct concentration value.

7 Test Report

Your test report should include:

(a) reference to this Standard Procedure;

(b) the concentration of potassium thiocyanate as determined by the procedure.