IEC 62021-1 Insulating liquids - Determination of acidity - Part 1: Automatic potentiometric titration
5 Reagents and auxiliary products
5.1 Reagents
Only reagents of recognized analytical grade and de-ionized water or water of equivalent purity shall be used.

5.2 Titration reagent
Standard volumetric alcoholic solution 0,05 mol/l potassium hydroxide.

Add 3,0 g of potassium hydroxide to 1000 ml +/- 10 ml of 2-propanol. Boil gently for 10 min to effect solution. Cool and stopper the flask.

Allow the solution to stand in the dark for 2 days and then filter the supernatant liquid through a 5 µm membrane filter. Store in a suitable amber glass bottle.

The concentration of this solution is approximately 0,05 mol/l and shall be standardized as described in 10.1. For periodic tests on equipment in service, faster titration may be achieved by the use of 0,1 mol/l potassium hydroxide by agreement between the laboratory and the equipment owner, although this may result in poorer precision and detection limit.

Store in such a manner that the solution is protected from atmospheric carbon dioxide by means of a guard tube containing soda-lime absorbent and in such a way that it does not come into contact with cork, rubber or saponifiable stopcock grease.

Commercial alcoholic potassium hydroxide solution may be used, if necessary diluting to 0,05 mol/l with 2-propanol. This shall be standardized as described in 10.1.

5.3 Titration solvent
2-propanol (isopropanol, IPA).

5.4 Potassium hydrogen phthalate, primary standard
This should be dried before use for 2 h at 105 °C.

NOTE A 0,1 mol/l solution of hydrochloric acid in de-ionized water, prepared as in ISO 6619, may be used. Other acids may be used, provided they are certified against a primary standard.

5.5 Potassium chloride reference electrolyte
Prepare a solution of potassium chloride in de-ionized water at the concentration recommended by the electrode manufacturer. Commercially available solutions may be used where available.

5.6 Aqueous buffer solutions
Buffer solutions of suitable pH for calibration of electrodes, for example, pH 4, pH 7 and pH 11.

5.7 Cleaning solution
Weigh 8 g of ammonium peroxydisulphate into a glass beaker. Carefully add 100 ml of 98 % sulphuric acid and gently stir. Before use, the solution should be left overnight for the solid to dissolve completely.

WARNING - Ammonium peroxydisulphate is a strong oxidizing agent. Sulphuric acid is a strong corrosive agent. Handle carefully.

Commercially available cleaning solutions as recommended by the electrode manufacturer may be used.

6 Apparatus
6.1 Potentiometric titration apparatus
An automatic pH titrimeter capable of titrating to a fixed end-point using either variable or fixed titrant increments.

The instrument shall be protected from stray electrical fields so that no change of the reading is produced by touching any part of the system with a grounded lead.

An automatic burette with a dispensing accuracy of +/- 0,005 ml or better is required.

A reservoir for the titrating solution. It shall be fitted with a guard tube containing soda lime or other carbon dioxide absorbing material.

6.2 Glass indicating electrode
A glass electrode specifically designed for non-aqueous titrations is recommended.

The electrode shall be connected to the potentiometer by means of a suitably screened cable such that the resistance between the screening and the entire length of the electrical connection is greater than 50 000 MΩ.

6.3 Reference electrode
The electrode shall be made of glass and shall be provided with a movable joint in the form of a sleeve or plug to facilitate easy washing of the reference electrolyte cell. It is recommended that a double junction design is used and the electrolyte cells filled with potassium chloride electrolyte (see 5.5). The electrode shall be reserved for non-aqueous titrations.

NOTE Certain alternative electrode-electrolyte combinations have been found to give satisfactory results, although the precision using these alternatives has not been determined. Combined electrodes may be used provided they otherwise conform to this standard and have at least a similar speed of response.

6.4 Stirrer
The stirrer should have a variable speed and be fitted with a propeller, paddle or magnetic bar of chemically inert surface material. It shall be electrically grounded to avoid any change in the meter reading during the course of the titration.

6.5 Titration vessel
This should be as small as possible, sufficient to contain the solvent, sample, stirrer and electrodes and be inert to the reagents. Glass vessels are preferred to prevent build-up of electrostatic charge.

6.6 Titration stand
A suitable stand to support the beaker, electrodes, stirrer and burette.

7 Sampling
Samples shall be taken following the procedure given in IEC 60475.

Ensure that the test portion is representative by thoroughly mixing, as any sediment present may be acidic or have adsorbed acidic material from the liquid phase.