ISO 3696 Water for analytical laboratory use - Specification and test methods
7 Methods of test
It is essential that the determinations specified in this clause are carried out in a dust-free, pure atmosphere and that suitable precautions are taken to prevent any contamination of the sample and test portions.
7.1 Measurement of pH
7.1.1 Apparatus
Ordinary laboratory apparatus and pH-meter, fitted with a glass measuring electrode and an Ag/AgCl reference electrode.

7.1.2 Procedure
Calibrate the pH-meter (7.1.1) in accordance with the manufacturer's instructions, using buffer solutions having pH values covering the range 4.0 to 8.0.

Transfer some of the laboratory sample to a suitable vessel and adjust the temperature of the water to 25 +/- 1 °C. Introduce the electrodes and determine the pH.

7.2 Measurement of conductivity
7.2.1 Apparatus
Ordinary laboratory apparatus and
7.2.1.1 Conical flask, fitted with a guard tube containing self-indicating granular soda lime.

7.2.1.2 Conductivity meter with cell for circulation, having an "in-line" conductivity cell, with automatic temperature compensation, for measurements on grade 1 and grade 2 water.

NOTE If the instrument is not temperature-compensated an in-line heat-exchanger, capable of adjusting the temperature of the water under test to 25 +/- 1 °C, should be fitted.

7.2.1.3 Conductivity meter, for measurements on grade 3 water.

7.2.2 Procedure
7.2.2.1 Grade 1 and grade 2 water
Using the conductivity meter (7.2.1.2), temperature-compensated to 25 +/- 1 °C, measure the conductivity.

7.2.2.2 Grade 3 water
Transfer 400 ml of the sample into the flask (7.2.1.1), insert the guard tube and adjust the temperature of the water to 25 +/- 1 °C. Measure the conductivity by means of the conductivity meter (7.2.1.3) in accordance with the manufacturer's operating instructions.

7.3 Limit test for oxidizable matter
NOTE The limits are equivalent to oxidizable matter, expressed as milligrams of oxygen (O) per litre, of 0.08 and 0.4 for grade 2 and grade 3 water respectively.

7.3.1 Reagents
Use grade 2 water to prepare the following reagent solutions.
7.3.1.1 Sulfuric acid, approximately 1 mol/l solution.

7.3.1.2 Potassium permanganate, standard volumetric solution, c(1/5 KMnO4) = 0.01 mol/l.

7.3.2 Procedure
7.3.2.1 Test portion
1000 ml of grade 2 water or 200 ml of grade 3 water.

7.3.2.2 Test
Add 10 ml of the sulfuric acid solution (7.3.1.1) and 1.0 ml of the standard volumetric potassium permanganate solution (7.3.1.2) to the test portion (7.3.2.1), bring to the boil and boil for 5 min. Check that the colour of the test mixture is not completely discharged.

7.4 Measurement of absorbance
7.4.1 Apparatus
Ordinary laboratory apparatus and
7.4.1.1 Spectrometer, with selectors for continuous variation, or

7.4.1.2 Spectrometer, with selectors for discontinuous variation, fitted with filters providing maximum transmission in the neighbourhood of 254 nm.

7.4.1.3 Optical cells, of the same silica material, 1 cm and 2 cm path length.

NOTE If the available spectrometer is not sufficiently sensitive, the sensitivity may be increased by using optical cells of greater path length.

7.4.2 Procedure
Fill the 2 cm optical cell (7.4.1.3) with some of the sample. Measure the absorbance of this optical cell with the spectrometer (7.4.1.1), at a wavelength of about 254 nm, or with the spectrometer (7.4.1.2) fitted with suitable filters, after having adjusted the instrument to zero absorbance against the same water in the 1 cm optical cell.

7.5 Determination of residue after evaporation on heating at 110 °C
7.5.1 Apparatus
Ordinary laboratory apparatus and
7.5.1.1 Rotary evaporator, with a flask of capacity about 250 ml.

7.5.1.2 Steam bath

7.5.1.3 Dish, of platinum, silica or borosilicate glass, of capacity about 100 ml.

7.5.1.4 Oven, capable of being controlled at 110 +/- 2 °C.

7.5.2 Procedure
7.5.2.1 Test portion
Transfer 1000 ml of the laboratory sample into a stoppered measuring cylinder.

7.5.2.2 Determination
Introduce 100 ml of the test portion into the clean and dry rotary evaporator (7.5.1.1) and distill on the steam bath (7.5.1.2) under reduced pressure. As the water evaporates, add successive quantities of the test portion until the entire test portion has been evaporated to about 50 ml.

Transfer the residue quantitatively to the dish (7.5.1.3), previously heated for 2 h in the oven (7.5.1.4) controlled at 110 +/- 2 °C, cooled in a desiccator and weighed to the nearest 0.0001 g. Use two approximately 5 ml portions of the sample to effect the transfer.

Using the steam bath, evaporate the residue to dryness. Transfer the dish and residue from the steam bath to the oven, controlled at 110 +/- 2 °C, and leave for about 2 h. Remove the dish from the oven, allow to cool to ambient temperature in a desiccator and weigh to the nearest 0.0001 g. Repeat the operation of heating, cooling and weighing until the difference between two consecutive weighings does not exceed 0.0002 g.

7.5.3 Expression of results
The residue after evaporation and heating at 110 °C, expressed in milligrams per kilogram, is numerically equal to the mass, in milligrams, of the residue, dried to constant mass.

7.6 Limit test for reactive silica
NOTE The limits are equivalent to silica contents, expressed as milligrams per litre of SiO2, of 0.01 and 0.02 for grade 1 and grade 2 water respectively.
7.6.1 Reagents
7.6.1.1 Silica, standard solution I (concentrated).
Weigh, to the nearest 0,000 1 g, 1 g of finely ground pure silica sand (> 99.9 % of SiO2), previously dried at 110 °C, into the platinum dish (7.5.2.3).

Add 4.5 g of sodium carbonate (anhydrous Na2CO3) and intimately mix the sample with a dry, smooth-ended glass rod. Ease the mixture into the centre of the dish and flatten it so that it covers an area about 30 mm in diameter. Cover the mixture with a further 0.5 g of the sodium carbonate, then gently brush any particles adhering to the glass rod into the dish.

Cover the dish with a platinum lid and place in the muffle furnace (7.6.2.3), controlled at 300 to 400 °C. Heat the mixture, gradually raising the temperature for about 10 min or until fusion is complete. Remove the dish from the furnace and gently swirl it to incorporate any particles. Allow to cool, wash into the dish any particles adhering to the underside of the lid with hot water, then dissolve the fused mass in hot water. Cool, transfer the solution quantitatively to a 1000 ml one-mark volumetric flask, dilute to the mark and mix. Transfer the solution to the plastics bottle for storage.

1 ml of this standard solution contains 1 mg of SiO2.

7.6.1.2 Silica, standard solution II (diluted).
Transfer a 5.0 ml aliquot portion of the silica standard solution (7.6.1.1) to a 1000 ml one-mark volumetric flask, dilute to the mark and mix. 1 ml of this standard solution contains 0.005 mg of SiO2.

Prepare this solution at the time of use.

7.6.1.3 Ammonium molybdate, 50 g/l solution.
Dissolve 5 g of powdered ammonium molybdate in a mixture of 80 ml of water and 20 ml of the sulfuric acid solution (7.6.1.5), without heating. Store in a plastics bottle.

7.6.1.4 4-Methylaminophenol sulfate (metol), indicator solution.
Dissolve 0.2 g of metol and 20 g of dipotassium disulfite (potassium metabisulfite) in 100 ml of water, without heating. Store in a plastics bottle.

Discard the solution after 4 weeks, or if it shows prior signs of decomposition.

7.6.1.5 Sulfuric acid, c(H2SO4)
approximately 2.5 mol/l.

Carefully add, with stirring, 135 ml of sulfuric acid solution, Q ≈ 1.84 g/ml, to sufficient water to produce 1000 ml of solution. Store in a plastics bottle.

7.6.1.6 Oxalic acid, 50 g/l solution.

7.6.2 Apparatus
Ordinary laboratory apparatus and
7.6.2.1 Dish, of platinum, of capacity about 250 ml.

7.6.2.2 Matched Nessler cylinders, of capacity 50 ml.

7.6.2.3 Muffle furnace, capable of being controlled at 300 to 400 °C.

7.6.2.4 Water-bath, capable of being controlled at approximately 60 °C.

7.6.3 Procedure
7.6.3.1 Test portion
Take 520 ml of grade 1 water or 270 ml of grade 2 water.

7.6.3.2 Test
Evaporate the test portion (7.6.3.1) in the dish (7.6.2.1) in successive portions, to produce a final volume of 20 ml. Add 1 ml of the ammonium molybdate solution (7.6.1.3). After exactly 5 min, add 1 ml of the oxalic acid solution (7.6.1.6) and mix well. After 1 min, add 1 ml of the metol solution (7.6.1.4), and heat for 10 min in the water bath (7.6.2.4), maintained at approximately 60 °C. Transfer the solution to one of the matched Nessler cylinders (7.6.2.2).

Prepare a standard colour solution following the same procedure, but using a mixture of 19.0 ml of the sample and 1.0 ml of the standard silica solution (7.6.1.2) instead of the 20 ml resulting from the evaporation of the test portion (7.6.3.1).

Transfer the solution to the other matched Nessler cylinder (7.6.2.2).

Viewing vertically downwards, check that the intensity of any blue colour produced in the test solution does not exceed that produced in the standard colour solution.

8 Test reports
Each test report shall include the following particulars:
a) an identification of the sample;
b) the reference of the method used;
c) the results and the method of expression used;
d) any unusual features noted during the determination;
e) any operation not included in this International Standard, or regarded as optional.