N.4 Sulfate content determination
N.4.1 Test Procedures
(1) The gravimetric method for analysis of acid or water extract or groundwater, mentioned here is suggested as the preferred one, unless it can be shown by parallel analysis that an alternative method has equal or better accuracy.
(2) The crystalline form of calcium sulfate, gypsum (CaSO4 · 2H2O) should be dried out at a temperature of 50 °C. Gypsum-bearing samples begin to lose their water of crystallisation at temperatures higher than about 65 °C, which can give rise to erroneously high measured water contents.
(3) The relationship between SO2–3 and SO2–4 is given by SO2–4 = 1,2SO2–3, with SO2–3 and SO2–4 contents expressed as a percentage.
NOTE Examples of test procedures for the: determination of sulfate content arc given in the documents listed in X.4.2.4.
N.4.2 Evaluation of test results
(1) The interpretation should consider that the solubility of calcium sulfate in water is low, but in geological time, appreciable quantities can dissolve as occurs, for example, in karstic formations. Particular care is needed when results are marginal with respect to classification categories.
(2) The presence of certain other substances (notably sulfides and sesqioxides) can affect the chemical reactions, which then influence the test results. Sulfides in soil can oxidise in the long term to produce additional sulfates.
N.5 pH value determination (acidity and alkalinity)
N.5.1 Test procedures
(1) Several methods are available for determining values of pH. Of these, the electrometric method, which gives a direct reading of pH either in a prepared suspension of soil or in groundwater, is recommended as the definitive method.
N.5.2 Evaluation of test results
(1) Erroneous test results can be caused by:
- omitting or wrong calibration of the pH meter before and after each set of tests;
- inadequate protection of electrodes when the instrument is not in use;
- failure to allow the pH meter to reach a stable condition before taking pH readings;
- contamination due to inadequate washing of containers for sampling groundwater.
NOTE Examples of test procedure for the determination of the pH value are given in the documents listed in X.4.2.5.
N.6 Chloride content determination
N.6.1 Test procedures
(1) Procedures for determination of chloride content include:
- Mohr's method for water-soluble chlorides;
- Volhard's method for acid-soluble or water-soluble chlorides;
- electrochemical procedures.
(2) The first two methods make use of the exchange reaction between the chlorides and silver nitrate, but different methods of analysis are used. Both methods require careful observation and weighing. The third method is based on the measurement of conductivity in dilutions of the sample with known water content.
(3) The presence of chlorides can be confirmed from a quick qualitative test: take about 5 ml of filtered groundwater, or of 1:1 soil-water extract, in a test-tube. If this is highly alkaline (pH 12–14), add a few drops of nitric acid to acidify it. Add a few drops of 1 %silver nitrate solution. Appreciable turbidity indicates that chlorides are present in a measurable quantity, which can be determined from one of the test procedures.
(4) Volhard's method is the basis of the tests given in 7.2 (water-soluble chlorides) and 7.3 (acid-soluble chlorides) of BS 1377-3:1990 and of the method given in BS 812-118:1988 for mineral aggregates. In principle, an excess of silver nitrate solution is added to the acidified chloride solution and the unreacted portion is back-titrated with potassium thiocyanate, with ferric aluminium used as an indicator.
(5) In Mohr's method, the test solution and a blank for comparison are each titrated with 0,02 N silver nitrate solution, potassium chromate being used as an indicator. This method is preferable for determining chlorides in groundwater.
NOTE Examples of test procedures are given in the documents listed in X.4.2.6.
N.6.2 Evaluation of test results
(1) The theoretical relationship between salinity expressed as sodium chloride content and chloride content need not hold due to the very mobile nature of the chloride anion.