AS 1289.4.1.1:2019 Methods of testing soils for engineering purposes
6  Estimation of the organic matter
Organic matter shall be estimated for each test portion as follows:
(a) Remove the bottle containing the dried soil from the oven, cool in the desiccator and determine its mass (m 4 ) to the nearest 0.001 g.
(b) Transfer a small quantity, between 0.2 g and 5.0 g, depending on the organic content, to a dry 500 mL conical flask. NOTE  Sample size: The size of sample for chemical analysis will vary with the amount of organic matter present in the soil. As much as 5.0 g may be required for soil low in organic matter and as little as 0.2 g with a very peaty soil. After a number of determinations have been made, experience will indicate the most suitable size of sample to be taken. (When in doubt, it is suggested that a series of samples of varying sizes and known masses be tested.) Take as the correct result, the determination giving a total of 5 mL to 8 mL dichromate solution reduced.
(c) Determine the mass of the bottle and contents (m 5 ) and calculate the mass (m 3 ) of soil removed by difference (m 4 − m 5 ).
(d) Run 10 mL of the 0.1667 M potassium dichromate solution into the conical flask containing the test portion from a burette and, very carefully, add 20 mL concentrated sulfuric acid from a measuring cylinder.
(e) Swirl the mixture thoroughly for about 1 min and then stand it on a heat insulating surface for 30 min to allow oxidation of the organic matter to proceed. Protect the flask from cold air and draughts during this period.
(f) Add to the mixture 200 mL of distilled water followed by 10 mL of orthophosphoric acid and 1 mL of the indicator and shake the resultant mixture thoroughly. If the indicator is absorbed by the soil, add a further 1 mL of the solution.
(g) Add iron II sulfate 0.5M solution from the second burette in 0.5 mL increments, swirling the contents of the flask, until the colour of the solution changes from blue to green.
(h) Add a further 0.5 mL of potassium dichromate, changing the colour back to blue.
(i) Add iron II sulfate 0.5M solution, drop by drop, with continued swirling until the colour changes from blue to green after the addition of a single drop.
(j) Note the total volume (V 2 ) of iron II sulfate 0.5M solution used, to the nearest 0.05 mL.