Easily oxidisable Carbon (Walkley-Black)

Chemicals

Potassium dichromate 1N (M/6) 98g to 2l
Ferrous sulphate N/2 (M/2) 139g to 1l + 10ml conc. Sulphuric Acid
Barium diphenylamine p-sulphonate 0.1g +16g Barium Chloride to 200ml (Warm to dissolve)

Method

The soil is first ground to 0.425mm using a pestle and mortar

A quantity of soil is then accurately weighed into a 250ml conical flask. The amount of soil to be weighed depends on the estimated carbon content. For highly organic soils the maximum amount is about 40-50mg but this can be raised to near 500mg for more mineral soils.

To each flask is now added accurately 10 ml of the 1N Potassium Dichromate. Blanks containing no soil also have to be done (3 blanks are suggested as this result is critical to the accuracy of the other determinations.)

To each flask is then added 20ml conc. sulphuric acid from a dispenser. This should be done in a fume cupboard wearing eye and hand protection as the reaction can be quite fierce. The flask is gently swirled so that all the soil is in contact with the acid-dichromate mixture and then allowed to gently mix by placing on the orbital shaker for exactly 30 minutes.

After the 30 minutes have elapsed 10ml of orthophosphoric acid are added to each flask (again using a dispenser) and also about 150ml of water.

The blanks are treated in exactly the same manner.

A magnetic follower is now placed in the flask and also a few drops of the indicator. The flask is then placed on a magnetic stirrer (the Imspins are preferred) under a 50ml burette containing the Ferrous sulphate solution. Ferrous sulphate is then added and the colour should go from a muddy brown through a blue stage to a green. As the green stage is approached the ferrous sulphate is best added more slowly. At this point a further 0.5 ml of Potassium dichromate is accurately added and the titration completed until the solution just goes green. The reading is then taken.

Calculation

a) Calculation of Ferrous Sulphate strength.

N(Fe) = 10.5/B where B is the titre for the blank

b) Calculation of Carbon content

1) Easily oxidisable carbon

%EOC = 300*N(Fe)*(B-T) / W

T = Titre for sample. W = Weight taken in mg

2) Total oxidisable carbon

%TOC=%EOC*1.3