Sodium sulphate is present in some soils. The salt is easily hydrated and dehydrated under the influence of climatic changes. There are enormous volume changes during this process of hydration and dehydration, which influence the engineering properties of soils.
Take the soil sample in a state in which it can be crumbled. If necessary, dry it in oven at 1050C to 1100C. Break the lumps in a mortar with a rubber covered pebble. Mix the sample thoroughly and sub-divide it by quartering.
Take 10 g of soil from the sample in a 250 ml bottle with 100ml of distilled water. Shake it occasionally for about 2 h with a mechanical shaker. Allow the soil to stand overnight. In case the soil is dispersive, add 0.5 to 1.0 g of pure potassium nitrate to flocculate the particles.
Filter and take 25 ml of filtrate in a beaker. See if the solution is alkaline from phenolphthalein indicator. If so, add concentrated hydrochloric acid to just neutralize the solution. Add further 4 ml of hydrochloric acid to make the solution acidic.
Boil the solution. Remove the solution from heat and add hot barium chloride solution in a fine stream with constant stirring, till there is no precipitation with further addition.
Place the beaker on a steam bath for a minimum period of 4 h and allow precipitate to settle. Further the precipitate through ashless filter paper wash free from chloride irons, dry and ignite filtration can be done through a pre-weighed sintered glass crucible or a Gooch crucible. In case of filter paper after drying, ashing shall be done. On a low flame and the precipitate then ignited over a burner or in a muffle furnace at 600oC to 700oC for half an hour.
Cool in a dessicator, weigh and note the residue. This is the weight of the barium sulphate.
Calculate the corresponding weight of sodium sulphate and determine its percentage as follows:
a) Sulphate (SO4) percent by mass = 41.15 W1/W2
b) Sodium Sulphate (Na2 SO4) percent by mass = 60.85 W1/W2
W1 = mass in g of the precipitate
W2 = Mass of g of the soil contained in the solution taken for precipitation.