By D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)
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2. Nitrazine yellow indicator: Since different batches of nitrazine yellow have different colours at a particular pH value, if the standard Tintometer disc is to be used, the nitrazine yellow should be that supplied by Tintometer. 3. Activated charcoal is used to clear the soil suspension. Procedure 1. 50 ml of the buffer solution and 20 g of soil are shaken together in a large test-tube for about a minute and then again at frequent intervals for 30 min. 2. T h e suspension is filtered; if the filtrate is coloured a small spatula full of activated charcoal is added, the mixture again shaken and filtered.
T h e first two methods for determining lime requirement are based on this calculation and do in fact determine exchangeable hydrogen. 36 A PRACTICAL COURSE IN AGRICULTURAL CHEMISTRY If an acid soil be shaken u p with a buffer solution of pH 7-0, the reaction of the suspension will become more acid. Woodruff's buffer solution is so constituted that the change in reaction is a linear relationship with the exchangeable hydrogen in the soil. Reagent required Woodruff's buffer consists of 8 g p-nitrophenol, 40 g of calcium acetate hydrate and 0-625 g of magnesium oxide all dissolved in 11.
T a p E is now closed; the pressure on the rubber bulb can be released and the level of water in the graduated limb will fall. 7. The conical flask is now taken out of the water bath and shaken so that the acid is spilt over the soil; shaking is continued vigorously for at least 1 min. 8. The flask is now replaced in the bath and the water is agitated by squeezing the bulb. T h e apparatus is now left for 2 or 3 min to equilibrate temperature. 9. By opening tap H and squeezing the bulb, the water in the graduated tube and that in tube G are brought to the same level, thus ensuring that the gas in the graduated tube is at atmospheric pressure.
A Practical Course in Agricultural Chemistry by D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)