Home Science Math • A Practical Course in Agricultural Chemistry by D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)

A Practical Course in Agricultural Chemistry by D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)

By D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)

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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. 3. 10 ml of the filtrate are poured into each of the comparator tubes, one of which is placed in the left-hand compartment of the comparator.

6. One or two drops of hydrogen peroxide are added until the solution turns colourless; this breaks down the excess potassium permanganate. 7. T h e solution is now washed into a 25-ml volumetric flask, made u p to the mark with distilled water, and well shaken. 8. Calcium is determined on the flame photometer with zero set with 0*2 normal hydrochloric acid and 100 set with 50 ppm calcium. 9. The meter reading multiplied by 7 gives the weight in mg CaO per 100 g of soil and the lime requirement is read from Table 5.

A stock solution of 500 ppm is first prepared by dissolving 0-4792 g of potassium dihydrogen orthophosphate in 500 ml of distilled water. 1 ml of this concentrated standard in 200 ml of 0-5 normal acetic acid gives 2*5 ppm. 2. Ammonium molybdate reagent is prepared by dissolving 5 g in 100 ml of distilled water and then adding 15 ml of concentrated sulphuric acid. 3. The reducing reagent is a solution of 0*2 g 1:2:4 amino-naphthol-sulfonic acid, 12 g of sodium metabisulphite and 2-4 g of sodium sulphite, in 100 ml of distilled water.

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