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QUANTITATIVE AGRICULTURAL ANALYSIS

by a direct titration method. With carbonates that are soluble
in water this is accomplished by dissolving a weighed sample,
adding methyl orange and titrating with standard acid. If the
carbonate is only slightly soluble in water an excess of standard
acid is added. This dissolves the carbonate and the unused
excess of standard acid is then titrated with a standard base.
In this case, if the solution has been boiled to remove carbon
dioxide, phenolphthalein or methyl red may be used as indicator
but the same indicator must have been used when standardizing
the base against the acid.
Soda Ash.—The standard acid that was used in the preceding
exercise is suitable for this determination. The soda ash may
be weighed on a counterpoised glass, if this is done quickly.
Determination of Alkalinity of Soda Ash.—Weigh about 2.5 gm of sample,
dissolve in a small beaker and rinse the solution into a 500-cc volumetric
flask. Dilute to the mark and mix well. By means of a pipette, measure
25 cc of the solution into a 250-cc Erlenmeyer flask or beaker, dilute with
50 cc of water, add a drop of methyl orange and titrate to the color change
with standard acid. Calculate the per cent of sodium carbonate in the
sample. This, of course, is upon the arbitrary assumption that no other
carbonate or basic substance is present. Sometimes the alkalinity is
expressed in terms of sodium oxide, Na20.
Limestone.—Powdered limestone is used for neutralizing the
acidity of soils. If the alkalinity is calculated in terms of calcium
carbonate the result may be greater than 100 per cent in case of
dolomitic limestones, on account of the presence of magnesium
carbonate, a substance of lower equivalent weight than that of
calcium carbonate. Although a figure so obtained would be
fictitious, in one sense, it is after all a practical basis for calculat-
ing the amount of stone required. If a determination of soil
acidity should indicate n pounds of pure calcium carbonate
required per acre, a sample showing by analysis a calculated per
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cent of 105 would be used in the ratio of :™ pounds per acre, no
matter what carbonates were actually present.
Determination of Alkalinity of Limestone.—Prepare a solution of sodium
hydroxide, equivalent to the standard acid already on hand, or use the basic
solution prepared for acidimetry, page 59. This should be made from
material as nearly as possible free from carbonates. Sodium hydroxide