Hydrolysis of ammonium citrate is due to the fact that both
ammonium hydroxide and citric acid are weak electrolytes.
Because of this fact it is very difficult to prepare a solution in
which the two electrolytes are present in exactly equivalent
quantities (a "neutral" solution), using an Indicator to determine
this condition. It is well to remember that the citrate Is always
largely hydrolyzed and that it is, therefore, rather a solution of
(at best) equivalent quantities of the two constituents, acid and
base,

Ammonium Citrate Solution.. — Two methods are approved
by the A. 0. A. C. for preparing Ci neutral " ammonium citrate.
In one of them a stated amount of citric acid in solution is
neutralized by ammonium hydroxide, using corallin (rosolic

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279

Volumetric Determination.— Wash - gin of the sample as clim-h-d :i!w»\v

r the gravimetric method. Measure the aliquot of the filtrate, and
neutralize as there directed. Dilute to C>0 cc and precipitate the phosphorus
as directed for the volumetric determination of total phosphorus. Calculate
the per cent of phosphorus pent-oxide of water-soluble compounds.

Citrate-insoluble Phosphorus. — The value of a fertilizer Is
frequently rated upon the degree of solubility or the availability

of its constituents to plants, as already explained. For this

puxpose it is desirable to imitate the solvent action of solutions

found In soils. The use of ammonium citrate solution provides

an approximate distinction between available and non-available

phosphates, although it should be noted that there is still con-

siderable disagreement among agricultural chemis s as to the

true availability of the different compounds of phosphorus.

The solvent action of this solution upon calcium phosphate is

largely due to the presence of free citric acid or of acid citrates,

caused by the hydrolysis of the ammonium citrate (i.e., to the ; I

fact that chemically equivalent quantities of ammonium hydrox-

ide and citric acid in solution yield an acid condition, P% being

less than 7).

(NH4)3C6H507 + H20 -» (]SrH4)2HC6H507 + NH4OH (1)

(NH4)3C6H507 + 2HS0 -> NH4H2C6H507 + 2NH4OH, (2)

(NH4)3C6H507 + 3H20 -> H8C6H507 + 3NH4OH, (3)

2CaHP04 •+ 2H3C6H507->Ca(H2P04)2 + Ca(H2C6H5C>7)2. (4)

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