The determination of the proportion of uric acid in urine was formerly
rather neglected by physicians. There is now, however, a growing
tendency in a certain class of diseases to attach considerable
importance to its accurate estimation, and, as some little trouble is
involved, pharmacists should be prepared to undertake the work. A
rough way is to concentrate somewhat, acidulate with hydrochloric
acid, and collect and weigh the precipitate thrown down on standing.
There are several objections, however, to this method, and many
attempts have been made to elaborate a more reliable process. One of
the most recent, and which has been pronounced the most practical and
successful, has been devised by Professor Haycraft. Although
apparently rather detailed and elaborate, the determination is easy
and extremely simple.
The following solutions must be prepared: 1. Dissolve 5 grammes of
nitrate of silver in 100 c.c. of distilled water, and add ammonia
until the precipitate first formed redissolves. 2. Dilute strong
nitric acid with about two volumes of distilled water; boil, to
destroy the lower oxides of nitrogen, and preserve in the dark. 3.
Dissolve about 8 grammes of ammonic thiocyanate (sulphocyanide)
crystals in a liter of water, and adjust to decinormal argentic
nitrate solution, by diluting till one volume is exactly equal to a
volume of the latter. Dilute the solution thus prepared with nine
volumes of distilled water, and label "Centinormal ammonic-thiocyanate
solution." 4. A saturated solution of ferric alum. 5. Strong solution
of ammonia (sp. gr. 0.880). The uric acid estimation is conducted as
follows: Place 25 per cent. of urine in a beaker with 1 gramme of
sodic bicarbonate. Add 2 or 3 c.c. of strong ammonia, and then 1 or 2
c.c. of the ammoniated silver solution. If, on allowing the
precipitate caused by the latter reagent to subside, a further
precipitate is produced by the addition of more solution, the urine
contains an iodide, and silver solution must be added till there is an
excess. The gelatinous urate must now be collected, the following
special procedure being necessary: Prepare an asbestos filter by
filling a 4 oz. glass funnel to about one-third with broken glass, and
covering this with a bed of asbestos to about a quarter of an inch
deep. This is best managed by shaking the latter in a flask with water
until the fibers are thoroughly separated, and then pouring the
emulsion so made in separate portions on to the broken glass. On
account of the nature of the precipitate and of the filter, it is
necessary to use a Sprengel pump, in order to suck the liquid through.
The small apparatus sold to students by chemical instrument makers
will answer the purpose admirably. Having collected the precipitate of
silver urate on the prepared filter, wash it repeatedly with distilled
water, until the washings cease to become opalescent with a soluble
chloride. Now dissolve the pure urate by washing it through the filter
with a few cubic centimeters of the special nitric acid. The process
is carried out thus: Add to the liquid in the beaker a few drops of
the ferric-alum solution to act as an indicator, and from a burette
carefully drop in centinormal ammonic thiocyanate until a permanent
red coloration of ferric thiocyanate barely appears. The number of
cubic centimeters used of the thiocyanate solution multiplied by
0.00168 gives the amount of uric acid in the 25 c.c. One milligramme
may be added to compensate for loss, and the whole multiplied by four
gives the percentage of uric acid in the urine. The whole process
depends on the fact that argentic urate fails to dissolve in ammonia,
but is soluble in nitric acid, and is thus easily obtained in the pure
state. By determining the amount of combined silver, the percentage of
uric acid can readily be calculated. The addition of sodic bicarbonate
prevents the otherwise inevitable reduction of the silver salt.