Glucose, so characteristic of _diabetes mellitus_, is not difficult of
detection or estimation. The facility with which it reduces alkaline
cupric, argentic, bismuthous, ferric, mercuric salts, indigo and
potassic picrate and chromate solutions has been utilized for the
preparation of several ready methods for its determination. Trommer's
test consists in adding enough cupric sulphate to color green, then
excess of alkali, and boiling. Yellow to brick-red cuprous oxide forms
as a heavy precipitate if glucose is present. The organic matter of
the urine prevents the precipitation of cupric hydrate on the addition
of the alkali. This test is delicate and deservedly popular. Fehling's
well-known solution contains sodio-potassic tartrate, which serves the
purpose chiefly of retaining the copper in solution. Unfortunately,
Fehling's original solution has a tendency to become hyper-sensitive
if kept long, a proneness to change that is much increased on
dilution. When so altered, the solution will yield a more or less
copious precipitate of cuprous oxide on merely boiling, and quite
independent of the presence of glucose. This decomposition is obviated
by preserving the copper salt in a separate solution from the tartrate
and alkali, and mixing before use. Schmiedeberg substitutes mannite
and Cresswell glycerin for the Rochelle salt, in order to render the
solution stable. Some prepared by the writer over twelve months ago,
according to the suggestion of the latter physician, has since shown
no signs of decomposition, and is now as good as it was then. For
qualitative purposes the solution may be prepared thus: Dissolve 35
gm. of recrystallized cupric sulphate and 200 c.c. of pure glycerin in
100 c.c. of distilled water. Dissolve separately 80 gm. of caustic
soda in 400 c.c. of water. Mix the solutions and boil for a quarter of
an hour. A small amount of reduction from impurity in the glycerin
takes place. Allow to stand till clear, decant, and dilute to 1,250
c.c. Ten cubic centimeters will then equal roughly 5 centigrammes of
glucose. For exact quantitative determination it is necessary to
standardize the solution with pure anhydrous dextrose.
To a practiced operator the indications yielded by the use of this
test are of great value; but beginners are exceedingly liable to
mistake its various reactions, and to report the urine as saccharine
when normal traces only of sugar are present. The bismuth test of
Bottger, as greatly improved by Nylander, is fairly delicate, and not
so easily misread as Fehling's. A large volume of reagent being used
with a comparatively small quantity of urine, the precipitate of
earthy phosphates does not interfere in the least with the reaction.
On boiling about 3 drachms of Nylander's solution and 20 minims of
urine for a minute or two, the liquid darkens with a trace of sugar,
and becomes opaque and black if the latter is present in quantity. The
reagent is prepared by dissolving 494 grains of caustic soda, 247
grains of Rochelle salt, and 154 grains of subnitrate of bismuth (free
from silver) in 13 fluid oz. of distilled water. It should be decanted
for use from any sediment.
[Illustration: DR. PAVY'S APPARATUS.]
In those cases where the amount of glucose present is required to be
determined, Dr. Pavy's ammonia cupric process distances all compeers
for ease of application and delicacy of end-reaction, combined with
considerable accuracy. His solution differs from that of Fehling in
containing ammonia, which dissolves the cuprous oxide as soon as it is
formed, yielding a colorless solution. It is only necessary,
therefore, to note the moment that the blue color of the liquid is
exactly discharged, in order to tell when all the copper present has
been reduced. Pavy's solution is prepared as follows: Dissolve 356
grains of Rochelle salt and the same weight of caustic potash in
distilled water; dissolve separately 73 grains of recrystallized
cupric sulphate in more water with heat. Add the copper solution to
that first prepared, and when cold add 12 fluid oz. of strong ammonia
(sp. gr. 0.880), and distilled water to 40 fluid oz. The estimation is
thus conducted: Dilute 10 c.c. of the ammoniated cupric
solution--equivalent to 5 milligrammes of glucose--with 20 c.c. of
distilled water, and place in a 6 or 8 oz. flask. Attach this by means
of a cork to the nozzle of an ordinary Mohr's burette, _b_, preferably
fitted with a glass stopcock, and filled previously with the diluted
urine. The small tube, _c_, which traverses the cork is intended to
permit the escape of steam. Now raise the blue liquid in the flask to
active ebullition--not too violent--by the aid of a spirit lamp or
small Bunsen flame. Turn the stopcock in order to allow the urine to
flow into the boiling solution at the rate of about 100 drops per
minute (not more or much less) until the azure tint is exactly
discharged. Then stop the flow, and note the number of cubic
centimeters used. That amount of dilute urine will contain 5
milligrammes of glucose. To render the determination as accurate as
possible, the urine should be diluted to such an extent that not less
than 4 or more than 7 c.c. are required to decolorize the solution,
and the proportions necessary will be found to vary from 1 part of
urine in 2½ to 1 in 30 or 40. The subsequent calculation is very
simple. If you wish to give the percentage of sugar, multiply 0.005 by
100, and divide the product by the number of cubic centimeters of
dilute urine employed. The figure thus obtained, multiplied by the
extent of dilution--i.e., if there is 1 of urine in 10, multiply by
10--gives the required percentage. The number of grains per fluid
ounce can of course be obtained by multiplying the percentage by
4.375. To observe easily the exact end-reaction a piece of white paper
should be placed behind the flask. If the analyst objects to the
escape of the waste ammoniacal fumes, they may be conducted by a
suitable arrangement into water or dilute acid. In addition to glucose
there are small quantities of other copper-reducing bodies present in
all urine, which always render the reading higher than strict accuracy
would demand. Their aggregate proportion, however, is, comparatively
speaking, so minute that for most medical purposes their presence may
be disregarded. Greater care must be exercised, though, in those
instances where such a deoxidizer as chloral hydrate is accidentally
present. In case of doubt, a little washed and pressed yeast should be
allowed to stand with the urine for a day or two in a warm place.
Alcoholic fermentation with evolution of carbonic acid gas soon sets
in, and the specific gravity of the liquid is lowered considerably.
This reaction points conclusively to the presence of sugar.
Based upon Braun's potassic picrate test, Dr. G. Johnson has devised a
colorimetric process for the estimation of sugar. On boiling an
alkaline solution of that salt with glucose, the former is reduced to
deep red-brown picramate, the color of the liquid, of course, varying
in intensity according to the proportion of sugar present. This
solution is diluted till it corresponds in tint with a ferric acetate
standard, and the percentage of sugar is then readily calculated. For
those who prefer this process the convenient apparatus manufactured by
Mr. Cetti, of 36 Brooke street, Holborn, is recommended, who will also
furnish full particulars of the test.