Being called upon to make a good many brief and rapid analyses of
urine on "clinic days" of our medical department, I devised the
following modification of Knop's method of estimating urea; and after
using it for a year with perfectly satisfactory results, venture to
describe and recommend it as especially adapted for physicians' use,
by reason of simplicity, cheapness, and accuracy. In perfecting and
testing it I was assisted greatly by J. Torrey, Jr., then working with
me.
[Illustration]
The apparatus consists of the glass tube, A, which is about 8 cm. long
and 2½ cm. in diameter, joined to the tube, B, which is about 25 or 30
cm. in length in its longer arm and 8 or 10 in its shorter, and has a
diameter of about 5 mm. Near the bend is an outlet tube, _c_, provided
with "ball valve" or pinch cock. _d_, _e_, _f_, _g_, are marks upon the
tubes. C is a rubber cork with two holes through which the bent tube,
D, passes. D is of such size and length as to hold about 1 c.c., and
one of its ends may be a trifle longer than the other.
The apparatus is used as follows: Remove the cork and pour in mercury
until it stands at _e_ and _g_, then fill up to the mark, _f_, with
sodium or potassium hypobromite (made by shaking up bromine with a
strong solution of sodium or potassium hydroxide). Next carefully fill
the tube in the cork with the urine, being careful especially not to
run it over or leave air bubbles in it. This can easily be done by
using a small pipette, but if accidentally a little runs over, it
should be wiped off the end of the cork with blotting paper. The cork
is then to be inserted closely into the tube; the urine tube being so
small, the urine will not run out in so doing. The mercury is then
drawn out through _c_ till it stands in B at _d_. Its level in A will
of course not be changed greatly. Now, incline the apparatus till the
surface of the hypobromite touches the urine in the longer part of the
urine tube, and then bring it upright again. The urine will thus be
discharged into the hypobromite, which will of course decompose the
urea, liberating nitrogen, which will cause the mercury to rise in B.
Shake until no further change of level is seen, and mark the level of
mercury in B with a rubber band, then remove the cork, draw out the
liquid with a pipette, dry out the tube above the mercury with scrap
of blotting paper, pour back the mercury drawn out, and repeat the
process to be sure that no error was made.
If now two or three marks have been made upon the tube, B, indicating
the height of the mercury when solutions containing known per cents.
of urea are used, an accurate opinion can be at once formed as to the
condition of the urine as regards urea.
As is well known, normal urine contains about 2.5-3 per cent. of urea,
so that graduations representing 2, 2.5, 3, and 4 per cent. are
usually all that are needed, though of course many more can be easily
made.
The results obtained with this apparatus have been repeatedly compared
with those of more elaborate ones, and no practical difference
observed. Evidently the same apparatus, differently graduated, might
be employed to determine the carbonate present in such a substance as
crude soda ash or other similar mixture. In such a case the weighed
material would be put upon the mercury with water and the small tube
filled with acid.
Bowdoin College Chemical Laboratory.--_F.C. Robinson, in Amer. Chem.
Jour._
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