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METHODS FOR THE DETERMINATION OF UREA. 089
Recently Folin and Farmer 1
have suggested a method for the estima-
tion of the total nitrogen in very small quantities of urine, 1 cc. dilute
urine. After hydrolysis with acid the ammonia formed is colorimetrically
determined by means of Nessler’s reagent.
Among the methods suggested f<<r the special estimation of urea,
that of Morner-Sjo ovist, in combination with Folin’s method, is the
one that is generally used.
Principle of M&rner-Sjoqvist’s Method.2
According to this method
the nitrogenous constituents of the urine, with the exception of urea,
ammonia, hippuric acid, creatinine, and traces of allantoin,3
are pre-
cipitated by a mixture of alcohol and ether after the addition of a solu-
tion of barium chloride and barium hydroxide, or in the presence of sugar
with solid barium hydroxide. The urea is determined in the concentrated
filtrate, after driving off the ammonia, by Kjeldahl’s nitrogen estima-
tion. The slight error due to the presence of hippuric acid and creatinine
can be prevented according to Morner by a combination of his method
with Folin’s method.
Principle of Folin’s Method.* On heating urea with hydrochloric
acid and crystalline magnesium chloride, which melts in its water of
crystallization at 112-115° C. and then boils at about 150-155° C, the
urea is completely decomposed, while no appreciable decomposition
of the hippuric acid and creatinine takes place. The ammonia produced
from the urea is distilled off and determined by titration. The amount
of ammonia previously existing in the urine must be specially determined.
Determination of Urea by the Mdrner-Sjoqvist and Folin Method. 5
Five cc. of the urine are treated with 1.5 grams of powdered barium
hydroxide, and when as much of this is dissolved as possible by gently
mixing, it is precipitated by 100 cc. of the alcohol and ether mixture
(I vol. ether). On the following day it is filtered and the precipitate
washed with the alcohol and ether mixture. The alcohol and ether
are distilled off from the filtrate at about 55° C. (not above 60° C). The
remaining liquid is treated with 2 cc. of hydrochloric acid of sp.gr.
1.124 (for 5 cc. urine), and carefully transferred to a flask of 200 cc.
capacity, and evaporated to dryness on the water-bath. Then add 20
grams of crytalline magnesium chloride to the contents of the flask
and 2 cc. of concentrated hydrochloric acid, and boil on a wire gauze
over a small flame for two hours, making use of a proper return cooler.
1
Journ. of biol. Chem., 11.
2
Skand. Arch. f. Physiol., 2, and Morner, ibid., 14, where the recent literature
may also be found.
3
According to Wiechowski, Hofmeister’s Beitriige, 11, the quantity of allantoin
is so great in urine that it must be considered in this method.
4
Zeitschr. f. physiol. Chem., 32, 3<> and 37.
8
See Morner, Skand. Arch. f. Physiol., 14.
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