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DETERMINATION OF UREA IN URINE.

cold water, with frequent agitation, for twenty-four hours: 100 cubic centimetres of such a solution contain 31.84 grammes of chloride of sodium. 10 cub. centim. of this solution (to 3.184 grms. of chloride of sodium) are poured into a small beaker, and mixed with 3 cub. centim. of a solution of urea, containing about 4 per cent. of urea, and also with 5 c.c. of a cold saturated solution of pure sodic sulphate; the solution of mercuric nitrate is then to be added to this mixture from a burette, with constant stirring, until a distinct precipitate is permanently formed. The number of cub. centim. of the solution poured from the burette indicates the amount of the liquid which corresponds to 3.184 grms. of sodic chloride. The strength of the mercurial solution having been thus ascertained, such a proportion of water must be added to it, that 100 cub. cent. may correspond to 1 gramme of chloride of sodium.

2. Preparation of the Solution of Nitrate of Silver employed for removing the Chlorine.-17436 grms. of fused nitrate of silver are dissolved in water, and diluted until the liquid amounts to 600 cub. centim.; 100 c.c. of this solution correspond to I gramme of chloride of sodium.

3. Preparation of the Solution of Nitrate of Mercury No. 2, for determining the Urea.-A concentrated solution of mercuric nitrate, containing about 2'5 grms. of the salt in 18 cub. centim., is prepared according to the directions already given. In order to graduate this solution, 5 grammes of pure urea are to be dissolved in water, and diluted till the volume of the solution amounts to exactly 250 cub. centim.: 10 c.c. of this liquid are to be poured into a beaker, and the mercurial solution is to be added. from a burette, till a few drops in a watch-glass produce a distinct yellow colour with sodic carbonate. If the solution were of the exact strength desired, it would require 20 cub. centim. of the mercurial solution; but if the latter be prepared of the strength above directed, a somewhat smaller quantity will be requisite, and a quantity of water exactly sufficient to reduce it to this strength must be added to the solution.

It will be found convenient, in executing analyses of urine by this method, to be provided, 1. with a pipette capable of delivering exactly 15 c.c. from a mark upon its stem, for measuring off the diluted urine; 2. with a small burette divided into tenths of a cubic centimetre, and capable of measuring 5 c.c., for the mercurial solution No. 1; and 3. with an ordinary alkalimeter burette, divided to cub. centim., for the mercurial solution No. 2.

Before proceeding to determine the amount of the urea in

urine, it is necessary to remove the phosphoric acid contained in the liquid. This is effected by means of a mixture of two volumes of cold saturated baryta water, and one volume of a cold saturated solution of baric nitrate. A glass cylinder of about 30 c.c. in capacity is filled to overflowing with urine, the excess being removed by causing a glass plate to slide over the mouth of the cylinder; two such cylinderfuls are to be poured into a beaker, and mixed with one cylinderful of the baryta solution. The precipitate thus formed is to be filtered off, and the amount of chloride of sodium in 15 c.c. of the filtrate (= 10 c.c. of urine) is to be determined by faintly acidulating by the addition of nitric acid, and then adding the standard solution of mercury No. 1, till a cloudiness appears: 30 c.c. more of the filtrate (= 20 c.c. of urine) are then to be measured off into a separate vessel, and mixed with a quantity of the standard solution of silver, equal to twice that of the mercurial solution employed in the preceding experiment. The liquid is to be filtered, and a bulk of the filtrate, equal to 15 c.c. + half the volume of silver solution used, is to be employed for the determination of the urea. This quantity, which corresponds to 10 c.c, of urine, is to be poured into a beaker, and the graduated mercurial solution No. 2 added from a burette with frequent stirring, until no further increase of the precipitate is perceptible. In order to ascertain whether a sufficient quantity of the mercurial solution has been added, a few drops of the turbid liquid are to be removed with a pipette into a watch-glass, and 2 or 3 drops of a solution of sodic carbonate allowed to flow from the edge of the glass into the liquid. If, after some minutes, the mixture retain its white colour, a further quantity of the mercurial solution is to be added, until a fresh sample plainly exhibits the yellow colour after the addition of the sodic carbonate.

Picard has successfully applied a modification of this method to the detection of urea in healthy blood, and he has even succeeded in estimating the difference in the quantity of urea contained in the blood of the renal artery, and in that of the renal vein after the blood has undergone the depurating influence of the kidney. (Comptes Rendus, Sept. 8, 1856.)

(1604) Compound Ureas.-A remarkable series of compounds may be obtained from urea by the displacement of a certain number of the atoms of hydrogen which it contains. The formation of these compounds is readily explained upon the hypothesis that urea is the diamide (1346) of carbonic acid: (CO)" would then occupy the place of H, in the double molecule of ammonia; thus

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gen would admit of displacement by an equivalent amount of some organic radicle. Compounds of this kind are readily formed in most cases by the action of cyanic acid upon the base which they represent, just as ordinary urea is formed from ammonia, by acting upon it with cyanic acid. They may also be obtained by decomposing the cyanic ethers with ammonia. The compound ureas combine, like ordinary urea, with acids, and form crystallizable salts. Examples of the formation of these compounds have already been cited in the case of the alcohol radicles (1164); thus we have :

It is obvious that this class of compounds admits of being multiplied and varied as extensively as the allied group of artificial bases. Some of the natural organic bases, such as nicotylia and conylia, when made to act upon the cyanic ethers, also re-act like ammonia, and give rise to bodies belonging to the class of ureas (1388).

(1605) Ureides.-Urea likewise gives rise to the formation of another class of compounds analogous to the amides, forming substances which have been called ureides; that is to say, these bodies may be represented as salts of urea from which the elements of water have been abstracted, or they may be regarded as com

pounds in which one or more of the atoms of hydrogen in urea have been displaced by an equivalent amount of the radicle of an acid :

When, for instance, a mixture of an atom of urea with an atom of one of the oxychlorides of the acids (1266) is heated, decomposition occurs, and a ureide is formed. For example, if urea be heated to 300°, or 311° (155° C.), and then gradually mixed with benzoyl chloride, taking care that the temperature shall not exceed 320° (160° C.), the mass becomes pasty, the odour of benzoyl chloride disappears, and benzureide, or benzoyl urea, is left in the form of a crystalline powder, which is soluble in alcohol, but insoluble in ether:

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Acetyl chloride, butyryl chloride, and valeryl chloride may be made in like manner to yield respectively acetureide [H,,,H,, N ̧¤0]; butyrureide [H,,,H,O,N,Є0]; and valerureide [H, ¤ ̧H ̧Ð ̧Ñ‚¤Ð]. (Zinin.) These bodies, in fact, belong to the class of secondary diamides (1346), and may be regarded as urea in which one of the atoms of hydrogen has been displaced by the acid radicle benzoyl, acetyl, or valeryl.

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Besides these ureides, there are various compounds known which bear a relation to urea similar to that of the amidated acids to ammonia :-for example, allophanic acid, known, however, only in its salts (1165; H¤‚Í ̧Ñ‚Ð ̧), may be regarded as carbureic acid, analogous to carbamic acid: oxaluric acid (1626) may in like manner be viewed as oxalureic acid; whilst parabanic acid (1625) would represent the imide corresponding to oxaluric acid (Gerhardt), as indicated by the following equations:

Oxaluric acid.

Hypothetical oxalate of urea and hydrogen.

H€H NᎾ,ᎾᎾ, = €HNO,HC.Ꮎ - ᎻᎾ ;

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Oxal. urea and hydrogen.

32 Parabanic acid.

(1605) Biuret (ЄH ̧Ñ‚Ð ̧‚μ‚).—This compound has the com

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position due to ammonium dicyanate; but it appears to be really a derivative from cyanuric acid, and to form one of a series, the relations of which may be thus represented (Hofmann) :—

And Finckh has indeed shown that biuret may be converted into a salt of guanidine (1611) by the prolonged action of gaseous hydrochloric acid at a temperature of 338° (170° C.).

Parallel with this series runs one derived from cyanuric ether, which indicates the successive stages by which this ether is converted into ethylia (1165) :—

Biuret is prepared by melting urea for some time in an oil bath at a temperature between 302° and 338° (150° and 170° C.). When the disengagement of ammonia has ceased and the mass has assumed a pasty consistence, it is treated with a small quantity of boiling water, and the solution, after filtration, is mixed with a solution of basic acetate of lead; cyanuric and melanuric acids are thus precipitated; the excess of lead is removed by sulphuretted hydrogen, and the filtered liquid, on evaporation, yields granular crystals of biuret. This compound is very soluble both in water and in alcohol. It may be obtained from its alcoholic solution in long, anhydrous, foliated crystals. It is a remarkably stable substance, since it may be dissolved by concentrated sulphuric or nitric acid without being decomposed. A characteristic reaction of biuret is the formation of an intensely red liquid when a few drops of a solution of a cupric salt, followed by the addition of a slight excess of caustic potash, are added to its aqueous solution. When heated strongly, it is decomposed into ammonia and pure cyanuric acid: