CHEMISTRY, PHARMACY, AND MATERIA MEDICA

1845 The Lancet  
622 beyond doubt. Dr. Warren had spoken of this accident as though it were not uncommon, but he (Mr. Fergusson) thought it did not often occur. Indeed, the only case which then occurred to his memory, was the one which was under the care of Mr. Colles, but in this instance the subclavian was tied on the right side, between the trachea and scaleni muscles, whilst in Dr. Warren's case the vessel had been secured over the first rib; he thought the pleura was in little risk from the latter
more » ... the latter operation, even though the anterior scalenus might, as it did in this case, require division. The statistics appended to the paper would correct him (Mr. Fergusson) if he were wrong regarding the frequency of wounds of the pleura. The second point he wished to refer to was, the peculiar sound of the heart which was mentioned-a sound which he might safely say was quite unknown to practitioners in this country. NONE of the constituents of urine claim the attention and interest, both of the chemist and physician, in a more eminent degree than urea. The occurrence of this substance in the blood after the extirpation of the kidneys, its presence in the serum, and partial or total absence from the urine of patients labouring under diabetes, cholera, or morbus Brightii, its presence in the liquor amnii, and in hydropical fluids, and the influence which certain articles of food, or certain diseases, exercise upon its proportional quantity, are circumstances which render it highly desirable that we should possess a safe and precise method of detecting the presence of urea, and of determining its quantity. With regard to the detection of very minute quantities of urea, it is well known that the microscope is particularly adapted to detectthis substance by the characteristic form which the recently separated crystals of the nitrate exhibit in the mother liquor. There is one point connected with this, however, to which I should like to call the attention of chemists, since it seems to me of some importance for the detection of urea by means of the microscope. The crystals of nitrate of urea which separate from aqueous or alcoholic solution, are seldom perfectly regular and distinct. Several experiments have proved to me that the most clearly defined crystals are obtained from a solution of nitrate of urea in moderately concentrated pure nitric acid. For this purpose the urea is, in the first place, to be separated from the substance under examination in the usual manner, by means of alcohol and subsequent concentration in the water-bath ; a small portion of it is then dissolved in about three or four drops of tepid nitric acid upon a watch-glass, and subsequently allowed to cool ; the crystals of nitrate of urea formed in this process are extremely well defined, and may be readily distinguished with a microscope of a linear magnifying power of 200. The experimenter should take care to use the nitric acid in sufficient quantity to leave a mother liquor upon the separation of the crystals, since without this the latter will not appear very distinctly. The crystals of nitrate of urea form very thin rhombic plates, or laminae, in which the acute angle is replaced by a small plane ; part of these plates swim isolated in the mother liquor, whilst another portion is found united into layers, after the manner of tiles. Another characteristic property of these crystals is, that they disappear with effervescence upon the addition of fuming nitric acid. Two methods have hitherto been pursued to effect the quantitative determination of urea,-viz., either the urea is separated as nitrate in the proper manner,* and its proportion calculated from the amount of the latter, or it is separated by means of oxalic acid, and the oxalate of urea subsequently decomposed by carbonate of lime; the urea is then finally weighed in its pure state. Both methods have this disadvantage, that nitrate and oxalate of urea are perceptibly soluble, which prevent, on the one hand, their complete precipitation, and, on the other hand, their thorough washing, on which latter account they retain extractive matter in admixture. No other insoluble compound of urea being known, adapted for the quantitative determination of this substance, I tried to apply the products of its decomposition for this purpose. After several experiments made to this effect, with chlorine and with nitrous acid, I found that concentrated sulphuric acid answers best for the decomposition of urea, with an ultimate view to the quantitative determination of the latter. For this purpose a mixture of one part of urea with from three * Zehmann. Berzelius' Tahresbericht, XXIII., p. 631. to four parts of concentrated sulphuric acid is introduced into a flask, and the latter exposed to the heat of a sand-bath, (which must not exceed 572°, to avoid loss of ammonia;) the decomposition of the urea commences at 383°, and the evolution of carbonic acid gas is very lively at 392°. In this process one equivalent of urea assumes the elements of two equivalents of water, and transposing with the latter, is converted into two equivalents of carbonic acid which escape as gas, and two equivalents of ammonia which remain in combination with the sulphuric acid. C2 N2 H4 O2 + 2 S O3 H 0 = 2 (N H3S O 3H O) + 2 C O 2 I decomposed in this manner, accurately-weighed portions of pure urea, dried at 2120, and determined the ammonia subsequently in the form of ammonio-chloride of platinum. The following figures will show how approximately urea may be determined in this manner. I. 0.2612 grammes of urea yielded 1.9323 grammes of ammoniachloride of platinum; this corresponds to 0.2598 grammes of urea. II. 0.3139 grammes of urea yielded 2.317 5 grammes of ammoniachloride of platinum; this corresponds to 0.3116 grammes of urea. III. 0.2761 grammes of urea yielded 2.0400 grammes of ammonio chloride of platinum ; this corresponds to 0.2743 grammes of urea. To ascertain how far the presence of extraneous matters might interfere with the accuracy of the results, I charred accuratelyweighed samples of urea mixed with sugar, with a proportionably large amount of sulphuric acid. I extracted the charred mass subsequently with water, evaporated the solution, and precipitated finally with alcohol and bichloride of platinum. The results thus obtained possessed the same degree of accuracy as those made upon pure urea. To render this method applicable for the determination of urea in urine, there remained now still to ascertain whether the so-called extractive matter of urine yield ammonia or not when charred with sulphuric acid. For this purpose I precipitated 120 grammes of fresh and healthy urina sanguinis with acetate of lead, after having previously separated the uric acid by means of some hydrochloric acid. The precipitate was subsequently washed, decomposed sulphuretted hydrogen, and the gold-coloured acid,. urinous fluid thus produced, evaporated to syrupy consistence, and charred with sulphuric acid, in the manner above described. The charred mass was afterwards extracted with water, the solution evaporated, and finally treated with alcohol and bichloride of platinum. This process gave no indication of the presence of ammonia. Having thus ascertained that the extractive matters which are normally present in urine exercise no adverse influence upon the quantitative determination of urea, by means of sulphuric acid and bichloride of platinum, I proceeded next to determine, by this method, the amount of urea present in divers samples of urine, in order to compare the results, both among each other and with those obtained by one of the old methods. I found, after several experiments, that 7 grammes of urine require about three and a half grammes of concentrated sulphuric acid. If less of the acid be taken, the charred mass will readily dry up, and some loss of ammonia will be incurred in consequence. The mixture of urine and sulphuric acid is kept in a moderate state of ebullition ; much water evaporates upon this, and the fluid turns black. The temperature rises higher and higher, until, at about 392, there ensues evolution of carbonic acid gas, in small bubbles; the cessation of this disengagement of gas indicates that the urea present in the analyzed urine is completely decomposed. The black residue is then thoroughly extracted with water, and the solution filtered ; the clear and urine-yellow filtrate is finally evaporated in the water-bath, and the sulphate of ammonia treated with alcohol and bichloride of platinum. Since urine contains potass salts and ammoniacal salts, which will, of course, likewise precipitate, upon the addition of bichloride of platinum, it is necessary to determine the exact proportion in which these salts are present in the urine under examination. For this purpose a separate weighed portion of the latter is precipitated with bichloride of platinum ; the amount of the precipitate obtained is subtracted from the amount of the ammoniobichloride of platinum produced by the processes of the method above described. Two samples of urine, of seven grammes each, treated according to this method, yielded 0.202 grammes of urea, (2.88 per cent.,) 0.199 grammes of urea, (2.84 per cent.) Fourteen grammes of the same urine were determined according to the old method, by means of nitric acid-they yielded 0.617 grammes of nitrate of urea, (dried at 212°,) which, according to Regnault, corresponds to 0.301 grammes of urea, (2.15 per cent.) The extractive matters of the same urine yielded no ammonia upon the application of the process which I have already described. These experiments prove that the method of determining urea n the form of ammonio-bichloride of platinum yields much more
doi:10.1016/s0140-6736(02)75092-7 fatcat:dsr3cj5txfaydbmdthhfrlfc3m