fRKA. 



I I! 1C OROUP. 



60k 



hermetically in glass tube, uul heats it to temperature of from 

 250* to 4<!U Kahr. (or several hours ; carbonate of baryta U thus pro- 

 duced, the weight of which indicate* the amount of urea originally 

 present, inaiimuch ai urea in under theae circumstances alwaya split up 

 into ammonia and carbonic acid : this process gives accurate result*, 

 but require* too much chemical skill ana experience to be very gene- 

 rally applicable. Liebig's process for the estimation of urea (and 

 chloride of sodium) in urine in the one now always used. It is a 

 volumetric method, and depends upon the facts already mentioned 

 concerning the combinations of urea with nitrate of peroxide of 

 mercury. 



From the following details it will be found that certain standard 

 solutions have to be made ; these require time and care, but, when once 

 obtained, the actual analytical operation is one requiring a few minute* 

 only and is very easy of execution. Solution of mercuric nitrate 

 occasions a white precipitate in pure solutions of urea, but not in mich 

 impure solutions as urine ; this is because chloride of sodium is present 

 in the urine, and the chlorides of the metals of cither the first or second 

 groups decompose mercuric nitrate, mercuric chloride, which does not 

 precipitate urea, being formed. But on adding mercuric nitrate to urine 

 until the whole of the chloride of sodium is converted into nitrate of 

 soda, further addition will cause a precipitate because nitrate instead of 

 chloride of mercury will then come in contact with the urea. Obviously 

 now if the strength of the mercuric nitrate solution which may be 

 distinguished as No. 1 be known, the amount of chloride of sodium in 

 the urine will be at once indicated. The means by which the No. 1 

 solution is standardised will be given presently. It might be thought 

 that the further addition of the same mercuric nitrate until no more 

 precipitate falls, a point easily ascertained, would afford the means of 

 determining the amount of urea present ; but it is not so, probably 

 because the mercuric chloride formed would keep some of the urea in 

 lolution, a definite compound of the two having, in fact, been pro- 

 duced. The exact amount of chlorine in the urine therefore having 

 lius determined, it must now be got rid of by adding to a fresh 

 portion of urine the requisite quantity of a solution of nitrate of silver 

 of known strength. The chloride of silver having been separated by 

 tiltrition, the filtrate in treated with mercuric nitrate solution (No. 2) 

 until the whole of the urea is precipitated, a point that is ascertained 

 by the yellow colour produced, on putting a drop of the mixture con- 

 taining the precipitate on to a drop of carbonate of soda solution placed 

 on a white plate ; the yellow colour (mercuric oxide) simply showing 

 that mercuric nitrate is then in solution, a state of things that ob- 

 viously cannot occur so long as urea exists in the liquid. 



The only remaining fact to be noticed in connection with this pro- 

 cess, before describing the method of preparation of the standard 

 solutions, is that phosphoric acid and phosphates must be removed 

 from the urine before commencing the analysis ; this is readily accom- 

 plished by adding to any portion of the urine half its bulk of a mixture 

 of two parts of cold saturated baryta water, and one part of cold 

 saturated aqueous solution of nitrate of baryta, and filtering ; remem- 

 bering, of course, that only two-thirds of the filtrate is urine. The 

 nitrate will probably be alkaline from excess of baryta, if so, nitric acid 

 must be added till it is exactly neutral to-test paper, and the determi- 

 nation of chloride of sodium and urea then proceeded with. The 

 Quantity of the nitrate most convenient for manipulation is 225 grains 

 = 150 grains of urine) for determining the chloride of sodium ; and 

 450 grains ( = 300 grains of urine) for determining the urea. The 

 indicated number of grains of nitrate of silver solution having been 

 added to the 450 grains just mentioned, exactly one-half of the filtered 

 product ( = 150 grains of urine), is the quantity recommended to be 

 i. i 'I for estimating the urea by the No. 2 mercuric nitrate solution. 



Solution of mercuric nitrate is obtained on dissolving pure mercury 

 in nitric acid, and boiling until the itolution gives no indication of mer- 

 curmn nitrate on the addition of alkaline chloride. It is then cvupo- 

 rated to drynes* over a water-bath and redissolved in seven or eight 

 time* iU bulk of water. For the No. 1 solution above described, the 

 strong liquid just mentioned is standardised by pure chloride of 

 sodium ; and for No. 2 by pure urea. Pure chloride of sodium rota- 

 tion is procured by digesting excess of pure rock-salt in distilled water 

 for one or two days, the whole being occasionally well agitated: 

 100 water grain measure* of such a solution contain, at all ordinary 

 temperatures, 31-84 grains of chloride of sodium : 15U grain measures 

 of this solution of salt are mixed with 45 grain measures of a solution 

 of pure urea containing about 4 per cent, of tliat substance, and also 

 with 75 grain measure* of a cold saturated solution of pure sulphate of 

 soda ; the solution of mercuric nitrate is then to be added to the mix- 

 ture until a distinct permanent precipitate is formed. The number of 

 grain measures of mercuric nitrate solution added corresponds, of 

 course, to 47 76 grains of chloride of sodium : it must then be so 

 diluted that 100 water grain measures may correspond to one grain of 

 i-hlnride of sodium, the resulting liquid being mercuric nitrate solution 

 No. 1. The No. 2 solution is prepared by adding the strong mercuric 

 nitrate solution to 150 grain measure* of distilled water containing 

 exactly 2 per cent, of pure urea until a yellow colour is produced, with 

 carbonate of soda in the manner already indicated ; the strong solution 

 must then be diluted until 100 water grain measures exactly correspond 

 to one grain of urea. Finally, the nitrate of silver solution, employed 

 for removing the chlorine of the chloride of sodium in urine after the 



amount of the Utter has bean indicated by the No. 1 solution. 

 pared by dissolving 174'38 grains of fused nitrate of silv 

 and diluting till the whole amounts to 6000 grain measures ; H" 

 grain measures of the remitting liquid will correspond to one grain of 

 chloride of Hodimii. 



l KKA I.STIMATION OF. [URKA.] 



IKF.AS [I 



rUKTHANK. [CABBAMIC ACID.] 



! KKTHKA. I lii.AUDtii, HIBKAKKS OP TUE.J 

 BYL \NK. [CAHBAMIC Am.. I 



riUC ACID. [UBIC GROIT.] 



1'KIC OROUP, a cluster of chemical compounds, derivatives, or 

 congeners of uric acid. Next to uric acid itself, the product* of its 

 artificial oxidation claim chief interest, inasmuch as they illustrate 

 some of the change* which oxygen effect* upon the exhausted tissues 

 of the human frame prior to their elimination as urea. Thus uric acid 

 plus moisture and oxygen yields urea and a body termed alloxan. 

 Alloxan plus moisture furnishes urea and mesoxalic acid ; and meaoxalic 

 acid contains no nitrogen, the latter element having thusartn 

 been thrown out from uric acid as urea. It is probable tli. 

 change, or a similar one, occurs naturally in animals; for although uric 

 acid is nearly always present in human urine, its quantity is much 

 modified by diet. It does not occur in the urine, but does in the 

 spleen, of the dog; and Professor Haughton, who has paid considerable 

 attention to the physiology of this body, says that " no uric acid what- 

 ever should occur in the urine of man in perfect health, but all the 

 nitrogen of the urine should pass off in the form of urea, a more 

 highly oxidated product than uric acid." 



The following list includes nearly all the members of the uric group. 

 The consideration of those of them derived from uric acid is supposed 

 to be simplified by assuming that body to contain the radii 

 (uryl) or cyan oxalic acid (C.NjO^CjO^Cy,) ; that is, oxalic anhydride, 

 in which two equivalents of oxygen are replaced by two of cyanogen : 

 but uril has not been isolated, and its usefulness is very slight. 



Uric (Hthic) acid . . . . 2HO, Cj.H.X.O, 



Uroxanicacid 2HO, C 10 H,X 4 O,. 



Alloxan C.H,N,O,, ! aq. 



Dialuric acid HO, C,U,X,O. 



Uramilic acid C|.Hio N !is 



Alloxantin CjHjNjO.+C.HjN.O., S q. 



Allituric acid C,HjN,O 4 



Dilituric acid . . . . . 



Amalic acid (dimethjl-aUoxantin) . C,,(C,H,) 4 N 4 O 14 



AUoxanic acid . . . . 2HO, C,H,X,o, 



Lcucoturic acid C 4 H,N,O 4 ! 



Difluan C,H 4 N,O, ! 



Mycomello acid HO, ' . aq. 



Mrsoxalk acid .... 2HO, C,O, 



I'aiabanic acid 211O, C,N,O 4 



Oxaluric acid HO, C.UjN.O, 



I'ranil (murexan or dialurainidr) . H,N, C,H,N,O, 

 Thionuric acid .... 2HO, C,H,N,O,, 2SO, 

 Murcxid (purpurate of ammonia) . . fi 4 n,,N ll ,O 1 , 



Allantoin C ,H,N 4 O, 



Hidaatoic acid HO, C,H,N,(), 



Allanturic odd . . . . HO, Cj^NjO, 



I.antanuric acid HO, <'1I 4 X,O S 



Xnnthlc (uric) oxide or nrou acid . C 10 H 4 N 4 O 4 



Hypoxantuin < H>H 4 X,O, 



Ouaninc C > )I II I N 1 O, 



ii C,H,NO 4 8, 



.!'</ (iiHO,C >0 H,N 4 4 ). This substance, sometimes called 

 lilhic acitl, was discovered by Scheele ; Vauquelin afterwards found it 

 in the excrements of serpents, Brugnatelli in that of silkworms, and 

 Roubiqtict in cantbarides. 



Uric acid is secreted by carnivorous animals, birds, and by several 

 insects. When in excess, it is deposited from human urine a* a 

 brownish-yellow powder, which is usually a compound of uric acid and 



ammonia. It occurs, in combination with soda ,- a lonia, in those 



gouty concretions commonly cull.. -, and it constitutes the 



principal portion of the calculi deposited in the Innnan bladder. The 

 semi-solid urine of serpents ly composed of urate of 



.in.iiiiiiiia ; and guano, the decomposed excrement of aquatic birds, and 

 which is imported from some islands in the South Sea and extensively 

 lined as a manure, contains a large quantity of urate of ammonia. 



Thin acid is conveniently obtained by dissolving the excrement of 

 HiT|M'iitB the boa constrictor, for example in a solution of potash, 

 and decomposing the clear solution by the addition of hydrochloric 

 .Kid. It may also be obtained from the excrement of pigeons and 

 other birds by the same process. According to Liebig, it is better to 

 employ borax as a solvent than a caustic alkali, it dissolving leas of 

 the animal matter. On the large scale, as a source of umrexia as a 



