CHAPTER XI.

FREE ACID OF THE URINE.

THE nature of the acid which imparts an acid reaction to urine is not exactly known, though it is commonly admitted to be most probably phosphoric acid in the form of an acid phosphate. Now, as it is desirable to know exactly the amount of free acid discharged in a given time, several observers have deemed it useful and convenient to analyse the amount of free acid by finding the equivalent of oxalic acid, and to express it as such.

Mode of determining the amount of Free Acid in the Urine.

The analysis is simply performed by a standard solution of caustic soda, graduated so that a given volume corresponds to a certain amount of oxalic acid. With this a known bulk of urine is exactly neutralized, and from the amount of standard solution used we find, by calculation, the amount of oxalic acid which would be equivalent to the amount of unknown acid actually contained in the urine.

Preparation of the standard solution of Caustic Soda according to Neubauer.

This solution is to be graduated so that every cubic centimètre indicates exactly 10 milligrammes of oxalic acid. For this purpose we require a solution of oxalic acid of known strength, which is shortly prepared by dissolving one gramme of dry oxalic acid in so much water that the solution exactly amounts to 100 c.c. Every 100 c.c. of this solution contain 10 gramme of oxalic acid. This quantity is now measured off, put into a small beaker, and coloured red with several drops of tincture of litmus. After being placed upon a piece of white paper, the dilute solution of caustic soda is cautiously

added until the red colour has been changed into the original litmus blue. Suppose we have used for effecting this 6 c.c. of the solution of caustic soda, then they would correspond to 1 decigramme of oxalic acid. We now add to every 6000 c.c. of the solution of soda 4000 c.c. of water, and thus obtain 10000 c.c. of standard solution, of which 10 c.c. exactly neutralizes 10 milligrammes of oxalic acid. If, after the addition of 100 c.c. of this solution to 100 c.c. of the solution of oxalic acid reddened by litmus, the blue colour appears, the solution is correct and ready for use.

The fluid applied to the urine.-To 50 or 100 c.c. of quite fresh urine the standard solution of soda is added in small portions, sayc.c. at a time, and after every new addition the fluid is tested by the aid of litmus paper, as the yellow colour of the fluid would not allow the tincture of litmus to show the transition from red to blue, and therefore excludes its use in the manner described for the preparation of the test-fluid. The testing with litmus paper is best effected by placing a drop of the mixture upon neutral blue litmus paper. If, after a time, the spot covered by the drop does not become red any longer, the analysis is completed. make sure, we may now test for an excess of alkali, and if the latter be found, a fresh analysis, guided by the experience of the first one, will lead to the exact point of neutrality.

To

Quantity of Free Acid discharged in a given time by healthy individuals.

As the result of numerous analyses Vogel designates 2 to 4 grammes of oxalic acid as the equivalent of the amount of free acid discharged by a healthy man during twenty-four hours. This amounts to 0.10 to 0.20 grammes per hour. The hourly quantities, however, were subject to considerable variations, dependent upon the time of the day, and these variations were quite parallel in four different individuals examined at one and the same time. The maximum amount of acid per hour was discharged during the night, the minimum during the forenoon, and a quantity intermediate between those of night and morning was secreted in the hours of the afternoon. Thus the frequent examination of the urine of one individual yielded 0.19 for the night, 013 for the forenoon, and 0-15 grammes of (oxalic) acid for the afternoon.

The amount of free acid in the urine is diminished by the ingestion into the system of caustic alkali, or of carbonates or substances capable of being transformed into carbonates.

All these substances have already been enumerated in the chapter on the chemical reaction of the urine.

The ingestion into the system of mineral acids increases the amount of free acid in the urine. As some organic acids, when taken in large doses, pass through the organism unchanged, as has been ascertained by Wöhler, and confirmed by Buchheim, they must also increase the amount of free acid in the urine.

A young man, labouring under hæmoptoe, was treated by Vogel with large doses of the sulphuric and hydrochloric acids. The daily amount of free acid in the urine was (the average of six days) 44 grammes, and on one day the quantity rose to 7.5 grammes.

All the causes which we have seen to determine the amount of phosphoric and sulphuric acid in the urine are, of course, active in determining the quantity of free acid.

Quantity of Free Acid in diseases.

In chronic and acute diseases Vogel found the amount of free acid mostly diminished, increased only in exceptional cases. The following are his results:

In males: In a case of pneumonia the free acid rose steadily from 0 to 1:50. Average of eight days, 0.5 grammes. Another case of pneumonia, terminating fatally, showed fluctuations between 0.9 and 30. Average of four days, 19 grammes.

In a patient labouring under gastric fever, the amount of free acid fluctuated between 06 and 16. Average of four days, 11 grammes.

A case of acute rheumatism gave 0·7 and 1·0 grammes for several days.

In a case of chronic bronchial catarrh, the amount of free acid fluctuated during eleven days between 0 and 0.8. Average, 0.5 grammes.

In females: Girl with strumous glands, 16 to 2:4. Average of four days, 20 grammes.

A woman, 30 years of age, suffering from spinal irritation, 0 to 0.8. Average of five days, 0.4 grammes.

A woman, 70 years of age, with ascites, the consequence of disease of the liver, 0 to 3.1. Average of eighteen days, 141 grammes,

KO 47.115 NaO 30.973

There is only a very small quantity of potash present in the urine, by far the greater part of the acids being in combination with soda and the alkaline earths. The value which an accurate knowledge of the quantities of potash and soda would have, is evident from their relations to different parts of the animal economy; the salts of potash almost exclusively predominating in the muscles, and occurring in the blood only in very small quantities, while the salts with soda as their base prevail in the blood, and, it may be well assumed, form no part of the solids of the juices of the flesh.'

Mode of determining the quantity of Potash and Soda in the Urine.

The rationale of this analysis is to remove sulphuric and phosphoric acid and organic matters, to transform the alkalies entirely into chlorides, and then to separate them by making one of them insoluble.

Of a solution of one part of acetate of baryta in 20 parts of water made strongly alkaline by means of ammonia, one volume, say 20 c.c., is mixed with two volumes, 40 c.c., of urine. After the precipitate has begun to settle, it is sepa

'Liebig, Unters. über das Fleisch.,' p. 85.

Rose, Analyt. Chemie. ii, 11. For some particulars see Neubauer, loc. cit. p. 138.

rated from the fluid by filtration. Of the alkaline filtrate, which contains an excess of baryta, 45 c.c., corresponding to 30 c.c. of urine, are evaporated on the water-bath in a platinum dish, dried, and exposed to red heat until the carbon is entirely burned. Water is now added to the ashes until the soluble part is entirely dissolved. After the addition of some ammonia, carbonate of ammonia is added, to precipitate all baryta in the form of carbonate. The fluid is now separated from the insoluble parts and precipitate by filtration, and the filter is washed carefully. The filtered fluids are then acidulated by means of hydrochloric acid, evaporated in a platinum capsule of known weight, exposed to a gentle red heat, and weighed. The alkalies, the weight of which in the form of chlorides has thus been ascertained, are dissolved in a little water; chloride of platinum is then added in great excess, the mixture is evaporated nearly to dryness on the water-bath, and digested for several hours with spirits of wine of 80 per cent. When the supernatant strata of the mixture indicate by a deep-yellow colour that a sufficient amount of chloride of platinum is present, and when after frequent stirring of the mixture all chloride of sodium and platinum is probably dissolved, the solution is filtered from the chloride of potassium and platinum, and the precipitate and filter are well washed with spirits of wine, dried and weighed. The weight, less the weight of the filter, is chloride of potassium and platinum; 100 parts of the latter correspond to 30:51 parts of chloride of potassium. By subtracting the amount of the latter from the amount of the united chlorides, the rest gives the amount of chloride of sodium. The amount of chloride of potassium found, gives, by multiplication with 0-6317, the corresponding amount of potash; and the amount of chloride of sodium found, multiplied by 0.5302, gives the corresponding amount of soda.

Physiological relations of Potash and Soda.

The facts adduced by Professor Liebig are of so remarkable a nature, that to follow them out promises to be a source of progress in practical medicine. The analysis of the urine. will always be the principal means for ascertaining the proportions of these alkalies to each other, and to the other ingredients of the juices of the body. The problem is therefore a physiological one; and the analysis above detailed should not be considered as a mere exercise in the laboratory. Many animals take with their food no phosphate of soda, a