CHAPTER IX.

SULPHURIC ACID AND SULPHATES.

THE chemical characters of sulphuric acid are so well known, that it is scarcely necessary to advert to them here for the purpose of establishing its diagnosis. There is one test which distinguishes sulphuric acid from all other substances that may occur in the urine, namely, that with a common soluble salt of baryta it forms a precipitate, which is not soluble in any acid, and has the good property of being distinctly visible when present in the smallest quantity. But, though we may be able to ascertain the amount of sulphuric acid discharged in the urine, yet we have at present no means of ascertaining the bases with which it is combined. It has been a general practice in analytical chemistry, separately to determine the acids and bases, and afterwards to distribute them according to the principle that the strongest acid takes the strongest base. But since it has been shown that this affinity is not the same under all circumstances, that mixtures and dilutions will materially alter it, we had better discontinue such a mode of arranging the constituting acids and bases according to their ordinary strength of affinity, and express in separate amounts the acids, leaving it undecided with which base, or with what amount of base, they may have been combined.

Formula: SO + HO, being the hydrate of sulphuric acid.

Method of determining the amount of Sulphuric Acid in Urine by Volumetrical Analysis.

This proceeding originated with Gay-Lussac, and has for a length of time only been used for testing the alkalies of commerce for their impurities. It has, however, been adapted to the analysis of the urine by different chemists; the adaptation consisting in giving to the test-fluid a sufficient degree of dilution corresponding to the small amount of sulphuric acid contained in urine.

The proceeding consists in adding to a measured quantity of urine a solution of chloride of barium of known strength, until a precipitate is no longer produced.

Preparation of test-fluid.-One equivalent of the test-fluid is to contain just so much barium as is required to neutralize 10 milligrammes of sulphuric acid. For that purpose we pulverize crystallized chloride of barium (of the formula CIBa+2HO; equivalent Cl=35'4, Ba=68.6, 2Aq=18=122), and expose it to the air until it no longer loses water. Of this powder, 30.5 grammes are dissolved in so much water that the solution will amount to 1 litre. Of this solution 1 c.c. will precipitate 10 milligrammes of anhydrous sulphuric acid.

Neubauer gives a ten times more dilute solution, for the purpose of finishing the analysis where great accuracy is desired. It is made by mixing 100 c.c. of the solution described with 900 c.c. of water. Of this dilute solution, 1 c.c. is required for precipitating 1 milligramme of sulphuric acid.

Application of the test-fluid to urine.-One hundred cubic centimètres of the urine to be examined are measured into a balloon, acidulated with 20 or 30 drops of hydrochloric or nitric acid, and heated to the boiling point. The balloon best suited for that purpose is a Florence oil flask. The graduated solution of baryta is now added to the boiling urine in small portions; and before every fresh addition of solution, the fluid is allowed to rest, for the subsidence to the bottom of the precipitate formed. When the upper strata of the fluid in the flask have become clear, more of the test-fluid may be added, and so on, as long as a fresh addition of testfluid produces a turbidity in the clear upper strata. But when it becomes doubtful whether any precipitate is yet formed

1 Schwarz, 'Die Maassanalysen,' p. 21.

or not, it becomes necessary to filter a small portion of the mixture, and to test it separately with a drop of the test-fluid in a very small test-tube. If a precipitate be still produced, the whole amount filtered must be poured back to the total quantity employed for the analysis, and a new quantity of test-fluid must be added, and so on, until a new filtration shows that there is no more sulphuric acid in solution. We have now to ascertain that no excess of the solution of baryta has been added to the mixture; this is found by adding a drop of a solution of sulphate of potash. If neither solution of chloride of barium, nor of sulphate of potash, produce any precipitate in separate portions of the filtered liquid, the analysis is perfect.

It is, however, very difficult to arrive at this point, because there is no means of preventing the addition of an excess of the solution of baryta. As soon, however, as an excess has been added, a new experiment becomes necessary. In conducting this, we have the advantage of knowing, from the first experiment, that, after the addition of, say 10 c.c., there was yet sulphuric acid uncombined with baryta, and that, after the addition of another cubic centimètre, there was an excess of barium present. The truth therefore lies between 10 and 11 c.c. of test-fluid, and the urine used contains from 100 to 110 milligrammes of sulphuric acid. Such a fact is sufficiently accurate for ordinary medical purposes. If we desire to make the analysis quite correct by a second analysis, we at once add to the 100 c.c. of acidulated boiling urine 10 c.c. of test-fluid, and complete the analysis with the dilute test-fluid in the manner described. For calculation, the quantities of the two test-fluids used are simply added, with the caution of putting the figures for the dilute fluid one decimal further back, and taking it into account as concentrated.

Direct analysis of Sulphuric Acid.

One hundred cubic centimètres, or any measured quantity of filtered urine, are acidulated with some hydrochloric acid, and, by the addition of chloride of barium in excess, the whole amount of sulphuric acid is precipitated. The precipitate is then boiled for some minutes in the acid fluid, filtered, washed, and exposed to a red heat, the filter being incinerated separately on the cover of the platinum crucible. BaO+SO, : SO=116·5 (116.59, Neubauer): 40 (Will.)

To prevent the formation of sulphuret of barium by the reducing influence of any animal matter which may have

been intimately mixed with the precipitate, it is advisable to add a little sulphuric acid to the sulphate in the crucible, after it has got cool from the first heating. It is then again heated for a short time, and, after cooling, may now be weighed.

Quantity of Sulphuric Acid discharged by healthy persons during twenty-four hours.

The results of the best observations are put together in the following table:

From these observations it follows that the average amount of sulphuric acid discharged during twenty-four hours, by healthy young men, fluctuates between 1.5 and 2.5 grammes. Vogel and Gruner have made some direct observations of the quantities of sulphuric acid secreted during every single hour of the day, and have found the average of one hour to be 0.090 grammes.

The average for every single hour of the forenoon they found 0.063 grammes, for one hour of the afternoon 0.108 grammes, for one hour of the night 0.070 grammes. From this Vogel abstracts the general law that the largest amount per hour of sulphuric acid is discharged a few hours after the principal meal of the day. The quantity then begins to decrease, and continues so with every hour up to the principal meal of the next day, after which it again rises.

In some individuals the discharge of sulphuric acid is ef fected more rapidly than in others, in whom the total average

'Gruner, Gottlieb, Die Ausscheidung der Schwefelsäure durch den Harn,' Giessen, 1852.

Clare, Waldemar, 'Experimenta de excretione acidi sulfurici per urinam, Dorpati Livonorum, 1854.

3 Vide Vogel, Semiotic,' p. 261.

for the twenty-four hours is spread more equally over that time. The former may have an hourly average varying between 0.317 grammes and 0·016 grammes, or between 0.165 grammes and so small a quantity that it cannot be determined.

Physiological origin of Sulphuric Acid.

Though the analysis of food has shown that a certain amount of sulphuric acid in the form of sulphates is being taken daily, yet this is not sufficient to account for the whole amount discharged by the urine. This excess of sulphuric acid over the amount ingested as such, is undoubtedly due to the oxydation in the body of the sulphur which, as we know, enters into the constitution of albuminous substances. But as the greater part of our albumen is taken in the form of meat, it is a reasonable supposition that the greater part of the sulphuric acid in the urine of well-living people is due to the oxydation of the sulphur contained in the meat they eat. In accordance with this it has been found that, under a diet consisting principally of meat, the amount of sulphuric acid discharged in the urine may be double or three times the amount of the ordinary average. On the other hand, a purely vegetable diet has been found to make the amount of sulphuric acid sink considerably below the average under ordinary mixed diet. Lehmann, while living on ordinary mixed food, found 7·026 grammes of sulphuric acid in the urine of twenty-four hours. But when, during twelve successive days, he confined himself to animal food exclusively, the sulphuric acid rose to 10.399 grammes per day. Under an exclusively vegetable diet it fell to 5.846 grammes for twenty-four hours. These quantities are very high, and, like the rest of Lehmann's observations upon himself, do not permit any conclusion as to the average quantities of ordinary mortals with a slighter appetite. But in itself the experiment is as conclusive as the following ones. Vogel examined the urine of a person whose ordinary average for twenty-four hours was 2.02 grammes of sulphuric acid. That person took a large supper of meat principally, in consequence of which the discharge of sulphuric acid by the urine rose to 0.50 grammes per hour between midnight and nine o'clock next morning; the ordinary average per hour for that time of the day having been 0.10. During the subsequent twenty-four hours the amount of sulphuric acid rose to 7.3 grammes, being more than three times the ordinary average of 2:02 grammes. It was observed by Vogel that the rise and fall of