356 STUDIES ON FERMENTATION. 



been added to absorb tbe oxygen in solution, and the volume 

 of the reagent required is in proportion to the quantity of 

 oxygen in solution in the water. To render the process 

 sensitive, we must dilute the hydrosulphite to such an extent 

 that 10 c.c, for example, may correspond very nearly with 

 1 c.c. of oxygen. If the reagent would keep we should only 

 have to determine directly, once for all, the volume of oxygen 

 that a known volume of the liquid could absorb ; but, in con- 

 sequence of its extreme liability to change through contact 

 with air, it is necessary to titrate the liquid every time before 

 using it. This is easily done in the following manner : — 



Accordino; to the observations of Messrs. Schiitzenberorer and 

 Lalande, the hydrosulphite decolourizes an ammoniacal solution 

 of sulphate of copper, reducing the copper to a lower state of 

 oxidation ; the sulphite and bisulphite having no action as long 

 as there is an excess of ammonia. We prepare a strongly 

 ammoniacal solution of sulphate of copper, containing such a 

 quantity of copper that 10 c.c. of the liquid will correspond, as 

 far as action on the hydrosulphite is concerned, with 1 c.c. of 

 oxygen. Calculation by equivalents gives us the correct value 

 verified by direct experiment* 



The object of the modification which M. Raulin has intro- 

 duced, is to avoid the loss of time thus occasioned by the changes 

 which take place in the titrated liquids by long keeping, 

 as well as certain errors which may arise from the acidity of 

 the wort. On this latter point M. Schlitzenberger has remarked 

 that the quantities of hydrosulphite of soda corresponding with 

 one and the same volume of oxygen vary with the acidity of the 

 liquid operated upon, a phenomenon which that skilful chemist 

 explains by the formation of oxygenated water, of varying 

 stability in media of different acidity. 



Instead of determining the strength of the titrated solution 

 of hydrosulphite before each operation, we take the solution as 

 it happens to be, and determine its strength b}^ causing it to 



* SchCtzenberger, Comptes rcndus de VAcademie dea Sciences, vol. 

 Ixxv., p. 880. 



