Determination of Cellulose in Drinking- Water, 465 



acid is usually added to the water, and some length of time 

 allowed to elapse before making the final readings. 



The modifications whereby we have completely altered the 

 character of the Forchhammer process are as follows : — 



Instead of simply mixing the standard solution of perman- 

 ganate with the water to be examined, we distil a given vo- 

 lume of the water (say, 1 litre) with a considerable excess of 

 standard solution of permanganate, and thereby get more oxi- 

 dizing action than in the ordinary operation. We find ad- 

 vantage in having the liquid strongly alkaline during the 

 distillation ; but we render acid before titrating the residue. 

 The following are the working details : — 

 A standard solution of permanganate of potash (strength 

 0*4 milligramme of active oxygen per 1 cubic centim.)is pre- 

 pared. 



A standard reducing '-solution, containing protosulphate of 

 iron, and of such a strength that each cubic centim. exactly 

 corresponds to the permanganate solution, is also prepared. 

 Solution of caustic potash, 5 per cent., is prepared. 

 Diluted sulphuric acid (1 volume of oil of vitriol with 9 

 volumes of w T ater) is prepared. 



These solutions having been got into order, the operator may 

 begin. 



A litre of the w r ater to be examined is placed in a retort 

 which is conveniently mounted, just as in the ordinary course 

 of water-analysis by the ammonia process. Into the retort 

 the operator drops with a graduated pipette 5 cubic centims. 

 of the solution of potash (to render the water alkaline), and 

 then 5 cubic centims. of very carefully measured standard 

 solution of permanganate ; and then the contents of the retort 

 are boiled by means of a large Bunsen burner, and the water 

 is rapidly distilled off. After the distillation has advanced, it 

 will very often happen that the permanganate will show T signs 

 of having been used up : if that happens, a second, and, if 

 necessary, a third 5 cubic centims. of standard permanganate 

 must be dropped into the retort, and the distillation continued 

 until 800 or 900 cubic centims. have distilled over, and only 

 some 200 or 100 cubic centims. of liquid remain behind in 

 the retort. When the distillation is stopped, the observation 

 that the contents of the retort still contain unacted-upon per- 

 manganate must hold good ; otherwise too little permanganate 

 would have been employed in the operation. 



The manner of finishing the operation is of a very obvious 



description. The operator acidities the residue in the retort 



(i. e. he adds 10 cubic centims. of the diluted sulphuric acid) ; 



then he adds a measured quantity of iron-solution, taking care 



Phil. Mag. $. 5. Vol. 5. No. 33. June 1878. 2 H 



