I 



CHEMICAL ANALYSIS 39 



Subtracting the amount of thiosulphate required by the 

 sample from that used by the blank, and multiplying this 

 number of cubic centimetres by o"oooo8, will give the weight of 

 oxygen consumed in the time by the volume of sample used, 

 and this is calculated to parts per 100,000. The standard 

 strength of permanganate solution is 0*395 gramme of the pure 

 crystallized salt per litre: i c.c. = o*oooi gramme of available 

 oxygen. 



The acid used is i part by volume of pure H2SO4 to 3 parts 

 of pure distilled water. Permanganate solution is added till a 

 faint red tint remains for some hours. 



As the oxidation is never quite final, it is important that 

 standard conditions of time and temperature should be 

 observed. Unfortunately, a uniform method of working has 

 not yet been agreed on between all observers ; therefore the 

 exact method used should be stated. The chief modifica- 

 tions are : 



1. Society of Public Analysts* standard, originally proposed 

 for waters. Two equal samples maintained at 80° F., one 

 titrated after fifteen minutes, the other after four hours : 

 250 c.c. liquid, 10 c.c. acid, 10 c.c. permanganate. 



2. Mersey and Irwell Joint Committee, specially for sewages 

 and effluents. Temperature 60° F. Two portions of 70 c.c. 

 of the sample with 10 c.c. of acid and 50 c.c. permanganate 

 are titrated, the one after three minutes, the other after four 

 hours. 



The three minutes' test shows nitrites, ferrous salts, sul- 

 phuretted hydrogen, and putrefying matter decomposing per- 

 manganate at once with acid. The difference between three 

 and fifteen minutes shows matter readily putrefying and rapidly 

 decomposing acid permanganate. The difference between 

 fifteen minutes and four hours gives matter capable of putre- 

 fying, though slow to decompose.^ 



Inasmuch as the important point is to discover how far the 

 sewage or efi^uent is deficient of complete oxidation, I have 

 preferred to obtain as quickly as possible a final figure by 

 taking 50 or 100 c.c. of the sample, making up to 250 c.c. with 

 pure water (a good tap water answers in most cases), adding 

 25 c.c. acid and 25 c.c. permanganate, heating on a water-bath 

 (along with a blank) to 80° C. for two and a half hours, and 

 titrating as described in the above example. 



^ F. ScMddev, Joimi. Soc. Chem. Ind., January and May, 1898. 



