NOTE ON THE ANALYSIS OF SODA-SULLTJ UK 



DIPS. 



By Bertka:\i Juseptt Hill. 



{Abstract.) 



Several soda-sulpluir concentrates are now on the Soiitlt 

 African market, sold primarily for use as sheep-dip in the 

 eradication of scab. One of these, indeed, has been marketed 

 as a lime-sulphur dip although calcium is practically absent. 

 Since sodium polysulphide is just as valuable as calcium poh - 

 sulphide (if not more so) in the treatment of scab, nothing is to 

 be gained by such confusion of sijeciticati(Mi. 



In the analysis of soda-sulphur dips, the direct iodine titra- 

 tion method, largely used in the United States for lime-sulphur 

 concentrates, is not applicable. In this process the " mono- 

 sulphur efjuivalent," or " l)ase in combination with sul])hur as 

 sulphide," is determined by runnin^^' in iodine luitil the yelKn\ 

 colour of ])olysulphide is completely discharged, and thiosuli^hate 

 then determined by continuing the titration to starch end-point. 

 In dealing with soda-sulphur concentrates a certain amount of 

 sodium carbonate is always presem, and although this is not 

 directly registered to iodine, it has the effect of raising and ob- 

 scuring the reading for "" monosulphur equivalent." If b\ 

 chance any free caustic soda is ])resent. Ijoth iodine readings ma}" 

 be vitiated. In calculating monosul]>hur ecjuivalent against 

 total sulphide sulphur, in order to determine the nature of the 

 polysidphides present., an erroneottsly narrow ratio may then be 

 obtained, and this may lead to tmjust suspicion that the harm- 

 less higher polysulphides are admixed with depilatory mono- 

 sulphide. It is therefore maintained that the older method O'f 

 determining monosulphide equivalent by titration Avith ammonia- 

 cal zinc, and subsequent determination of thiosulphate in the 

 filtrate by iodine titrati<:^n, gives more reliable results. An 

 ordinary acid titration to methyl orange should also be carried 

 oiu in order to obtain a rough indication of the amount of car- 

 bonate present. If acid titration is higher than zinc titration, 

 the difference is usually to be attrilnued to carbonate, although 

 theoretically it might be hydroxide. If the zinc titration is 

 higher than the acid titration, hvdrosulphide is indicated. If 

 the two titrations are the same, it indicates that all the titratablc 

 base is in combination with sulphur, or that carbonate is in 

 equilibrium with hydrosulphide. 



Carbonate, hydroxide, a,nd polysulphide (tetra or penta i 

 can co-exist together in solution without immediate interaction 

 A little carbonate is therefore of no consequence in a freshly- 

 made dip, but it is considered at least possible that on prolonged 

 storage slow reaction with polysul|)hide may take place with pos- 



