30 
Proceedings of Royal Society of Edinburgh. 
ferric salt either mixed with the mud or in the solution, then it is 
reduced to ferrous salt, with the destruction of the equivalent 
amount of H 2 S and separation of sulphur. If the ferric salt be in 
excess, no sulphuretted hydrogen makes its appearance at all. The 
reaction is very simple — 
because the 2HC1 appears on both sides of the equation, and is in 
fact unnecessary. A trace of free acid is no doubt necessary, and 
it is turned over and over again in the reaction of indefinite 
quantities of FeS on Fe 2 Cl 6 , after the manner of a catalytic action. 
The same reaction takes place if we use ferric sulphate in place 
of ferric chloride. 
It is evident, therefore, that if we have a sample of mud contain- 
ing sulphide, and we mix it thoroughly with a solution of Fe 2 Cl 6 
or Fe 2 (S0 4 ) 3 , we shall have in the ferric salt reduced a measure of 
the decomposable sulphide present. The ferrous salt can be readily 
determined by permanganate of potash or otherwise. It will be seen 
from the above equation that one molecule FeS decomposes one 
molecule Fe 2 Cl 6 with the formation of three molecules FeCl 2 , so 
that the FeS in the mud is one-third of the ferrous salt found. 
In order to make some preliminary experiments, a mixture of 
100 grms. alum and 30 grms. ferrous sulphate were dissolved in 
about f litre of water and precipitated with ammonia and sulphide 
of ammonium. The precipitate was thoroughly washed by repeated 
decantations, the flask being always filled up to the neck, and corked 
and allowed to settle. When it was completely washed the surplus 
water was poured off, and the precipitate, suspended in about \ litre 
of water, was preserved in a well-stoppered reagent bottle. The 
precipitate consists of alumina and sulphide of iron, and may 
therefore be taken as an imitation of a simple form of mud. I 
made some experiments to see with what amount of agreement in 
the results one could titrate a number of different samples of the 
same mud. 
Three flasks were placed side by side, and into each 50 c.c. sus- 
pended FeS mud were measured. The mixture of FeS + A1 2 0 3 was 
and 
by addition 
FeS + 2HC1 =FeCl 2 +H 2 S 
H 2 S + Fe 2 Cl 6 = 2FeCl 2 + 2HC1 + S 
FeS + Fe 2 Cl 6 = 3FeCl 2 + S 
