the Ferrocyanide of Potassium in Chemical Analysis. 215° 
to obtain the substance to be used for this purpose in a suf- 
ficiently pure and dry state, and secondly, to form a solution 
of it the exact strength of which may be known: for though 
it may appear avery simple operation to dissolve a known weight 
of a certain substance in a given bulk of water or other solvent, 
yet, when this has to be done with such great precision as is 
necessary in these cases, it is a tedious and troublesome opera- 
tion, and any inaccuracy in the graduation ofthe standard solu- 
tion will render all determinations made with it more or less 
inaccurate. It is obvious, therefore, that it would be most de- 
sirable that the substances which are intended to be used as re- 
agents in volumetric analysis should be easily obtained in a pure 
state, and that where considerable time and trouble have been 
expended in graduating solutions of those substances, they should 
not be liable to undergo changes whereby their strength would 
be more or less altered, but that when standard solutions have 
once been made, they might be kept and used for a great num- 
ber of determinations. 
The ferrocyanide of potassium fulfils both those conditions ; 
for it is in general met with in commerce almost chemically 
pure, and in a state in which it can at once be employed asa 
volumetric reagent ; andif at any time it should happen to occur 
not quite so pure, it can readily be purified by recrystallization ; 
and in addition to these important considerations, its solution is 
not prone to change, especially if it be not left exposed to the 
action of the light. In this latter respect it has a decided 
advantage over several of our most useful volumetric reagents, 
viz. the permanganate of potash, the protosalts of iron, sul- 
phurous acid, &c., which, from their bemg so prone to undergo 
spontaneous decomposition, must be either freshly prepared, 
or the strength of their solutions accurately ascertained every 
time they are used, if a day or so has elapsed between each de- 
termination. 
The employment of the ferrocyanide of potassium as a volu- 
metric reagent depends on the following circumstances : viz., that 
it is readily converted into the ferrideyanide of potassium (red 
prussiate of potash) under different circumstances, and that the 
point where the whole of the former salt has been changed into 
the latter may easily be known, either by the use of a diluted 
solution of a persalt of iron (which gives with a drop of the mix- 
ture a blue or green coloration as long as any of the ferrocyanide 
remains unchanged) or by some other simple indication. Thus, 
for example, when chlorine is brought in contact with the ferro- 
cyanide of potassium, this change, as is well known, takes place, 
which is expressed by the following symbols :— 
2 (K2, Fe Cy’) + Cl=(K? Fe? Cy®) +K Cl. 
