February 24,1872.] THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS 
683 
Cjmpto fiorr Statafs* 
CHEMICAL NOTES TO THE PHARMACOPOEIA. 
BY WILLIAM A. TILDEN, D.SC. LOND. 
DEMONSTRATOR OF PRACTICAL CHEMISTRY TO THE 
PHARMACEUTICAL SOCIETY. 
Potassii Bromidum. —Bromine is added in suc¬ 
cessive small portions to any convenient quantity of 
solution of potash, until there is a slight excess, as 
indicated by the pale brown colour assumed by the 
liquid. The change which ensues on the solution of 
the bromine consists in the partition of the potassium 
between the bromide and bromate which result, the 
latter consuming half of the oxjrgen. 
5KHO + KHO -j- 3Br 2 = 
5KBr -f KBrO, + 3H 2 0. 
The solution is evaporated to dryness, the residue 
mixed with a little charcoal and thrown into a red- 
hot crucible. The oxygen of the bromate is then 
carried off in the form of carbonic oxide gas. 
5KBr + KBr0 3 + 3C = CKBr + 3CO. 
The resulting bromide is then dissolved out and 
crystallized. 
The action of bromine upon hydrate of potassium 
is exactly parallel to that of chlorine, and also to 
that of iodine. In each case corresponding com¬ 
pounds are formed. 
Chloride . KC1 Chlorate . KC10 3 
Bromide . KBr Bromate . KBr0 3 
Iodide . . KI Iodate . . KI0 3 . 
It may here be observed that the order of relative 
stability is not the same in the two classes of com¬ 
pounds, for bromine will displace iodine from an 
iodide, and chlorine will displace bromine from a 
bromide, so that evidently in the chloride the che¬ 
mical affinities concerned are the most powerful. On 
the other hand, iodine can, under suitable conditions, 
displace bromine from a bromate, or chlorine from a 
chlorate, producing an iodate. This is in fact the 
method usually adopted for making potassic iodate. 
(See B. P. App. II.) Whilst iodic acid, HI0 3 , and 
anhydride, I 2 0 5 , are easily prepared, the bromic and 
chloric acids are bodies of extreme instability, and 
the anhydrides, Br 2 0 5 and C1 2 0 5 , are at present 
unknown. 
Bromide of potassium crystallizes in cubes which 
are indistinguishable from those of iodide of potas¬ 
sium. . 
Of the following tests from the Pharmacopoeia, the 
first indicates that it is a salt of potassium, the 
second that it is a bromide, and the third that it is 
free from iodide. [§ Its aqueous solution gives a 
white crystalline precipitate with tartaric acid; when 
its solution in water is mixed with a little chlorine, 
chloroform agitated with it, on falling to the bottom, 
exhibits a red colour. A solution of the salt mixed 
with mucilage of starch and a drop of an aqueous 
solution of*bromine or chlorine, does not exhibit any 
blue colour.] 
Potassii Iodidum. —Substituting iodine for bro¬ 
mine in the process last described, the details are 
precisely the same. 
Iodide of potassium generally forms opaque cubes, 
the faces of which are regularly excavated, but occa¬ 
sionally it is met with in transparent octahedra. 
[§ It commonly has a feeble alkaline reaction; its 
solution mixed with mucilage of starch gives a blue 
colour on the addition of a minute quantity of chlo¬ 
rine. It gives a crystalline precipitate with tartaric 
acid. The addition of tartaric acid and mucilage of 
starch to its watery solution does not develope a blue 
colour.] The last test requires explanation; the 
object of it is to detect iodate of potassium, which is 
an exceedingly objectionable and even dangerous 
impurity. Neither iodide nor iodate of potassium 
alone give free iodine on the addition of tartaric 
acid, but simply hydriodic and iodic acids respectively. 
KI + H 2 C 4 H 4 0 6 = HI + KHC 4 H 4 0 6 
KI0 3 4- H 2 C 4 H 4 0 6 = HI0 3 + KHC 4 H 4 0 6 . 
But when they are mixed, these two acids being 
generated in presence of each other react with for¬ 
mation of water and iodine. 
SHI + HI0 3 = 3I 2 + 3H 2 0. 
[§ Solution of nitrate of silver added in excess 
forms a yellowisli-white precipitate which, when agi¬ 
tated with ammonia, yields by subsidence a clear 
liquid, in which excess of nitric acid causes no tur¬ 
bidity.] If chloride were present, it would be dis¬ 
solved out of the silver precipitate by the ammonia, 
and the addition of nitric acid to the clear liquid 
would throw it down again. 
There should be but traces of carbonate present 
in good iodide of potassium, and therefore solution 
of lime should be almost unaffected by it. 
Iodate of Potash. — |_§ Appendix II. B. P.] Finely 
powdered iodine and chlorate of potassium are 
boiled together with water very slightly acidulated 
with nitric acid. When the colour of the iodine has 
disappeared, the solution is evaporated to dryness, 
in order to drive off’ the nitric acid, and the residue 
is redissolved in water. 
The reaction is interesting, as illustrating what 
has already been indicated as to the relative sta¬ 
bility of the oxygenated acids of chlorine, bromine 
and iodine. Iodine and potassic chlorate, heated 
together in the absence of nitric acid, do indeed 
give iodate and free chlorine, or rather chloride of 
iodine— 
KC10 3 -f I 2 = KI0 3 + I Cl; 
but the reaction takes place very slowly. The addi¬ 
tion of a little nitric acid hastens the reaction. It 
liberates from the chlorate successive small portions 
of chloric acid, which are in turn converted by 
the iodine into iodic acid, until the whole is so 
changed. Each portion of iodic acid, as it is pro¬ 
duced, decomposes the nitrate of potassium which 
was temporarily formed, and so ultimately, potassic 
iodate, chlorine and nitric acid are the only resul¬ 
tants. 
2 KC10 3 + I 2 -f aHN0 3 
= 2 KI0 3 + CL* -f x HN 0 3 . 
Iodate of potassium is employed as a test for sul¬ 
phurous acid in acetic and other acids. 
2 KI0 3 + 5 HoS0 3 
Iodate of Sulphurous 
Potassium. Acid. 
= Io + 2 K H S 0 4 + 3 H 2 S 0 4 + H 2 0. 
Iodine. Acid Sulphate Sulphuric Acid. Water, 
of Potassium. 
The iodine thus liberated is recognizable by its 
colour or by the usual reaction with starch. 
