ON THE SOLUBILITY OF METALLIC OXIDES. 
375 
lution of citrate of ammonia is mentioned. I am not aware, however, that any direct 
experiments have hitherto been made with other metallic oxides, or that attention has 
been called to the pharmaceutical importance of the subject. I was first led to examine 
the action of the alkaline citrates on metallic oxides by a paper published in the ‘ Ame¬ 
rican Medical Times,’ by a Mr. Lalor. Mr. Lalor, having reason to suspect the presence 
of lead in a specimen of ammonio-citrate of iron, examined it, and found the metal to the 
enormous proportion of 25 per cent. Seeing that the explanation given in this case, 
namely, that the lead was derived “ from the use of an impure sulphate of iron,” was 
manifestly erroneous, the simple experiment of allowing a solution of ammonio-ferric 
citrate to remain for a few days in contact with some clippings of sheet lead, at once 
supported my own view of the matter, and showed that this dangerous impurity was in 
all probability due to the employment of leaden vessels in the preparation of the citrate. 
This result led to the further experiments, which I shall now, as briefly as possible, de¬ 
tail. I have not attempted to obtain precise quantitative results, but simply such data 
as may be useful to the practical pharmaceutical chemist. I may suggest that it would 
be desirable to ascertain what is the limit of solubility in the cases detailed; how this 
solubility is affected by differences of molecular condition; and whether the alkaline ci¬ 
trates form with the metallic oxides definite and crystalline double salts. 
My own experiments have only been carried so far as to demonstrate the following 
facts:— 
(a.) Solutions of the neutral citrates of ammonia, soda, and potash dissolve r sensible 
quantities of metallic copper and lead and their dry oxides at ordinary temperatures. 
(7>.) The same solutions dissolve a comparatively large quantity of the moist hydrated 
oxides of iron, copper, lead, and bismuth, and solution is in these cases accelerated by heat, 
and its activity is dependent upon the quantity of alkaline citrate present, and upon the 
molecular state of the oxide, e. g. a recently-precipitated oxide is more easily soluble than 
an oxide which has been for some time prepared. 
(c.) The ammonia and soda citrates have greater solvent power in relation to metallic 
oxides than the citrate of potash. 
( d .) Solutions of the oxides of bismuth, lead, copper, and iron in the alkaline citrates 
are unaffected by many of the reagents, which either produce precipitates or marked 
change of colour in solutions of the same oxides in the mineral acids. 
Solutions of the citrates of ammonia, potash, and soda were prepared, containing in 
each 1000 fluid grains 250 grains of citrate respectively. The moist metallic oxides were 
suspended in water in such proportion that 2000 grains of water contained a quantity 
equivalent to 33‘3 grains of anhydrous oxide. To this 1000 fluid grains of the alkaline 
citrate solution were then added, and where solution did not immediately take place, heat 
was applied. The application of heat was found necessary in all cases, except those of 
the oxides of lead and copper in the soda and ammonia citrates. In the potash citrate it 
was found impossible to dissolve the entire of any of the oxides even after continued 
ebullition. With this exception, all the solutions contained in 300 fluid grains 33*3 grains 
of metallic oxide, and in the case of the potash citrate I should think that about one-half 
of each remained undissolved. 
In the tabular statement given below the action of several ordinary reagents upon each 
solution is shown. The addition of a weak acid, as citric or acetic, was always sufficient 
to enable the characteristic reaction of iodide of potassium, or bichromate of potash upon 
lead salts to manifest itself:— 
I .—Peroxide of Iron. 
Citrate of Ammonia. 
Citrate of 
Soda. 
Citrate of Potash. 
Ammonia . 
No change. 
As before 
5J 
5> 
» 
As before. 
5? 
5? 
5? 
Carbonate of ammonia ... 
Potassa . 
Carbonate of potassa . 
Ferrocyanide of potassium 
Phosphate of soda . 
Sulphide of ammonium ... 
Gallic acid . 
Precpt. (Fc,0 3 ) . 
No change. 
Dark-green coloration . 
No change. 
Black precipitate . 
Greenish-brown coloration 
