543 
CITRINE OINTMENT OF THE BRITISH PHARMACOPOEIA. 
Oxide of mercury, like other metallic oxides, forms a soap with the fatty acids. 
Berthollet produced such a compound by decomposing corrosive sublimate by means of 
solution of soap; but it was not permanent, in process of time it became slate-coloured. 
A soap may be formed of an analogous constitution by heating finely-levigated peroxide 
of mercury with lard ; but the soap thus produced undergoes the same changes as that 
of Berthollet, first becoming brown, and ultimately slate-coloured. Hence, the ointment 
of red precipitate cannot be kept unchanged in the apothecary’s shop. Citrine ointment 
is similarly acted on by time; if its colour be ever so bright a yellow at first, it soon 
becomes dull, and by degrees tends to the same alterations as those already mentioned. 
These changes are induced by the gradual decomposition of the mercurial nitrate and 
evolution of the oxide. If the decomposition be assisted by heat, the oxide is not only 
evolved but decomposed, and vapour of mercury is freely discharged. 
Citrine ointment is of French origin. It was first introduced at the Hotel Dieu, 
where it was employed for the cure of itch—a disease which raged there ruthlessly at 
the time. The basis of it was lard, with one pound of which an ounce of mercury 
dissolved in nitric acid was incorporated ; it was found very effectual. This is the same 
formula that was introduced into the Dublin Pharmacopoeia of 1807 ; but it was so hard 
that it could not be mixed with other ointments so as to be smooth, without the 
greatest trouble, and melting could not be used without changing its chemical constitu¬ 
tion. With a view of remedying this defect, the Colleges introduced oil, forgetting that 
nitrate of mercury solidifies olive-oil, and renders it even hard. Changes in the ratio of 
nitric acid, in its strength, in the temperature at which the solution of the mercury was 
effected, and at which the solution was presented to the fatty matter, were made, but 
failed in attaining and preserving the qualities required. 
I believe that the following is the theory of these changes. As there are two oxides 
of mercury, so there are two nitrates, which by certain agencies becomes basic. When 
mercury is dissolved in nitric acid in the ratio directed by the Colleges, the solution will 
be found to contain both the protoxide and peroxide, one or other predominating, accord¬ 
ing to the temperature at which the solution was affected. The two salts soon separate; 
the protonitrate crystallizes, the pernitrate remains in solution, retaining, however, a 
little of the former. If either of these nitrates, or its solution, be mixed gradually with 
water, its basic salt will sooner or later be precipitated: sooner, the less free acid it 
contains; later, as it contains much; but in every case the basic salt at length will be 
precipitated. If the protonitrate had been decomposed by a small quantity of water, 
the resulting basic salt will be white ; if by a large quantity, yellow ; if by a large 
quantity of boiling water, it will be bluish dark-grey. These degrees of colour are 
due to the abstraction of successive quantities of acid. If pernitrate be decomposed by- 
cold water, it affords an orange-brown salt; but according to Thenard and others, if 
much boiling water be used, the whole acid is washed away and leaves pure red oxide. 
Much the same series of changes may be observed when, instead of water, the 
mercurial nitrates are exposed to the action of the fatty acids; but owing to the con¬ 
siderable excess of strong nitric acid and the high temperature present, additional 
phenomena are induced. When the nitric acid holding the mercurial salts in solution 
is mixed with the melted fatty matter, the chemical action induced is first to form 
Fourcroy’s pomatum; the uncombined acid is withdrawn by decomposition, and the 
basic mercurial salts, which are now yellow, communicate that colour to the ointment. 
But the decomposition of the basic salts does not stop here; the abstraction of nitric 
acid still slowly continues, until at length the mercurial salts are resolved into the two 
original oxides—slate-grey and orange-red—by the admixture of which colours an 
ointindnt of a brownish hue, lighter or deeper according to age and circumstances, will 
result. Heat will bring about these changes more rapidly; without heat a very long 
time will be required. 
This series of changes took place in a remarkable manner when I repeated the process 
of the British Pharmacopoeia for making citrine ointment. When the hot mercurial 
solution was poured into the hot lard and oil, and well stirred, an effervescence was 
excited which would soon have overflowed but that the vessel was capable (as the Phar¬ 
macopoeia directs) of containing six times the volume of the ointment. During this 
violent effervescence the colour of the liquid ointment became continually darker, until, 
the effervescence having ceased, the ointment was found to be of a dark mahogany 
colour. When perfectly cold, it did not solidify, but remained of the consistence of 
