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Volume 9, 1876
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Art. LXXXIII.—On a modification of the Mercuro-iodide test for the detection of Alkaloidal or Albumenous matters.

[Read before the Wellington Philosophical Society, 24th February, 1877.]

While engaged in making an exhaustive research into the causes which prevent the proper action of the mercuro-iodide test when applied to weak solution of certain alkaloids, I found that a number of salts are able to dissolve certain of the mercuro-iodides of these alkaloids, and thus prevent the formation of those precipitates, which are the indications (unto this test) of their presence. Among the salts which have notable effects of this kind are potassic iodide and mercuric chloride, and it was therefore found to be a matter of great importance that in the preparation of the mercuro-iodide of potassium an excess of either be avoided. This is readily accomplished by adding the solution of the alkaline iodide to the bichloride of mercury until a small quantity of the red precipitate which forms is seen to be permanent, even when well shaken about through the liquid.

The test liquid prepared in this manner generally has two or three times the delicacy of that prepared in the usual way, that is by mixing the necessary salts in the supposed ratio of their equivalents, this ratio being in practise not easy to observe.

Prosecuting this subject further I found that several of the alkaloids and two or three of the albuminoids do not respond to this test, even as thus prepared, except they are present in quantity comparatively large, and I then ascertained that by the application of this test in a still more modified form, even such alkaloids and albuminoids can be indicated, though present only in minute quantity. This modification is a very simple one, and consists in just following up the addition of the mercuric iodide to the suspected liquid with one of mercuric chloride in successively minute quantities until the liquid is permanently turbid, when in case either of these substances is present the precipitate which ultimately forms is of a pale yellow colour, instead of a brilliant red, as when alkaloidal or albuminous matters are absent.

A good way to apply the mercuric chloride is to take a little of its weak solution (aqueous) up by a thin glass rod, and dip the rod, with attached

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liquid, into the solution to be tested. Usually the precipitate thus made will at first dissolve. In such cases more of the test liquid is added until the precipitate or turbidity ensuing is permanent.

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The instances to which this modifiation of the mercuro-iodide test has been found to be applicable are aconite, atrophia, colchicum, morphia, and veratria, among the alkaloids; caseine and gelatine among the albuminoids. It is also applicable to solutions of digitalin and urea. Those to which I have ascertained it to be unapplicable are codea, narcotina, nicotina, quinine, strychnia and albumen, this last group forming such insoluble compounds with iodide of mercury, prepared as first indicated, that mercuric chloride is not needed with them; strychnia, for instance, giving a distinct cloudiness with it when present in a solution as but 1/480-000 part of it, quinine affecting this when only in a 1/500-000 part, narcotina as 1/100-007 part; and albumen, hydrous as it exists in eggs, reacting in a similar way when present in only the 1/120-000 part.

I append a few approximate determinations as to the degree of delicacy this test (the mercuric iodide and chloride conjoined) admits of in the case of a few of the substances cited in the first group.

Atrophine 1/30.000 a distinct cloudiness.
Morphia 1/200.000 "
Veratria 1/250.000 "

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For comparison of this test with the best of those hitherto used I may state that morphia and gelatine do not give a decided precipitate with these, except it is present in not less than the 1/5-000 part of the liquid to which the test is applied.

I should state that the liquid suspected to contain an alkaloid should be made neutral, or care should be taken that no free mineral acid is present; this is readily accomplished by adding thereto sufficient acetate of soda.

The exact composition of the various precipitates formed in this way has not yet been ascertained; they of course mainly, if not altogether, consist of the alkaloid or albuminoid used together with iodine and mercury, but whether in any case chlorine forms a part, I do not yet know, but I believe it does not. Various useful and interesting facts in connection with this subject have been ascertained in the course of this investigation, and which I will shortly state.

In the first place, I find that the compounds of the alkaloids generally with iodine and mercury are readily soluble in ether, and to such an extent that they can be removed from their aqueous solution acidified with acetic acid, by this liquid, as in “Stas's process,” and the etherial solution leaves the triple compound in a granular form; this, dissolved in alcohol, and allowed to stand a few hours with thin strips of copper, or agitated a short

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time with mercury, is decomposed, the organic base present being dissolved in a pare or nearly pure state by the alcohol, and the remainder of the compound precipitated in an insoluble state.

The mercuro-iodide of strychnia is, I observe, readily decomposed by cold concentrated sulphuric acid, and the addition of the proper oxidizing agents to this solution develope the chromatic reaction of strychnine.

The mercuro-iodide of morphia, when recently formed, gives the morphia reaction with ferric chloride, that is, a blue colouration, and what is remarkable, this colour is not evanescent as is that which is got by operating with this test upon a solution of morphia, the colour being maintained for several days, but it is destroyed by bi-chloride of mercury; that it is destroyed in this way explains in part the reason why solutions of morphia so weak as to require the aid of mercuric chloride in conjunction with mercuric iodide of potassium give a precipitate which does not colour blue in presence of ferric chloride.

The mercuro-iodide of digitalin, which I believe I have produced, was especially sought for as an insoluble substance, because tannic acid is known to form an insoluble compound with this principle; this mercuro-salt has a great tendency to pass through filter paper; it is easily and completely soluble in alcohol, also in ether and iodide of potassium, and manifests a distinct pink colouration when placed in contact with cold strong sulphuric acid.

Adverting to particulars in the case of the behaviour of potassic iodide with the alkaloids, I have to state that the strychnia and narcotina compounds with mercury and iodine are but feebly soluble in it. I should state that albuminous compounds of this nature are readily distinguished from alka-loidal ones by their insolubility in alcohol or ether, only the mercury and iodine dissolving, a viscid mass being left, as the albumen, etc.

I have now to relate a few facts which, as being connected with the subject in hand, and arrived at collaterally with its prosecution, I think I am at liberty to do.

The remarkable aptitude to form insoluble compounds with iodides of the alkaloids which mercuric iodide displayed to me during my pursuit of the subject of this paper, led me to seek substitutes for these organic iodides among our metallic ones, and this under the hope that the exact position of the alkaloids themselves, in regard to the metallic bases, might, in the case of success, be easier to define than it is now. In this I believe I have been successful; at any rate, I find that the iodides of silver, thallium, and lead, combine with iodide of mercury. The thallium and lead compounds of this kind do not manifest any great or notable difference in colour from that of their simple iodides. A careful comparison however, of the salts side by

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side, shows the more complex ones are of a distinct yellower shade of colour than the others.

In the case of silver iodide, however, the change which ensues when mercuric iodide combines with it is so striking, that it seems very remarkable this combination has not before been proved to be possible. The iodide of silver is, as you are aware, of a very pale yellow colour, and the effect of a solution of mercuric iodide upon it is to heighten its colour instantly to a very deep good yellow. As in the case of the detection of alkaloidal matter, however, by the mercuric iodide test, it is necessary to avoid excess of either potassic iodide or mercuric chloride in this reagent when we wish to prepare these double metallic iodides, a circumstance which may account for the fact that they have not hitherto been formed.