Art.—LVI.—On the Alkalinity or Acidity of certain Salts and Minerals, as indicated by their Reaction with Test Paper.
[Read before the Wellington Philosophical Society, 28th October, 1871.]
A knowledge of the reaction of various substances with test paper is justly esteemed of considerable importance, since it enables us at once to refer them to one or other of three distinctive groups, each of which has strict regard to the molecular structure of those substances falling within it, as manifested by the chemical combinations they are most prone to form. These groups are, as is well known, the alkaline or basic, the acidic, and the neutral, and properly prepared test paper indicates the one to which any particular substance belongs by suffering certain colourations when brought in contact with it, these changes being the result of chemical ones, by which the combinations previously existing among the colouring matter of such paper are ruptured, and new ones superinduced.
The terms alkalinity and acidity, therefore, have a signification expressive of condition, and their real meaning is only this, that as applied to any substance they indicate a tendency in such to form combinations with acid or alkaline bodies, as the case may be. Neutrality, however, has a relation more to the breaking point (if I may so term it) of these combinations in the litmus paper, than to absolute condition of the substance tested; for it is easy to conceive that a substance may be acid or alkaline, and still, by reason of the feebleness of either these characters, be unable to overturn the combinations referred to, and so manifest either of these reactions.
I only point this out that I may not be misunderstood in the use I make of the term neutrality, and not for the sake of opening a question, as I do not attempt bere to remove the neutral point thus arbitrarily located to a position nearer to the true one, preferring to take up ground less subject to criticism, and of more immediate interest, viz., that set forth in the heading of this paper, to which, after the above necessary explanations, I now address myself.
These reactions being in general so easy to obtain in the case of bodies capable of manifesting them, I may perhaps be deemed hypercritical when, in the course of this statement, it is found that my remarks tend to show that the condition of certain of these bodies, as demonstrated by such tests, has been mis-stated in our popular works on chemistry, and that this has tended inferenti-ally to involve us in further errors relative to the numerous substances chemically allied thereto; at the risk of being thought so, however, I do not hesitate to make the following remarks, and the exact condition of such bodies shall be the principal subject of this paper.
The ores I particularly object to as having their true characters in these respects mis-stated, or inferentially liable to be misapprehended, belong to a class of salts insoluble, or nearly so, in water. They are the carbonates, borates, silicates, phosphates, and arseniates of the alkaline earths (lime, baryta, strontia), also of magnesia, silver, and lead. Theoretically they should be alkaline, from the following considerations.
Taking an equivalent of any of these bases and combining it with one of sulphuric or any of the stronger acids, we have a salt corresponding to those of the alkaline mono-sulphates in being neutral. Thus, under these conditions, the bases referred to are, equivalent for equivalent, equal in degree of alkalinity to that of the alkalies (potash, soda, etc.), and, therefore, the corresponding salts of these two classes throughout should possess one common character in respect to this particular reaction.
Now the alkaline carbonates, borates, and their common phosphates, etc., give a very decided alkaline reaction with litmus paper properly prepared; they are indisputably alkaline, but the corresponding salts of lime, strontia, etc., are, as a rule, accepted either from experimental results direct, or from inferences based on them, as being neutral, although, from the above considerations, they ought to be alkaline.
The importance of ascertaining which of these two assumptions is correct is obvious, for, if these salts are in reality neutral, we learn, and must take cognizance of, a radical difference existing either in the acids or the bases of which these salts are made up, according as the other portion of the salt is possessed of powerful or weak affinities. In such a case lime, for instance, would not retain the same degree of saturating power (quantivalence) through all its combinations with the acids, the degree of this in any case being deter-
mined by the strength of the acid employed; a strong one being thus necessary, as it were, to draw out its highest capabilities in this respect.
Our recently acquired knowledge of the mobility (if I may term it so) of the component molecules of bodies in respect to each other, even in the case of simple elements, and the great tendency many of them manifest to form inter-combinations among themselves, dispose us favourably towards a belief which, by ascribing a variable potentiality of this nature to these bodies, explains away the apparent anomaly I have just referred to as pertaining to them in their present reputed condition, and make it very desirable to have experimental results, by which to enable us to decide between theory and our present belief. Results, therefore, having for their sole purpose this object I now relate, from which it will be seen, I think, that theory is in this case our safer guide.
The ground taken up by these results has been already just broken in upon, as will perhaps be remembered, in a communication to this Society, entitled “The Alkalinity of Carbonate of Lime,”* and while the criticism which it evoked has been already useful in stimulating me to this inquiry, it will be useful again, but in a different manner, by supplying us with a knowledge of the precise conditions deemed necessary, by a well-known chemist, to insure reliable indications when testing substances generally in respect to their behaviour with the test I am employing—litmus paper. Using the precautions recommended in this criticism, I prepared the test (litmus paper) for use by simply washing it in water free from ammonia till it acquired a pale violet colour, in which condition “it is a delicate test for either acids or alkalies.”
Thus prepared, the test when pressed upon them in a moist state indicated the conditions of the following substances to be as stated below:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Carbonate of magnesia (magnesite)||Phosphate of alumina (wavellite)||Quartz|
|" lime (calcareous spar)||Phosphate of zinc cryst. (Zn O)3 + PO||Clay (purest-washed)|
|" strontia||Phosphate of iron, (proto- and sesqui-)||clay slate|
|" baryta||Arsenite of zinc|
|Borate of magnesia (datholite) crystallized|
|Tribasic phosphate of lime cryst. (Ca O)2 + HO + PO|
|Tribasic phosphate of magnesia (Mg O)2 PO|
|Apatite (Ca O)3 + PO|
|Phosphate of silver (A.9 O)3 + PO|
|Silicate of magnesia (olivine)|
|" " (serpentine)|
[Footnote] * See Art. LV.
If the terms acidity and alkalinity have any meaning, or if the test here applied to discover these properties is trustworthy, we cannot refrain from classifying those substances specified in the first and second columns of the foregoing table as alkaline and acidic respectively.
That the test used is trustworthy, in the case of alkalinity at least, and these results consequently so far correct, is in the highest degree probable, from the fact that it has been approved of, and I may say recommended, for this latter object, by one who attempts to demonstrate the condition of neutrality in a case for which, as aforesaid, I have assigned alkalinity.
The correctness of this table being allowed, we may safely and largely add to it by filling in with those salts analagous in chemical composition to the ones stated, or we may at once deduce from it the following general conclusions:—
1. That those salts of the earthy oxides, as also those of the oxides of silver and lead, which contain single equivalents of carbonic, phosphoric, arsenic, or boracic acids, are alkaline.
2. That the common silicates of these oxides are also alkaline.
3. That the salts of the sesqui-oxides and the remaining metallic protoxides are acid when containing one equivalent or more of phosphoric or arsenic acid.
4. That the silicates of the sesqui-oxides are neutral.
The salts of the oxides, therefore, enumerated under the foregoing Nos. 1 and 2 appear to agree, in respect to the characters under investigation, with the corresponding salts of the alkalies; the oxides themselves, as compared to those of the so-called alkalies, thus exhibiting an equal alkalinity through all their combinations, and therefore each oxide is, as far as we can judge, similar in molecular arrangement throughout all such combinations.
In the case of those salts comprised in section 3, it is seen that they compare with the sulphates and chlorides of the same or corresponding bases in being acid; but the degree of this acidity is, we know, dissimilar, and may be inferred from the character of alkaline salts with these acids respectively, which has been already described.
The facts above stated have a great significance in respect to the relative potency of the alkalies as compared with that of the alkaline earths; thus, the perfect equality of the bases magnesia and lime as compared with potash (equivalent for equivalent) in respect to alkalinity (here shown) will, if fully recognised, oblige us to dispute the title which this base now holds—that of being the most powerful of any we are yet conversant with.
In reality lithia has far better claims upon this position, as having the lowest combining number, and being, equivalent for equivalent, undoubtedly equal to potash in basicity, it has, therefore, for similar weights, the greatest saturating power for acids.
Next to lithia is magnesia, then lime, soda, and afterwards potash. In this connection it is proper to remark that lithia is the only base which readily attacks platinum when fused upon it—a pretty good test of strength one would think, and proving, as far as a single result has weight, that the relative position I here assign to this base is correct.
Both potash and soda, however, have certainly an appearance of being far more powerfully alkaline than any of the bases just compared with them, but this is due simply to the fact that they dissolve in water to a larger extent and with far greater speed than these bases, whereby they are enabled to act with greater facility, and effect more, in a given time.
The fact is that in our use of the term alkalinity hitherto we have not expressed absolute potentiality, but rather energy or speed of action, and this speed being dependent (other circumstances being equal) upon the degree of solubility in water of the substance tested, we have thus unconsciously perverted the true meaning of the term (alkalinity), by making it denote a certain degree of solubility—a quality which we do not know is the least related to it.
It only remains to notice that in relation to rocks the terms basicity and acidity have, by the facts above stated, their significance enlarged and their appropriateness rendered still more apparent than before, while the term neutrality is now shown to be predicable of certain of them and to be equally significant.
The character of rock masses, or portions of them, in these respects may be discoved in a very direct and simple manner, by just pounding a portion of them upon litmus paper moistened and properly prepared, when, according to the results and manner of sampling, we know off-hand the true condition of the specimen as a whole, or of any particular portion of it; and, knowing this, we learn at once the general affinities of such rocks or portions of rocks, or to particularise whether they are absorbent of acid silicates or silica, or of basic silicates (as the earthy or alkaline ones), or whether, as in the case of clay or-clay slate, they are negative to both these classes of bodies.
I may add, in conclusion, that the rationale of the new process for the retention of the fertilizing constituents of sewage by means of phosphate of alumina is readily explicable by the fact that this is an acid salt (see table); it is thus enabled to chemically absorb all the more basic organic portions of such sewage, these being generally the most valuable for manure, as they are, I believe, the most noxious to animal life.