commence our study of mordants by a consideration of the nature of
salts. I have already told you that acids are characterised by what we
term an acid reaction upon certain vegetable and artificial colours,
whilst bases or basic substances in solution, especially alkalis,
restore those colours, or turn them to quite another shade; the acids do
the one thing, and the alkalis and soluble bases do the opposite. The
strongest and most soluble bases are the alkalis--soda, potash, and
ammonia. You all know, probably, that a drop of vitriol allowed to fall
on a black felt hat will stain that hat red if the hat has been dyed
with logwood black; and if you want to restore the black, you can do
this by touching the stain with a drop of strong ammonia. But the use
of a black felt hat as a means of detecting acidity or alkalinity would
not commend itself to an economic mind, and we find a very excellent
reagent for the purpose in extract of litmus or litmus tincture, as well
as in blotting paper stained therewith. The litmus is turned bright red
by acids and blue by alkalis. If the acid is exactly neutralised by,
that is combined with, the alkaline base to form fully neutralised
salts, the litmus paper takes a purple tint. Coloured reagents such as
litmus are termed indicators. A substance called phenolphthalein, a
coal-tar product, is a very delicate indicator; it is more sensitive to
acids than litmus is. Now there are some salts which contain a
preponderance of acid in their composition, _i.e._ in which the acid has
not been fully neutralised by the base; such salts are termed acid
salts. Bicarbonate of soda is one of these acid salts, but so feeble is
carbonic acid in its acid properties and practical evidences, that we
shall see both monocarbonate or "neutral" carbonate of soda and
bicarbonate or "acid" carbonate of soda show evidences of, or, as
chemists say, react with alkalinity towards litmus. However,
phenolphthalein, though reacting alkaline with monocarbonate of soda,
indicates the acidity of the bicarbonate of soda, a thing which, as I
have just said, litmus will not do. We will take two jars containing
solution of monocarbonate of soda, and in the first we will put some
phenolphthalein solution, and in the second, some litmus tincture. The
solution in the first jar turns rose coloured, and in the second, blue,
indicating in each case that the solution is alkaline. If now, however,
carbonic acid be blown into the two solutions, that in the first jar,
containing the phenolphthalein, becomes colourless as soon as the
monocarbonate of soda is converted into bicarbonate, and this
disappearance of the rose colour indicates acidity; the blue solution in
the jar containing litmus, on the other hand, is not altered by blowing
in carbonic acid. Furthermore, if to the colourless solution containing
phenolphthalein, and which is acid towards that reagent, a little
reddened litmus is added, this is still turned blue, and so still
indicates the presence of alkali. We have, therefore, in bicarbonate of
soda a salt which behaves as an acid to phenolphthalein and as an alkali
to litmus. Another extremely sensitive indicator is the coal-tar
dyestuff known as "Congo red"; the colour changes produced by it are
exactly the inverse of those produced in the case of litmus, that is, it
gives a blue colour with acids and a red colour with alkalis.
We have now learned that acids are as the antipodes to alkalis or bases,
and that the two may combine to form products which may be neutral or
may have a preponderance either of acidity or of basicity--in short,
they may yield neutral, acid, or basic salts. I must try to give you a
yet clearer idea of these three classes of salts. Now acids in general
have, as we have seen, what we may call a "chemical appetite," and each
acid in particular has a "specific chemical appetite" for bases, that
is, each acid is capable of combining with a definite quantity of an
individual base. The terms "chemical appetite" and "specific chemical
appetite" are names I have coined for your present benefit, but for
which chemists would use the words "affinity" and "valency"
respectively. Now some acids have a moderate specific appetite, whilst
others possess a large one, and the same may be said of bases, and thus
as an example we may have mono-, di-, and tri-acid salts, or mono-, di-,
and tri-basic salts. In a tri-acid salt a certain voracity of the base
is indicated, and in a tri-basic salt, of the acid. Again, with a base
capable of absorbing and combining with its compound atom or molecule
several compound atoms or molecules of an acid, we have the possibility
of partial saturation, and, perhaps, of several degrees of it, and also
of full saturation, which means combination to the full extent of the
powers of the base in question. Also, with an acid capable of, or
possessing a similar large absorptive faculty for bases, we have
possibilities of the formation of salts of various degrees of basicity,
according to the smaller or larger degree of satisfaction given to the
molecule of such acid by the addition of a base. We will now take as a
simple case that of hydrochloric acid (spirits of salt), which is a
monobasic acid, that is, its molecule is capable of combining with only
one molecule of a monoacid base. Hydrochloric acid may be written, as
its name would indicate, HCl, and an addition even of excess of such a
base as caustic soda (written NaOH) would only yield what is known as
common salt or chloride of sodium (NaCl), in which the metal sodium (Na)
has replaced the hydrogen (H) of the hydrochloric acid. Now chloride of
sodium when dissolved in water will turn litmus neither blue nor red; it
is therefore neutral. Such simple, neutral, monobasic salts are mostly
very stable. By "stable" we mean they possess considerable resistance to
agencies, that, in the case of other salts, effect decompositions of
those salts. Such other salts which are decomposed more or less readily
are termed "unstable," but the terms are of course only comparative.
Now let us consider a di- or bi-basic acid. Such an one is vitriol or
sulphuric acid (H_{2}SO_{4}). The hydrogen atoms are in this case an
index of the basicity of the acid, and accordingly the fully saturated
sodium salt is Na_{2}SO_{4} or neutral, or better normal, sulphate of
soda. In like manner the fully saturated salt of the dibasic acid,
carbonic acid (H_{2}CO_{3}), is Na_{2}CO_{3}, ordinary or normal
carbonate of soda. But we must observe that with these dibasic acids it
is possible, by adding insufficient alkali to completely saturate them,
to obtain salts in which only one hydrogen atom of the acid is replaced
by the metal of the base. Thus sulphuric and carbonic acids yield
NaHSO_{4}, acid sulphate or bisulphate of soda, and NaHCO_{3},
bicarbonate of soda, respectively. An example of a tribasic acid is
phosphoric acid, H_{3}PO_{4}, and here we may have three different
classes of salts of three various degrees of basicity or
base-saturation. We may have the first step of basicity due to
combination with soda, NaH_{2}PO_{4}, or monosodium phosphate, the
second step, Na_{3}HPO_{4}, or disodium phosphate, and the third, and
final step, Na_{3}PO_{4}, or trisodium phosphate. Now let us turn to the
varying degrees of acidity, or rather the proportions of acid radicals
in salts, due to the varying appetites or combining powers of bases.
Sodium only forms simple monoacid salts, as sodium chloride (NaCl),
sodium sulphate (Na_{2}SO_{4}); calcium forms diacid salts, _e.g._
calcium chloride (CaCl_{2}); and aluminium and iron, triacid salts, for
example, aluminium sulphate [Al_{2}(SO_{4})_{3}] and iron (ferric)
sulphate [Fe_{2}(SO_{4})_{3}]. Now in these triacid salts we can remove
some of the acid groups and substitute the elements of water, OH, or
hydroxyl, as it is called, for them. Such salts, then, only partly
saturated with acid, are termed basic salts. Thus we have
Al_{2}(OH)_{2}(SO_{4})_{2}, Al_{2}(OH)_{4}SO_{4}, as well as
Al_{2}(SO_{4})_{3}, and we can get these basic salts by treating the
normal sulphate [Al_{2}(SO_{4})_{3}] with sufficient caustic soda to
remove the necessary quantities of sulphuric acid. Now it is a curious
thing that of these aluminium sulphates the fully saturated one,
Al_{2}(SO_{4})_{3}, is the most stable, for even on long boiling of its
solution in water it suffers no change, but the more basic is the
sulphate the less stable it becomes, and so the more easily it
decomposes on heating or boiling its solution, giving a deposit or
precipitate of a still more basic sulphate, or of hydrated alumina
itself, Al_{2}(OH)_{6}, until we arrive at the salt
Al_{2}(SO_{4})_{2}(OH)_{2}, which is quite unstable on boiling;
Al_{2}(SO_{4})(OH)_{4} would be more unstable still. This behaviour may
be easily shown experimentally. We will dissolve some "cake alum" or
normal sulphate of alumina, Al_{2}(SO_{4})_{3}, in water, and boil some
of the solution. No deposit or precipitate is produced; the salt is
stable. To another portion of the solution we will add some caustic
soda, NaOH, in order to rob the normal sulphate of alumina of some of
its sulphuric acid. This makes the sulphate of alumina basic, and the
more basic, the more caustic soda is added, the sodium (Na) of the
caustic soda combining with the SO_{4} of the sulphate of alumina to
form sulphate of soda (Na_{2}SO_{4}), whilst the hydroxyl (OH) of the
caustic soda takes the position previously occupied by the SO_{4}. But
this increase of basicity also means decrease of stability, for on
boiling the solution, which now contains a basic sulphate of alumina, a
precipitate is formed, a result which also follows if more caustic soda
is added, production of still more basic salts or of hydrated alumina,
Al_{2}(OH)_{6}, taking place in either case.
_Mordanting or Fixing Acid (Phenolic) Colours._--But what has all this
to do with mordanting? is possibly now the inquiry. So much as this,
that only such unstable salts as I have just described, which decompose
and yield precipitates by the action on them of alkalis, heat, the
textile fibres themselves, or other agencies, are suitable to act as
true mordants. Hence, generally, the sources or root substances of the
best and most efficient mordants are the metals of high specific
appetite or valency. I think we have now got a clue to the principle of
mordants and also to the importance of a sound chemical knowledge in
dealing most effectively with them, and I may tell you that the man who
did most to elucidate the theory of mordanting is not a practical man in
the general sense of the term, but a man of the highest scientific
attainments and standing, namely, Professor Liechti, who, with his
colleague Professor Suida, did probably more than any other man to clear
up much that heretofore was cloudy in this region. We have seen that
with aluminium sulphate, basic salts are precipitated, _i.e._ salts with
such a predominance of appetite for acids, or such _quasi_-acids as
phenolic substances, that if such bodies were present they would combine
with the basic parts of those precipitated salts as soon as the latter
were formed, and all would be precipitated together as one complex
compound. Just such peculiar _quasi_-acid, or phenolic substances are
Alizarin, and most of the natural adjective dyestuffs, the colouring
principles of logwood, cochineal, Persian berries, etc. Hence these
substances will be combined and carried down with such precipitated
basic salts. The complex compounds thus produced are coloured substances
known as lakes. For example, if I take a solution containing basic
sulphate of alumina, prepared as I have already described, and add to
some Alizarin, and then heat the mixture, I shall get a red lake of
Alizarin and alumina precipitated. If I had taken sulphate of iron
instead of sulphate of alumina, and proceeded in a similar manner, and
added Alizarin, I should have obtained a dark purple lake. Now if you
imagine these reactions going on in a single fibre of a textile
material, you have grasped the theory and purpose of mordanting. The
textile fabric is drawn through the alumina solution to fill the pores
and tubes of the fabric; it is then passed through a weak alkaline bath
to basify or render basic the aluminium salt in the pores; and then it
is finally carried into the dye-bath and heated there, in order to
precipitate the colour lake in the fibre. The method usually employed to
mordant woollen fabrics consists in boiling them with weak solutions of
the metallic salts used as mordants, often with the addition of acid
salts, cream of tartar, and the like. A partial decomposition of the
metallic salts ensues, and it is induced by several conditions: (1) The
dilution of the liquid; (2) the heating of the solution; (3) the
presence of the fibre, which itself tends to cause the breaking up of
the metallic salts into less soluble basic ones. Thus it is not really
necessary to use basic aluminium sulphate for mordanting wool, since the
latter itself decomposes the normal or neutral sulphate of alumina on
heating, an insoluble basic sulphate being precipitated in the fibres of
the wool. (4) The presence of other added substances, as cream of
tartar, etc. The best alumina mordant is probably the acetate of
alumina ("red liquor"), and the best iron mordant, probably also the
acetate ("iron liquor") (see preceding lecture), because the acetic acid
is so harmless to the fibre, and is easily driven off on steaming, etc.
A further reason is that from the solution of acetate of iron or
alumina, basic acetates are very easily precipitated on heating, and are
thus readily deposited in the fibre.
_Mordanting and Fixing Basic Colours._--Now let us ask ourselves a very
important question. Suppose we have a colour or dyestuff, such as
Magenta, which is of a basic character, and not of an acid or phenolic
character like the colours Alizarin, Hæmatein (logwood), or carminic
acid (cochineal), and we wish to fix this basic dyestuff on the tissue.
Can we then use "red liquor" (acetate of alumina), acetate of iron,
copperas, etc.? The answer is, No; for such a process would be like
trying to combine base with base, instead of base with acid, in order to
form a salt. Combination, and so precipitation, would not take place; no
lake would be formed. We must seek for an acid or acid body to use as
mordant for our basic colour, and an acid or acid body that will form an
insoluble precipitate or colour-lake with the dyestuff. An acid much
used, and very valuable for this purpose, is tannic acid. The tannate of
rosaniline (colour principle of Magenta) is a tolerably insoluble lake,
which can be precipitated by Magenta from a solution of tannate of soda,
the Magenta being capable of displacing the soda. But tannic acid,
alone, does not form very fast lakes with Magenta and the other basic
dyestuffs, and so a means of rendering these lakes more insoluble is
needed. It is found that tannic acid and tartar emetic (a tartrate of
antimony and potash) yield a very insoluble compound, a tannate of
antimony. Perchloride of tin, in a similar manner, yields insoluble
tannate of tin with tannic acid. These insoluble compounds, however,
have sufficient acid-affinity left in the combined tannic acid to unite
also with the basic aniline colours, forming very fast or insoluble
colour lakes. This principle is extensively used in practice to fix
basic aniline colours, especially on cotton. We should first soak the
piece of cotton in a solution of tannic acid, and then pass it into a
solution, say, of tartar emetic, when the tannic acid will be firmly
fixed, as tannate of antimony, on the cotton. We then dip the mordanted
piece of cotton into the colour bath, containing, for instance, Magenta,
and it is dyed a fine red, composed of a tannate of antimony and
Magenta. You now see, no doubt, the necessity of sharply discriminating
between two classes of colouring matters, which we may term _colour
acids_ and _colour bases_ respectively. There are but few acids that act
like tannic acid in fixing basic aniline dyestuffs, but oleic acid and
other fatty acids are of the number. A curious question might now be
asked, namely: "Could the acid colour Alizarin, if fixed on cotton
cloth, combine with a basic aniline colour, _e.g._ Aniline Violet, and
act as a mordant for it, thus fixing it?" The answer is, "Certainly";
and thus an Alizarin Purple would be produced, whilst if Magenta were
used in place of Aniline Violet, an Alizarin Red of a crimson tone would
result.
_Chrome Mordanting of Wool and Fur._--In studying this subject I would
recommend a careful perusal of the chapter on "Mordants" in J.J.
Hummel's book, entitled _The Dyeing of Textile Fabrics_, and pages 337
to 340 of Bowman's work on _The Wool-Fibre_.
In the treatment of wool or fur with bichrome (potassium bichromate) we
start with an acid salt, a bichromate (K_{2}Cr_{2}O_{7}) and a strong
oxidising agent, and we finish with a basic substance, namely, oxide of
chromium, in the fibres of the wool or fur. If we desire to utilise the
whole of the chromic acid in our mordanting liquor, we must add to it
some sulphuric acid to set free the chromic acid from the potassium with
which it is combined. Bichromate of potash with sulphuric acid gives
sulphate of potash and chromic acid. The question of the proper
exhaustion of bichromate baths is an important economic one. Now we must
remember that this chromic acid (CrO_{3}) oxidises our wool or fur, and
must oxidise it before it can of itself act as a mordant by being
reduced in the process to hydrated chromic oxide, Cr_{2}O_{3} + 3
H_{2}O. [2 CrO_{3} (chromic acid) = Cr_{2}O_{3} (chromic oxide) + O_{3}
(oxygen).] It is this hydrated chromic oxide in the fibre that yields
with the Hæmatein of the logwood your logwood black dye. Mr. Jarmain
finds that it is not safe to use more than 3 per cent. (of the weight of
the wool) of bichromate; if 4 per cent. be used, the colour becomes
impaired, whilst if 12 per cent. be employed, the wool cannot be dyed at
all with logwood, the phenomenon known as "over-chroming" being the
result of such excessive treatment. I think there is no doubt, as
Professor Hummel says, that the colouring matter is oxidised and
destroyed in such over-chroming, but I also think that there can be no
doubt that the wool itself is also greatly injured and incapacitated for
taking up colour. Now the use of certain coal-tar black dyes in place of
logwood obviates this use of bichrome, and thus the heavy stress on the
fibre in mordanting with it. It also effects economy in avoiding the use
of bichrome, as well as of copper salts; but even thus, of course, other
problems have to be solved before it can be finally decided which is
best.
LECTURE IX
DYESTUFFS AND COLOURS