418 
LEEDS CHEMISTS’ ASSOCIATION. 
equal to,—that is, that a certain known quantity of every substance will combine with 
a certain known quantity of every other, capable of such combination. Thus 40 parts of 
S0 3 , neither more nor less, will equally combine with and neutralize 31 parts of soda, or 
28 parts of lime, or 17 parts of ammonia, and so on. Thus, if you know the quantity 
of sulphate of soda you have, you know exactly the quantity both of the S0 3 and the 
NaO that compose it; and also, if you know how much S0 3 you have in a quantity of 
NaOSOg, you also know how much soda and how much sulphate. It would be out of 
place for me to dwell further on this part of the subject, it is doubtless familiar to many 
of you ; a clear understanding of it is, however, of the greatest advantage. Another 
principle of Volumetric Analysis is that there must be some well-defined indication, 
evident to the senses, showing the exact termination of the various operations, thus 
enabling us to use only just so much material or reagent as is necessary to effect our 
object, and so to know exactly the quantity required. Thus, if we are decomposing soda 
ash by means of S0 3 , we must be able to know the exact point when we have added 
enough acid to neutralize the soda. Various chemical reactions are taken advantage of 
with this view, such as the fact that acids turn vegetable blues (e. g. litmus) to red, 
while alkalies, on the other hand, restore this reddened litmus to blue ; also, that iodine 
forms a blue colour with starch, ferrocyanide of potassium gives a blue with persalts of 
iron, ferricyanide a blue with protosalts of the same metal. Again, the commencement 
of formation of a precipitate in a clear liquid, or the cessation of production of a pre¬ 
cipitate on addition of more reagent, are among the indications which, as I have said, are 
necessary. It is this necessity that limits in some degree the applications of volumetric 
analysis; as it is not in all cases that we can obtain this clear and well-defined mark of 
the end of the operation. New suggestions in this direction are however frequently being 
made. Another condition required in volumetric analysis is the possession of various 
solutions of a known chemical value, that is, that so much by measure contains a known 
quantity of the dissolved substance by weight. And again, we require intruments by 
which we can accurately and conveniently know how much of any of these solutions we 
have had to employ to effect our object. We will first describe the standard solutions. 
Strictly speaking, the actual strength of these is of no consequence, so long as it is known . 
In practice, however, it is found that the necessary calculations are, in most cases, very 
much simpler if they are of such strength that one atom or one equivalent of each sub¬ 
stance is contained in the same measure of solution 
Two systems of weights and measures are in use for preparing and using these solu¬ 
tions : the French or decimal system, which has the great advantage that its weights and 
measures bear a simple decimal relation to each other,—the unit of weight, the gramme, 
and the cubic centimetre, which in volumetric analysis may be considered as the unit of 
measure, being identical. In the other system the grain is taken as the unit both of 
weight and measure. I much prefer the decimal system, and always work by it; but 
as the other is perhaps in more general use here, and is the one adopted in the Pharma¬ 
copoeia, I shall take it as the basis of the descriptions and examples that follow. In 
the decimal system, 1000 cubic centimetres of solution are generally made to con¬ 
tain an equivalent of each of the substances in grammes. In the grain system 
1000 grain-measures contain an equivalent of substance expressed in grains. Thus, the 
standard S0 3 solution will contain 40 grains in 1000; the soda solution 31 grains in 1000, 
and so on. It will be seen that the solutions are of the same strength whether prepared 
on one system or the other, both having one equivalent in 1000, and therefore they may 
be used for either method indiscriminately. One great advantage of this uniformity in 
the strength of the solutions is that they are mutually and equally convertible. Thus, 
10 grain-measures (a quantity which it has been proposed by Mr. Sutton, to whose work 
on volumetric analysis I am indebted for much contained in this paper, to call a decern 
and to accept as the unit of measure in this, as the C. C. is in the decimal system) 
of the standard or “normal” S0 3 will exactly equal or exactly neutralize 10 grain- 
measures of the standard soda solution, and vice versa ,, and so through the whole list. 
Sometimes more dilute solutions than these are required ; such are generally made one- 
tenth of the strength of the others, and are called decinormal solutions. 
With regard to the apparatus, we require a means of delivering the solutions in such a 
way as not to waste any, and also of readily and accurately knowing how much we 
have used when the operation is finished. There are several forms of “ burettes,” which 
is the name given to the apparatus for this purpose. The best of them for general 
