18 ACIDIMETRY 



and sulphurous acid, havo no hydrogen in their composition ; that property cannot, 

 therefore, be connoted by the term, unless those substances are no longer to be con- 

 sidered acids. Causticity and fluidity havo long since been excluded from the 

 characteristics of the class by the inclusion of silica and many other substances in it ; 

 and the formation of neutral bodies by combination with alkalis, together with such 

 electro-chemical peculiarities as this is supposed to imply, are now the only diffe- 

 rentia which form the fixed connotation of the word acid as a term of chemical 

 science.' 



The term ALKALI, though it is included by Mr. J. S. Mill in connection with acid 

 in his remarks, does not stand, oven as a scientific term, in the objectional position in 

 which we find acid. Alkali is not, strictly speaking, a common name to which any 

 definite idea is attached. Acid, on the contrary, is a word commonly employed to 

 signify sour. The highest chemical authorities, following Gerhardt, now define 

 ACIDS to be Salts of Hydrogen, or compounds in which the hydrogen may bo readily 

 replaced by a metal so as to form an ordinary salt. 



An acid must now be defined to be a body' which has the power of destroying more 

 or less completely the characteristic properties of alkalis at the same time losing 

 its own distinguishing peculiarities. See ALKALI ; ANHYDRIDES. 



In this Dictionary all the acids named will be found under their respective heads, 

 as ACETIC, NITBIC, SULPHTTBIC Acids, &c. 



ACXDXFZER. Any body whose presence appears to bo necessary for the pro- 

 duction of an acid. 



ACIDI1METER. An instrument for measuring the strength or quantity of real 

 acid contained in a free state in liquids. The construction of that instrument is 

 founded on the principle that the quantity of real acid present in any sample is pro- 

 portional to the quantity of alkali which a given weight of it can neutralise. The 

 instrument, like the alkalimeter (see ALKALIMETER), is made to contain 1,000 grains in 

 weight of pure distilled water, and is divided accurately into 100 divisions, each of 

 which therefore represents 10 grains of pure distilled water; but as the specific 

 gravity of the liquids which it serves to measure may be heavier or lighter than pure 

 water, 100 divisions of such liquids are often called 1,000 grains' measure, irrespec- 

 tively of their weight (specific gravity), and accordingly 10-20 &c. divisions of the 

 acidimeter are spoken of as 100-200 &c. grains' measure ; that is to say, as a quantity 

 or measure which, if filled with pure water, would have weighed that number of grains. 



ACIDIlVTETRTr. Acidimetry is the name of a chemical process of analysis by 

 means of which the strength of acids that is to say, the quantity of pure free acid 

 contained in a liquid can be ascertained or estimated. The principle of the method 

 is based upon Dalton's law of chemical combinations ; or, in other words, upon the fact 

 that, in order to produce a complete reaction, a certain definite weight of reagent is 

 required. 



If, for example, we take 1 equivalent, or 49 parts in weight, of pure oil of vitriol of 

 specific gravity 1'8485, dilute it (of course within limits) with no matter what quantity 

 of water, and add thereto either soda, potash, magnesia, ammonia, or their carbonates, 

 or in fact any other base, until the acid is neutralised that is to say, until blue 

 litmus-paper is no longer, or only very faintly, reddened when moistened with a drop 

 of the acid liquid under examination it will be found that the respective weights of 

 each base required to produce that effect will greatly differ, and that with respect to 

 the bases just mentioned these weights will be as follows : 



Soda (caustic) 1 eqi.iiv. = 31 parts in weight"} Saturate or neutralise 1 

 Potash (caustic) =47 ,, eqv. =49 parts in weight 



Ammonia =17 ( of pure oil of vitriol (sp. 



Carbonate of soda ,, =53 gr. T8485), or 1 equiv. 



Carbonate of potash =69 J of any other acid. 



This being the case, it is evident that if we wish to ascertain by such a method the 

 quantity of sulphuric acid or of any other acid contained in a liquid, it will be necessary, 

 on the one hand, to weigh or measure accurately a given quantity of that liquid to be 

 examined, and, on the other hand, to dissolve in a known volume of water the weight 

 above mentioned of any one of the bases just alluded to, and to pour that solution 

 gradually into that of the acid until neutralisation is obtained ; the number of volumes 

 of the basic solution which will have been required for the purpose will evidently in- 

 dicate the amount in weight of acid which existed in the liquid under examination. 

 Acidimetry is therefore exactly the reverse of alkalimetry, since in principle it depends 

 on the number of volumes of a solution of a base diluted with water to a definite 

 strength, which are required to neutralise a known weight or measure of the different 

 samples of acids. 



