ACIDIMETRY 



19 



The solution containing the known weight of base, and capable therefore of satu- 

 rating a known weight of acid, is called a ' test-liquor ; ' and an aqueous solution of 

 ammonia, of a standard strength, as first proposed by Dr. Ure, affords a most exact 

 and convenient means of effecting the purpose, when gradually poured from a gra- 

 duated dropping-tube or acidimeter into the sample of acid to bo examined. 



The strength of the water of ammonia used for the experiment should be so adjusted 

 that 1,000 grains' measure of it (that is, 100 divisions of the alkalimeter) really con- 

 tain one equivalent (17 grains) of ammonia, and consequently neutralise one equi- 

 valent of any one real acid. The specific gravity of the pure water of ammonia 

 employed as a test for that purpose should be exactly 0'992, and when so adjusted, 

 1,000 grains' measure (100 divisions of the acidimeter) will then neutralise exactly 



40 grains, or one equivalent, of sulphuric acid (dry). 



49 oil of vitriol, sp. gr. 1'8485. 



37'5 

 54 

 60 

 45 

 150 

 51 



hydrochloric acid (gas, dry), 

 nitric acid (dry), 

 crystallised acetic acid, 

 oxalic acid, 

 tartaric acid, 

 acetic acid. 



And so forth with the other acids. 



A standard liquor of ammonia of that strength becomes, therefore, a universal 

 acidimeter, since the number of measures or divisions used to effect the neutralisation 

 of 10 or of 100 grains of any one acid, being multiplied by the atomic weight or 

 equivalent number of the acid under examination, the product, divided by 10 or by 

 100, will indicate the per-centage of real acid contained in the sample ; the propor- 

 tion of free acid being thus determined with precision, even to ^th of a grain, in the 

 course of five minutes, as will be shown presently. 



The most convenient method of preparing the standard liquor of ammonia of that 

 specific gravity is by means of a glass bead, not but that specific-gravity bottles and 

 hydrometers may, of course, be employed ; but Dr. Ure remarks, with reason, that 

 they furnish incomparably more tedious and less delicate means of adjustment. The 

 glass bead, of the gravity which the test-liquor of ammonia should have, floats, of 

 course, in the middle of such a liquor at the temperature of 60 F. ; but if the 

 strength of the liquor becomes attenuated by evaporation, or its temperature increased, 

 the attention of the operator is immediately called to the fact, since the difference of 

 a single degree of heat, or the loss of a single hundredth part of a grain of ammonia 

 per cent., will cause the bead to sink to the bottom a degree of precision which no 

 hydrometer can rival, and which could not otherwise be obtained, except by the 

 troublesome operation of accurate weighing. Whether the solution remains uniform 

 in strength is best ascertained by introducing into the bottle containing the ammonia 

 test-liquor two glass beads, so adjusted that one, being very slightly heavier than the 

 liquid, may remain at the bottom ; whilst the other, being very slightly lighter, 

 reaches the top, and remains just under the surface as long as the liquor is in the 

 normal state ; but when, by the evaporation of some ammonia, the liquor becomes 

 weaker, and consequently its specific gravity greater, the bead at the bottom rises 

 towards the surface, in which case a few drops of strong ammonia should be added 

 to restore the balance. 



An aqueous solution of ammonia, of the above strength and gravity, being pre- 

 pared, the acidimetrical process is in every way similar to that practised in alkali- 

 metry ; that is to say, a known weight, for example, 10 or 100 grains of the sample 

 of acid to be examined, are poured into a sufficiently large glass vessel, and diluted, 

 if need be, with water, and a little tincture of litmus is poured into it, in order to 

 impart a distinct red colour to it; 100 divisions, or 1,000 grains' measure, of the 

 standard ammonia test-liquor above alluded to, are then poured into an alkalimeter 

 (which, in the present case, is used as an acidimeter), and the operator proceeds to 

 pour the ammonia test-liquor from the alkalimeter into the vessel containing the acid 

 under examination, in the same manner, and with the same precautions used in alkali- 

 metry (see ALKATIMETHT), until the change of colour, from red to blue, of the acid 

 liquor in the vessel indicates that the neutralisation is complete and the operation 

 finished. 



Let us suppose that 100 grains in weight of a sample of sulphuric acid, for example, 

 have required 61 divisions (610 water-grains' measure) of the acidimeter for their 

 complete neutralisation, since 100 divisions (that is to say, a whole acidimeter full) of 

 the test-liquor of ammonia is capable of neutralising exactly 49 grains one equiva- 

 lent of oil of vitriol, of specific gravity 1'8485, it is clear that the 61 divisions 

 employed will have neutralised 29'89 of that acid, and, consequently, the sample of 



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