80 ALKALIMETRY 



case, the operator should determine for himself how many drops are necessary to mako 

 up one division, and take account of them in the assay according to the ratio thus 

 found. In the example given before, and supposing 10 drops to form one alkalimo- 

 trical division, then the per-centage value of the sample of potash under examination 

 would probably bo as follows : 



Number of divisions of acid employed . . 60 -0 

 2 drops acid in excess 0'2 



Real per-centage of potash . . .49-8 



47. When the alkalimeter described in jig. 23 is employed, the tost acid may, at 

 the beginning of the experiment, be poured from the larger opening, E ; but towards 

 the end that is, when the neutralising point is approaching the acid should be 

 carefully poured from the point, D, in single drops, or only two drops at a time, until 

 the saturating point is hit, as we have just said. If the operator wishes to pour only 

 one drop, he should close the larger opening, E, of the bulb with the thumb, and then 

 fill the bulb with the test acid by inclining the alkalimeter ; putting now the alkalimeter 

 in an upright position, and removing the thumb, a certain quantity of acid will be 

 retained in the capillary point, D ; and if the thumb be now pressed somewhat 

 forcibly against the opening, E, the acid contained in the capillary point will be 

 forced out and form one drop, which will then fall into the alkaline solution if it be 

 held over it. If the saturation be complete, the operator, without removing the bulb 

 stopper, may, by applying his lips to the large opening, E, suck the acid engaged in 

 the capillary point back into the alkalimeter. 



48. If there should be in the mind of the operator any doubt as to what is meant 

 by the onion-red colour which the liquor tinged blue with tincture of litmus acquires 

 when slightly supersaturated, he may pour into a glass beaker a quantity of pure water 

 equal to, or even larger than, the alkaline solution operated upon, and tinge it blue 

 with a little tincture of litmus, to about the same degree of intensity as the alkaline 

 liquor under examination. If he now pour into the pure water coloured blue with 

 litmus, one single drop of the test acid, it will acquire at once, by stirring, the onion- 

 red colour alluded to, and which he may now use as a standard of comparison. 



49. Considering the rapidity with which these alkalimetrical operations can be 

 performed, the operator, unless he have acquired sufficient practice, or unless a great 

 degree of accuracy be not required, should repeat the assay two or three times, look- 

 ing upon the first determination only as an approximation, and as a sort of guide as 

 to the quantity of acid which will bo required in the subsequent experiments, whereby 

 he will now be enabled to proceed with increased caution as he approaches the point 

 of saturation ; but, at any rate, if he will not take the little extra trouble of a repeti- 

 tion, he should, before he begins to pour the acid, take a little of the filtered alkaline 

 solution out of the glass beaker, as a corps de reserve, which he adds to the rest after 

 the saturating point has been approximated, and from that moment he may proceed, 

 but with great care, to complete the neutralisation of the whole. 



50. Do not forget that as the test sulphuric acid must always be added in slight 

 excess to obtain a distinct red streak on the litmus-paper, a correction is absolutely 

 necessary ; that is to say, the excess of sulphuric acid employed must be deducted if 

 a strictly accurate result is sought. 



61. If, instead of the special alkalimeter for potash above described, the operator 

 prefers using that prepared of such a strength that 100 divisions of tho alkalimeter 

 (1,000 water-grains' measure) contain exactly one equivalent of each alkali or base, 

 which test sulphuric acid, as we have seen, has a specific gravity of 1'032 (see 

 31-36), he should proceed exactly as indicated in 38 and following; and the 

 alkalimeter being filled with that test acid, of specific gravity 1-032, up to 0, it (the 

 acid) should be poured carefully into the aqueous solution of the alkali tinged blue 

 with litmus, until exact neutralisation is attained, precisely in the same manner as in 

 38 and following. 



62. The neutralising point being hit, let us suppose that tho whole of the contents 

 of the alkalimeter have been employed, that the aqueous solution tinged blue with 

 litmus is not yet saturated, and that, after having refilled the alkalimeter, the 

 4 divisions more (altogether 104 divisions) have been required to neutralise the alkali 

 in the aqueous solution; then, since 100 divisions (1,000 water-grains' measure) of the 

 test acid now employed saturate exactly one equivalent, that is, 47 of potash, the 

 question is now, What quantity of potash will have been saturated by the 104 divi- 

 sions of acid employed ? Tho answer is found, by a simple rule of proportion, tc b, 

 nearly 49. 



100 : 47::104 :a 



