ALKALIMETRY 79 



40. Or, instead of the pipotte just described, tho operator may measure 1,000 grains 

 by taking an alkalimeter full of the alkaline solution, and emptying it into the glass 

 beaker in which the neutralisation is to take place, rinsing it with a little water, and 

 of course adding the rinsing to the mass in the said glass beaker. 



41. Whichever way is adopted, a slight blue colour should be imparted to the 1,000 

 grains' measure of the alkaline solution, by pouring into it a small quantity of tincture 

 of litmus. The glass beaker should then be placed upon a sheet of white paper, or a 

 slab of white porcelain, in order that the change of colour produced by tho gradual 

 addition of the test acid may be better observed. 



42. This being done, if the operator have decided upon using the test sulphuric acid 

 for potash ( 17-22), he should take one of the alkalimeters represented in figs. 21, 22, 

 23, or 24, and fill it up to (taking the under lino of the liquid as the true level) ; 

 then taking the alkalimeter thus charged in his right hand, and in his left the glass 

 beaker containing the alkaline solution coloured blue by tincture of litmus, ho should 

 gradually and carefully pour the acid liquor into the alkaline solution in the glass 

 beaker, to which a circular motion should be given whilst pouring the acid, or which 

 should be briskly stirred, in order to insure the rapid and thorough mixing of the two 

 liquors, and therefore their complete reaction ; moreover, in order at once to detect any 

 change of colour from blue to rod, the glass beaker should be kept over the white 

 sheet of paper or the white porcelain slab, as before stated. 



43. At first no effervescence is produced, because the carbonic acid expelled, in- 

 stead of escaping, combines with the portion of the alkaline carbonate as yet unde- 

 composed, which it converts into bicarbonate of potash, and accordingly no sensible 

 change of colour is perceived ; but as soon as a little more than half the quantity of 

 the potash present is saturated, the liquor begins to effervesce, and the blue colour of 

 the solution is changed into one of a vinous, that is, of a purple or bluish-red hue, 

 which is due to the action of the carbonic acid upon the blue colour of the litmus. 

 More acid should be still added, but from this moment with very great care and with 

 increased caution, gradually as the point of neutralisation is approached, which is 

 ascertained by drawing the glass rod used for stirring the liquor across a slip of 

 blue litmus-paper. If the paper remains blue, or if a red or reddish streak is thereby 

 produced which disappears on drying the paper and leaves the latter blue, it is a 

 proof that the neutralisation is not- yet complete, and that tho reddish streak was due 

 only to the action of the carbonic acid ; more acid must accordingly be poured from 

 the alkalimeter, but one drop only at a time, stirring after each addition, until at last 

 the liquor assumes a distinct red or pink colour, which happens as soon as it contains 

 an extremely slight excess of acid ; the streaks made now upon the litmus-paper will 

 remain permanently red, even after drying, and this indicates that the reaction is 

 complete and that the assay is finished. 



44. If the potash under examination were perfectly caustic, the solution would 

 suddenly change from blue to pink, because there would be no evolution of carbonic 

 acid at all, and consequently no vinous or purple colour produced ; if, on the other 

 hand, the potash was altogether in the state of bicarbonate, tho first drops of test 

 acid would at once decompose part of it and liberate carbonic acid, and impart a 

 vinous colour to the solution at the very outset, which vinous colour would persist as 

 long as any portion of the bicarbonate would remain undecomposod. 



45. The neutralising point being attained, the operator allows the sides of the alka- 

 limeter to drain, and he then reads off the number of divisions which have been 

 employed. If, for example, 50 divisions have been used, then the potash examined 

 contained 50 per cent, of real potash. See observ., 48-49. 



46. Yet it is advisable to repeat the assay a second time, and to look upon this 

 first determination only as an approximation which enables the operator, now that he 

 knows about where the point of neutralisation lies, to arrive, if need be, by increased 

 caution as he reaches that point, at a much greater degree of precision. He should 

 accordingly take again an alkalimeter full (1,000 water-grains' measure) that is to 

 say, another tenth part of the liquor left in the 10,000 grains' measure and add 

 thereto at once 48 or 49 alkalimetrical divisions of tho test acid, and after having 

 thoroughly agitated the mixture, proceed to pour the acid carefully, two drops only 

 at a time, stirring after such addition, and touching a strip of litmus-paper with the 

 end of the glass rod used for stirring ; and so he should go on adding two drops, 

 stirring, and making a streak on the litmus-paper until tho liquor assumes suddenly 

 a pink or onion-red colour, and the streak made on the litmus-paper is red also. The 

 alkalimeter is then allowed to drain as before, and the operator reads off the number 

 of divisions employed, from which number 2 drops (or T 2 5 ths of a division) should be 

 deducted ; Gay-Lussac having shown that, in alkalimetrical assays, tho sulphates of 

 alkalis produced retard the manifestation of the red colour in that proportion. Ono 

 alkalimetrical division generally consists of 10 (bops, but as this ia not always the 



