ALKALIMETRY 85 



75. The alkalimetrical assay of soda is performed exactly in the samo manner as 

 that of potash that is to say : From a fair average sample of the soda to be examined, 

 take 1,000 grains' weight (or less if that quantity cannot be spared), and boil it five 

 or six minutes in about eight fluid ounces of water, filter in order to separate the 

 insoluble portion, and wash the residue on the filter with boiling water until it no 

 longer drops from the filter with an alkaline reaction, and the bulk of the filtered 

 liquid and the washings received in a graduated glass cylinder form 10,000 grains' 

 measure. Should the water which may have been required to waeh the residue have 

 increased the bulk of the solution beyond that quantity, it should be evaporated to 

 reduce it to the bulk mentioned. 



76. This being done, 1,000 water-grains' measure that is to say, ith part of the 

 aqueous solution of the soda ash above mentioned ( 75) are transferred to the 

 glass beaker or vessel in which the saturation is intended to take place : it is tinged 

 distinctly blue with tincture of litmus, and the operation is performed in the same 

 manner and with the same precautions as for potash, the glass beaker containing the 

 blue alkaline solution being placed upon a sheet of white paper, or a slab of white 

 porcelain, the better to observe the change of colour which takes place when the 

 saturating point is approaching. 



77. Having put into a glass beaker the 1,000 grains' measure of the aqueous solu- 

 tion of soda ash to be examined ( 75), and if the test sulphuric acid for soda, 

 described before ( 23-27), the alkalimeter, fig. 22, 23, or 24, should be filled with 

 that test acid up to the point marked (taking the under lino of the liquid as the 

 true level), and poured therefrom with the precaution already indicated, stirring 

 briskly, at the same time, the liquid in the beaker. As is the case with the alkali- 

 metrical assay of potash, the carbonic acid expelled by the test acid reacting upon the 

 as yet undecomposed portion of the soda ash, converts it into bicarbonate of soda, so 

 that at first no effervescence is produced ; but as soon as half the quantity of the soda 

 in the solution is saturated, a brisk effervescence takes place. At first, therefore, the 

 operator may pour at once, without fear, a pretty large quantity of the test acid into 

 the alkaline solution, but as soon as this effervescence makes its appearance he should 

 proceed with increased precaution gradually as the saturating point is approached. 

 The modus operandi is, in fact, precisely as already detailed for the assay of potash, 

 precisely the same kind and amount of care is requisite, and the assay is known to be 

 terminated when the streaks made upon the litmus-paper with the stirring rod 

 remain distinctly and permanently of a pink colour. 



78. After saturation, and after having allowed the sides of the alkalimeter to drain, 

 the number of divisions at which the test acid stands in the alkalimeter indicate at 

 once the per-centage of the soda assayed, since, as we said, each division of this par- 

 ticular test acid represents one grain of pure soda. If, therefore, the test acid stands 

 at 52 in the alkalimeter, then the soda assayed contained 52 per cent, of real soda. 

 See, besides, the observations of 48 and following, and also 81* 



79. If, instead of the special test acid for soda just alluded to, the operator em- 

 ploys that which has a specific gravity of 1'032, and 100 alkalimetrical divisions of 

 which saturate one equivalent of each base, the modus operandi is the same that is to 

 say, the alkalimeter is filled with it up to 0, and it is poured therefrom carefully into 

 the alkaline solution ; but as the equivalent of soda is 31, and 100 alkalimetrical 

 divisions of the test sulphuric acid now employed are capable of saturating only that 

 quantity of soda, it is clear that with the soda ash taken as an example in the pre- 

 ceding case, and containing 52 per cent, of real soda, the operator will have to refill his 

 alkalimeter with the same test acid, and that a certain number of divisions of this 

 second filling will have to be employed to perfect the saturation. In this instance 

 the operator will find that nearly 68 divisions more, altogether 168 divisions (cor- 

 rectly, 167'74) have been required to effect the saturation. 



80. If, instead of the special test sulphuric acid for soda ( 23-27), or the test 

 sulphuric acid for potash, soda, and other bases ( 31-34), the operator uses the 

 potash and soda alkalimeter ( 28-35), the method is always the same ( 74, 75) 

 that is to say, the aqueous solution of the soda ash is poured into the glass beaker, 

 the difference being merely, that instead of the alkalimeter being quite filled up 

 with the test sulphuric acid, which, in the present instance, has a specific gravity of 

 1-268 ( 29), the said test acid is poured into the alkalimeter only up to the point 

 marked 'soda' (taking the under-line of the liquid as the true level), and the re- 

 maining divisions of the alkalimeter are carefully filled up with water. The mouth 

 of the tube should then be thoroughly closed with the thumb of the left hand, and 

 the whole violently shaken until perfectly mixed, taking great care, of course, not to 

 squirt any of the acid out of the tube, which evidently would cause an amount of 

 error proportionate to the quantity of the test acid which would have thus been lost. 

 The acid should then be poured from tho alkalimeter with the usual precaution 



