ALKALIMETRY 81 



The sample of potash examined contained, therefore, nearly 49 per cent, of pure 

 potash. 



53. If instead of the special test sulphuric acid for potash ( 17), or of the tost sul- 

 phuric acid for potash, soda, and other bases ( 28), the operator uses the potash and 

 soda alkalimeter ( 31-36), the method to be followed is exactly similar to that 

 described in 42 and following. Some of the test sulphuric acid, of specific gravity 

 1*1268, is to be poured into the alkalimeter until it reaches the point marked 'potash' 

 (that is to say, 48'62 divisions of the alkalimeter), taking the under line of the liquid 

 as the true level, and the remaining divisions up to are carefully filled with water. 

 The operator then closes the aperture of the alkalimeter with the thumb of his left 

 hand, and the whole is violently shaken so as to obtain a perfect mixture. 



54. The acid so mixed must now be carefully poured from the alkalimeter into the 

 alkaline solution of the potash under examination until neutralisation is attained, 

 precisely as described in 42 and following. 



55. The neutralising point being hit, the operator allows the sides of the alkalimotor 

 to drain, and he then reads off the number of divisions employed in the experiment, 

 which number indicates the per-centage of real potash contained in the sample. 



Had the operator wished to estimate the quantity of potash as carbonate of potash, 

 he should have poured the test acid into the alkalimeter up to the point marked ' car- 

 bonate of potash,' filled the remaining divisions of the alkalimeter up to with water, 

 and proceeding exactly as just mentioned, the number of divisions of acid employed 

 would indicate the per-centage of potash contained in the sample as carbonate of potash. 



56. The most accurate and expeditious method of determining the value of a sample 

 of an alkali is by means of a standard acid and a solution of a caustic alkali of cor- 

 responding strength contained in two burettes (fig. 26). The solution of soda ash, for 

 instance, having been prepared as directed in 38-40, is tinted with litmus solution, 

 and a quantity of the standard acid more than sufficient to saturate the whole of the 

 alkali is run into it, and the mixture boiled till the carbonic acid is entirely expelled 

 from it and the clear red of the litmus solution is seen. The number of divisions of 

 acid added is then noted and the neutral tint restored by the careful addition of the 

 solution of caustic alkali. From the absence of carbonic acid the reaction is very 

 sharp and decided, and even if the neutral point be overshot, the addition of a few 

 drops of acid followed by the more cautious use of the alkaline solution will enable a 

 correct result to be obtained. If the two standard solutions are of exactly equal 

 strength, it is only necessary to subtract the number of divisions of the alkaline solu- 

 tion used, from that of the acid one, to give at once the number of those of the latter 

 required for the neutralisation of the substance tested, and hence by a simple calcu- 

 lation the per-centage of real alkali. 



67. If a Schuster's alkalimeter (fig. 25) be used, and supposing, for example, that 

 the acid to be employed therewith is so adjusted that 10 grains weight of it neutralise 

 exactly 1 grain in weight of potash, proceed as follows : Take 100 grains in weight 

 of a fair average of the sample, previously reduced to powder, dissolve them in water, 

 filter with the precautions which have already been described before ( 38 and follow- 

 ing), and pour this solution into a glass cylinder graduated into 100 parts, and capable 

 of containing 10,000 water-grains ; fill it up with water exactly as described before ; 

 of this take now 100 alkalimetrical divisions, that is to say, v^th of the whole solution, 

 and pour it in a glass beaker. On the other hand, charge the Schuster's alkalimeter 

 with a certain quantity of the test acid, and weigh it, along with the alkalimeter itself, in 

 a good balance. This done, proceed with the neutralisation of the solution in the glass 

 beaker, by pouring the acid from the alkalimeter in the usual way, and with the usual pre- 

 cautions, until the saturation is completed. Keplace the alkalimeter, with the quantity 

 of unconsumed acid, in the scale of the balance, weigh accurately, and since every grain 

 of acid represents ith of a grain of potash, the number of grains of acid used in the 

 experiment indicates at once the per-centage of real potash present in the sample. 



58. When, however, potash is mixed with soda, as is frequently the case with the 

 potash of commerce, either accidentally or for fraudulent purposes, the determination 

 of the amount of the cheaper alkali could not, until a comparatively recent period, bo 

 estimated, except by the expensive and tedious process of a regular chemical analysis. 

 In 1844, however, M. Edmund Pesier, professor of Chemistry at Valenciennes, pub- 

 lished an easy and commercial method for the estimation of the quantity of soda which 

 potash may contain, by means of an areometer of a peculiar construction, to which the 

 name of ' Natrometer ' has been given by the talented professor. 



59. The rationale of the method is grounded upon the increase of specific gravity 

 which sulphate of soda produces in a solution saturated with pure sulphate of potash, 

 and is deduced from the fact that a solution saturated with neutral sulphate of potash 

 possesses a uniform and constant density when the saturation is made at the same 

 temperature, and that the density of such a solution increases progressively in proper- 



VOL. I. G 



