272 Researches on the Commercial Potash 



1. There are now, I believe, three sorts of potash recognized by 

 the inspectors ; but if my information be correct, the samples pas- 

 sed as " first sort," always contain a considerable proportion of lime, 

 to say nothing of soluble impurities. Another brand designated 

 " pure" or by any other convenient term might, in ray opinion, be 

 advantageously introduced. Samples thus branded, should contain 

 no lime or salt, and at least eighty five per cent of carbonated potash. 



2. The insoluble impurities may be easily determined, by dissolv- 

 ing a known quantity of potash, as five hundred or one thousand 

 grains, in pure rain water, conducting the operation in a glass flask 

 and applying a gentle heat to facilitate the solution. Then filtering 

 the solution, washing the precipitate if any, and drying it carefully, ' 

 the per centage of insoluble matter can at once be ascertained. 

 When the proportion exceeds two or three parts in the hundred, it 

 is probably due to the carelessness of the manufacturer, or to the 

 addition of lime to the alkaline liquor during its evaporation. 



3. The soluble impurities are more difficult of detection ; and it 

 is by no means easy to reduce the process to a single trial. The 

 mode ordinarily prescribed in chemical works, consists in determin- 

 ing the saturating power of the specimen under examination, and 

 very convenient instruments for this purpose are described by Mr. 

 Faraday, and by MM. Descroizilles and Gay Lussac. In these 

 instruments sulphuric acid of known specific gravity, is employed in 

 a fixed quantity and added to a given weight of the sample, (pre- 

 viously dissolved in water, and the solution filtered,) until by a test 

 paper, it is shown, that the alkali is exactly neutralized. The amount 

 of acid required to produce this effect, will if its strength be exactly 

 known, indicate the proportion of pure alkali, contained in the spe- 

 cimen under examination. 



It is evident however that this process is insufficient to detect the 

 presence of soda, and when that substance is in large proportion, it 

 is too objectionable to be relied on, for the reason that the satura- 

 ting powers of potash and soda are very different. For example, 

 fifty parts of soda will saturate as much of any given acid as seven- 

 ty five parts of potash ; so that in practicing with this test, a mixture 

 of twenty five parts of soda and 37.5 parts of potash =62.5, would 

 give the same result, as seventy five parts of potash. In this instance 

 then, there would be an error of 12.5 per cent, to say nothing of the 

 difference in the value of soda and potash. 



