ON THE ESTIMATION or POTASH ANJ) PHOSPHORIC ACID. 9 



Second Report of a Committee, consist imj of E. C. C. Stanford, 

 James Dewar, Alfred E. Fletcher, E. W. Parnell, T. R. 

 OgilviEj and Alfred II. Allen (Secretary), appointed to inquire 

 into the Methods employed in the Estimation of Potash and Phos- 

 phoric Acid in Commercial Products and the mode of stating the 

 Results. Drawn up by Alered H. Allen *. 



Determination of Potash. 



The evidence on this subject obtained previously to the lust meeting of the 

 Association (Bristol) showed that the method of determining potassium by 

 precipitation as a platinum salt was almost universally employed by chemists 

 of large experience in the assay of potash salts. The Committee thought it 

 desirable, therefore, to subject the process to an exhaustive examination, with 

 a view of ascertaining the origin of the discrepancies known to occur 

 between the results of chemists using different modifications of the general 

 method of estimation by platinum chloride. 



Tho process of determining potassium by platinic chloride is well known 

 to depend on the sparing solubility of chloroplatinatc of potassium and tho 

 easy solubility of the ehloroplatinates of the associated metals. The pre- 

 cipitate is crystalline, of a bright yellow colour, and is easily dried. On 

 account of its solubility in aqueous liquids it is necessary to operate on con- 

 centrated solutions and to employ alcohol for washing. When the precipitate 

 is produced suddenly by addition of platinic chloride to a concentrated solu- 

 tion of potassium chloride, or by rapidly cooling a hot saturated aqueous 

 solution of potassium chloroplatinatc, it is obtained in a finely granular or 

 pulverulent form. When the chloroplatinatc is formed by gradual concen- 

 tration of a dilute aqueous solution, or by adding chloride of platinum to a 

 dilute solution of chloiide of potassium and then concentrating, the precipi- 

 tate assumes the form of dense crystalline scales, the subsequent manipulation 

 of which is very easy. 



The following modifications of the general process have been employed by 

 the Committee with the view of testing their comparative accuracy under 

 various conditions likely to occur in practice. 



The information forming the basis of the experiments was communicated 

 to the Committee chiefly dining last year, and to a great extent was 

 incorporated in the Eeport presented at the Bristol Meeting. 



Modification I. — Essentially the process of Professor Presenilis described 

 in his ' Manual of Quantitative Analysis,' being shortly as follows : — The 

 solution of mixed chlorides of potassium and sodium, freed, if necessary, 

 from calcium, magnesium, and sulphates, was evaporated nearly to dryness 

 with excess of solution of platinic chloride. (In many of the experiments 

 a considerable excess of platinum was employed beyond the quantity required 

 to convert both the alkali metals into ehloroplatinates.) The evaporated 

 solution was then treated with alcohol of about 80 per cent., allowed to 

 stand for some time, transferred to a small filter, washed with alcohol of 

 80 per cent., and carefully dried. The bulk of the precipitate was then 

 transferred to a weighed capsule, dried at 100° C, and weighed. Tho filter 

 with from 1 to 3 milligrammes of adherent precipitate was ignited, the 

 weight of the filter-ash (-0004 gramme) subtracted, and the residue of 

 Pt+2KC1 calculated to rtCl, + 2KCl, the amount thus obtained being added 

 to the main quantity. 



* Kc;d at the Meeting at Glasgow, 1876. 



