In the ultimate micro-analysis of organic compounds there is a great need for a reliable and convenient method for the determination of sulphatc in amounts of a few mg. While searching for such a method we have tried out various reagents, amongst which was the new reagent, 4-amino-4'-cl~lorocliphes~yi, proposed by Ris~ctrarz, NUT-TEN ASD STEPW~S~'*. Although strictly adhering to the recoxnmcnded procedure WC obtained inconsistent and low results. Similar cspcrienccs have been reported by BAUMISGIEK~, and in a short communication GEILMANS ANI) GKH~\WIII~~ state that the excellent results' obtained with the reagent are due to a compensation of crrors,thc losses resulting from incomplete precipitation and washing the precipitate being balanccd by adsorption of the reagent by the precipitate. Recently, BELC~IIII< and co-worker+ have reportccl that the solubility of the freshly precipitated amine sulphatc appears to bc higher than was first estimated for the dried material&.
On reviewing the papers on the metliocl -1 7, little useful inforn~ation could be gained for estimating whether this reagent is thcorctically suitable for the above-mentioned determination.
The only two papers containing the appropriate data arc those published by BI:LCIIEI< and co-workcrs*~G, who detcrniincd the solubility of the amine sulphate in water and the sensitivity of the reagent towards sulphatc. From the solubility data given in the first paper 6, the solubility product is less than 10-l~ if the per of the saturated solution is high enough to justify the approximation that all the sulphate present occurs DS divalcnt sulphate ions, and if solution proceeds normally. From the diagram given in the second paper4 and assuming the ionisation constant of HSO,-to be 1.2 -x0-g and using the Dcbyc-Hiickel formula" for estimating the mean activity coefficients, the solubility product is calculated to be between 1.3 * ro'll and 2.3 * 10-11. The figures for the estimated solubility product of 4-amino-4'"chlorodiphenyl sulphatc arc found to differ by a factor of ro2. The uncertainty in the estimation of the activity coefficients could not account for this large discrepancy, and, according to our observations, the crystals are of the soft type; practically no supersaturation is to be cxpcctcd nor has any been observed.
In view of the foregoing it was decided to make an experimental determination of the quantities governing the equilibrium of the system 4-amino-4'-chlorodiphenyl sulphate-water.
The ionisation constant and the solubility of the. amine were detcrmined, and the solubility of the amine sulphate was redetermined. It was found that the reportecl figure for the solubility of the amine sulphate is about ten times too low, thus explaining the discrepancy mentioned. As will be shown later, the technique used Rcjere*rces p. 3.