The quantitative determination of sulphuric acid--the detection and estimation of which in leather is important--is considerably influenced by the presence of Neradol D. Practically all methods in vogue dealing with its determination were based on the estimation of the sulphur introduced into leather by sulphuric acid. The presence of Neradol D, the main constituent of which is dicresylmethanedisulphonic acid, renders it impossible by such methods to determine whether the combined sulphur owes its origin to sulphuric or sulphonic acid. It remains yet to be determined whether the sulphonic acid influences the leather substance to the extent that sulphuric acid does; it must, however, be borne in mind that Neradol D in addition to free sulphonic acid also contains sulphonates and sulphates, which may enter into the leather and thus increase the sulphur contents of the latter. A method must hence be devised which estimates the free acid only and provides the means of distinguishing this from all other acids of organic and inorganic acids. Paessler, [Footnote: _Collegium_, 1914, 527, 126; 531, 509; 532, 567.] by extracting the leather and dialysing the filtrate, has effected a separation of the acids and the tanning and colouring matters and quantitatively estimated the sulphuric acid in the dialysate. Immerheiser [Footnote 1:_Collegium_, 1918, 582, 293.] devised a method, based upon the property of sulphuric acid of combining with ether, for the purpose of determining free sulphuric acid in leathers:--10 gm. of the leather, cut into small pieces, are extracted three times with 200 c.c. distilled water at room temperature, the time of each extraction being ten to twelve hours, and the combined extracts evaporated to dryness on the water bath, 5 gm. of quart sand being added. The dry residue is now powdered, introduced into an Erlenmeyer flask provided with a glass stopper, and 200 c.c. of anhydrous ether [Footnote 2: To be tested for water by shaking with anhydrous copper sulphate.] added. After about two hours, during which the flask is occasionally shaken, the ether is poured through a filter, the residue washed with a little ether, and the operation repeated twice with each 40 c.c. anhydrous ether, using the same filter. To the combined ether extracts (about 200 c.c.) HCl and [Greek: b]aCl_2 are added, the ether distilled off and the residue evaporated on the water bath, in order to decompose the ether-sulphuric acid compound. 50 c.c, of hot water acidified with HCl are now added, the precipitate allowed to settle, filtered, washed, dried, and weighed. The sulphuric acid thus estimated was present in the leather as _free sulphuric acid_. That present as sulphates soluble in water is estimated in the residue on the filter: the residue is extracted with hot water, the sand filtered off, the filtrate acidified with HCl, boiled for one quarter hour and filtered if necessary. The clear filtrate, which may be coloured, is brought to boil and