Inorganic analysis

1920 The Analyst  
hferior qualities of lithopone and other pigments oDmposed essentially of zinc sulphide and barium sulphste sometimes contain natural barytes. Thie reduces the opadty and oovering power of the pigment, and oauses it to give a yellowish white when used in linoleum. The natural mineral may be detected by mixing a trace of the sample with a drop of water, drying the mixture, and examining it under the microscope with a 2 to + inch objective, the diaphragm being closed so as to form a dark
more » ... orm a dark background; Precipitated barium sulphate will then appear as a fine powder composed of minute orystals of uniform size, whilst in tho case of natural barytes transparent irregular fragments of larger size will be seen. The test is rendered more certain by first removing the zinc sulphide by treatment with dilute hydrochloric mid and potassium chlorate, and examining the insoluble residue, C, A. M. Titration of Chlorides with Silver Nitrate. H. W. Bolam. @em. N~w s , 1920, 20, 292.)-In the c a~e of a chromate, such as barium chromate, leas soluble than silver chromate, the titration of the chloride with silver nitrate becomes difficult as regards the end-point of the reaction. This may be overoome by adding potassium chromate in quantity slightly larger than is sufficient to precipitate the barium radicle ; the titration can then be carried out with silver nitrate as usual. The formation of a, faint brown tinge, which is not discharged on shaking the flask, shows that the reaotion is complete. , 1920,3, 433-446.)-An apparatus is described with a hydrogen electrode, the change in the potential of which in acidimetric titrations can be measured with snffioient accuracy with a millivoltmeter. The electrode consists of a tube of unglazed porcelain, 6 mm. in diameter, the exterior of which is electrolytically coated with a thin film of gold, and then at its lower end for a length of about 7 om. with a thin film of palladium black. For acidimetric titrations two suoh electrodes of equal size are combined to form an element. One is introduced into a suitabIe liquid (+$ sodium bicarbonate solution, or a citrate solution with hydrogen exponent p = 5) as a comparison electrode, whilst the other is immersed to the depth of about 2 cm. in the liquid to be titrated. The upper ends of the two eleotrodes are passed through a small glass bell-jar, which will form a water seal over the vessel containing the liquid. Ordinary hydrogen moistened with water is passed through the cornparison electrode at the rate of 2 to 3 bubbles per second, thence through the titration electrode, and finally into the air through a capillary tube bent upwards. A ahort capillmy tube filled with potassium sulphate connects the two eleotrodes. The liquid is titrated through a, small opening in the bell-jar, of such size that it is completely filled by the capillmy tube of the burette tap. The titration may also be effected in an open vessel, only the electrode (contained in a tube through whioh hydrogen is passed) being Jlowed to dip into the
doi:10.1039/an9204500341 fatcat:me2x2lahyvdcha4wzmfutgoudq