xv, i Witt: Portland Cement Raw Mixture 109 



sodium thiosulphate. A neutral solution of a calcium salt may 

 be titrated with standard sodium carbonate solution. 17 



Determination of constituents other than calcium. — Since the 

 calcium in a cement mixture is usually largely present as the 

 carbonate, the carbon dioxide evolved on ignition, or on treating 

 with acid, is often made the basis for determining the calcium 

 carbonate content. The following are among the many methods 

 that may be used : 



The gas may be absorbed in soda lime or in an alkali hydroxide 

 solution, 18 and the resulting mixture either weighed or titrated 

 with standard acid. The weight of the carbon dioxide evolved 

 may be found by the use of any one of a number of apparatus 

 such as Geissler's, 19 or by weighing the sample before and after 

 ignition. 20 Barker 21 has designed a special form of hydrometer 

 in which the sample is placed and treated with hydrochloric acid. 

 The decrease in weight due to the evolution of carbon dioxide is 

 shown by the position of the apparatus in the water in which it 

 floats. Chapin M calculates the volume of the evolved gas from 

 the increase in pressure in a closed vessel, as indicated by a 

 monometer. The volume of evolved gas is frequently measured 

 in a gas burette, or some type of calcimeter M is employed. 



A method of a different type consists in treating the sample 

 with standard acid and, when the reaction is complete, titrating 

 the excess acid with standard alkali. The details of this method 

 differ in various laboratories. 2 * In some cases the calcium car- 

 bonate content may be estimated from the acid-insoluble matter. 25 ' 



The United States Government official method for the deter- 

 mination of calcium in raw mixtures specifies that the sample 

 be ignited over a blast lamp for fifteen minutes and then treated 

 with hydrochloric acid. 26 The silica is dehydrated and filtered 



"Vizern, Journ. Chem. Soc. 66 A2 (1894) 161. 



"Fresenius, C. R., Zeitschr. f. Anal. Chem. 2 (1863) 49, 341. 



"Journ. f. Prak. Chem. 60 (1853) 35. 

 Treadwell and Hall, Analytical Chemistry. New York, John Wiley and 

 Sons 2 (1907) 292. 



" Barker, J. P., Journ. Ind. Eng. Chem. 9 (1917) 786. 



"Chapin, W. H., Journ. Ind. Eng. Chem. 10 (1918) 527. 



■ For description of the Scheibler-Dietrich calcimeter see George Lunge, 

 Technical Methods of Chemical Analysis. New York, D. van Nostrand Co. 

 1 (1908) 659. 



"Kluge, F., Chem. Zeitg. 20 (1896) 372; Meade, R. K., Portland Cement. 

 The Chemical Publishing Co., Easton. Pa. (1911) 313. 



- 6 Meade, op. cit. 326. 



" If anything remains undecomposed it is separated, fused with sodium 

 carbonate, dissolved, and added to the original solution. 



