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 22 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 23 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. 24 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. 28 The silica is dehydrated and filtered 
17 Vizern, Journ. Chem. Soc. 66 A2 (1894) 161. 
78 Fresenius, C. R., Zeitschr. f. Anal. Chem. 2 (1863) 49, 341. 
19 Journ. f. Prak. Chem. 60 (1853) 35. 
20 Treadwell and Hall, Analytical Chemistry. New York, John Wiley and 
Sons 2 (1907) 292. 
21 Barker, J. F., Journ. Ind. Eng. Chem. 9 (1917) 786. 
22 Chapin, W. H., Journ. Ind. Eng. Chem. 10 (1918) 527. 
23 For description of the Scheibler-Dietrich calcimeter see George Lunge, 
Technical Methods of Chemical Analysis. New York, D. van Nostrand Co. 
1 (1908) 659. 
24 Kluge, F., Chem. Zeitg. 20 (1896) 372; Meade, R. K., Portland Cement. 
The Chemical Publishing Co., Easton, Pa. (1911) 313. 
“ Meade, op. cit. 326. 
M If anything remains undecomposed it is separated, fused with sodium 
carbonate, dissolved, and added to the original solution. 
