xv, i Witt: Portland Cement Raw Mixture \\\ 



they have some effect on the accuracy of the results. One such 

 modification is to precipitate and reprecipitate the iron and alu- 

 minium from hydrochloric acid solution without first removing 

 the silica. The calcium in the nitrate is precipitated once as 

 the oxalate, and the precipitate is titrated with permanganate. 3 - 



The method may be still further shortened by neutralizing 

 the original hydrochloric acid solution with ammonia and adding 

 some organic acid in which calcium oxalate is not appreciably 

 soluble. The calcium may be precipitated as oxalate without 

 the removal of iron and aluminium. The calcium oxalate is 

 filtered out, and the determination is completed in the regular 

 manner. Citric, 1 " tartaric,* 4 and oxalic""' acid have been used. 



Of the methods noted above, several are used extensively for 

 controlling the calcium carbonate content of raw mixtures in 

 cement plants. These include some modification of the determi- 

 nation of calcium by precipitation with ammonium oxalate and 

 titration with permanganate, the determination of carbon dioxide 

 with the calcimeter. the determination of total alkalinity by 

 titration with standard acid and alkali, and the determination 

 of acid-insoluble matter. The last named is used much less fre- 

 quently than the others. The procedure in each case that I have 

 found best suited to the raw mixture discussed in this paper is 

 as follows: 



In determining the calcium, the usual method of igniting the 

 raw mixture before adding hydrochloric acid at the beginning 

 of an analysis is not sufficient. ™ Results are the same either 

 with or without ignition, but almost 2 per cent too low. It is 

 necessary to fuse with sodium carbonate. To avoid one filtra- 

 tion, the whole sample is fused instead of simply the acid- 

 insoluble residue. The melt is dissolved in hydrochloric acid and 

 the iron and the aluminium are removed by twice precipitating 

 with ammonia. The calcium in the filtrate is precipitated once 

 with ammonium oxalate, and the precipitate is dissolved in dilute 

 sulphuric acid and titrated with permanganate. The titration 

 method has been found more satisfactory than ignition, for use 



"Enright, B., Journ. Am. Chem. Soc. 26 (1904) 1003. 



"Passon, Max, Zeitschr. f. Ang. Chem. (1898) 776. 



14 Blum, L.. Zeitschr. f. Anal. Chem. 39 (1900) 152. 



"Meade, R. K., Chem. Eng. 1 (1904) 21. 



M With reference to igniting the sample, W. F. Hillebrand, Journ. Soc. 

 Chem. Ind. 21 (1902) 25, says: "Whether all cement mixtures can be thus 

 rendered wholly soluble I am unable to say, but doubtless most of them 

 can be, and if so, this method should always be resorted to in order to avoid 

 the use of alkali carbonates." 



