544 



witli citrates and substitated citrates, but onr experiments were 

 left unpublislied. I liave again occupied myself with this problem, 

 and performed a number of new determinations as a supplement 

 and clieck. The results follow. 



2. Experiments. 



The examined Ca-solution, which was strongly split up into ions, 

 was prepared as follows. A saturated solution of Ca SO, (puriss. 

 pro avel.) in distilled water was diluted with the 2,3 fold volume 

 of distilled water. In a narrow-mouthed glass jar of 250 cm*, capa- 

 city 50 cm' of this liquid was pipetted off and mixed with 50 cm' 

 of distilled water or with cm' of an aqueous solutions of the sub- 

 stance under consideration. These 100 cm' were titrated in tlie 

 same glass jar by Clark's method (with a solution of soap in 

 alcohol of 56 volume percentages.') In the titration a finely divided 

 precipitate of calcium moleate is formed in the bottle. The endpoint 

 has been reached when by the side of this precipitate so much 

 alkali-oleate remains in the solution that, after shaking, the solution 

 exhibits a not disappearing soap froth. As endpoint was taken the 

 condition at which after a from six to eight times repeated vigorous 

 shaking in the longitudinal axis of the bottle, the soap froth appears 

 at the rim of liquid and bottle, as a white ring, 1 mm. high and 

 from 1 to 2 mm. broad, and remains thus for five minutes. This 

 endpoint can be determined pretty sharply, when the necessary 

 practice has been obtained; when comparing experiments are 

 always carried out in the same way, repeated determinations of the 

 same liquid with a quantity of titration liquid of about 45 cm.' 

 deviate only some tenths of cm.' from the mean of the determina- 

 tions. For our determinations this accuracy is amply sufficient. 



Without citrate the 100 cm.' of calcium sulphate solution require 

 from 45 to 47 cm.' of titration liquid to reach this end-point; 

 hence the total volume of the liquid at the end of the titration 

 amounts to 145 or 147 cm.'. If in consequence of the addition of 

 citrate the liquid required considerably less titration liquid, 1 added 

 so much alcohol of 56 volume percentages (spec. gr. 0,921) from a 

 burette to the 100 cm.' that was to be examined, that at the end 

 of the titration the total volume would again be between 145 and 



in which the calcium ions are dissolved If, however, only small differences are 

 measured, in other words if about an equal amount of alcohol is added, this does 

 not prevent us from obtaining comparable results. 



') I refer for an accurate description, of Clark's method to Jahresberichte f. 

 Chemie 1850, p. 608; to Lunge and Berl, 6th edition. Vol. II, p. 23i2. 



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