J. F, ToCHEH 



VI. Ii;XPERBrEXTS WITH CONSTANT VOLUME AND CONSTANT CONCEN- 

 TRATION OF ACID BUT WITH VARYING QUANTITIES OF MINERAL 

 PHOSPHATES. 



In the first set of experiments fairly high constant vakies of mjnu 

 were used. We shall now consider the effect of making mjm^ small, 

 nij^ and iii^ being in this case constant and m.^ the variable. If m^Jm^ 

 is made small enough we should have a citric solubility practically zero 

 for all dilutions. An example indicating the approach to the latter con- 

 dition is given in the following table (Table VIII) where the third set 

 of conditions is observed. (See also Diagrams 7 and 8.) 



Table VIII. Amount of citric acid and volume constant — amount of 

 mineral phosphate (»(.,) used varied. 



Exp 



I 



3 

 4 



Wt. of min- 

 eral phos- 

 phate taken 



5 grams 

 10 ., 

 20 „ 

 40 „ 



■-= = 50 



(10 grains citric aciil = mj 

 ^500c.c. volume = Hi, 



Ratio 



1 



0-.5 



0'25 



Acidity ex- 

 pressed as 

 citric acid 



at end of 30 

 minutes 



8-87 

 8-50 

 7-6.5 

 5-98 



Amount of 



pliosphate 



dissolved as 



CajP.O, 



0-970.5 

 0-86(30 

 0-6070 

 0-4110 



Mol. cone, of 



phospliate 



dissolved at 



end of 30 



minutes 



-UU(i2Ul 

 -00.5.5S7 

 -003916 

 -002652 



Citric solubility 

 expressed as 



Ca3P3*^)a per cent. 



of pliosphate 



taken 



19-41 

 8-66 

 3-04 

 1-03 



.a 

 a 



60 



3 cd 



go 



a « 



o > 



Wei;;lU of Miner; 

 taken — < Jciini-- 



1 l'lios|iluite 



5 10 IE 

 Diasrram 7. 



20 25 30 35 

 See Table VIII. 



20 



l-i 

 ^ 3 fl 



IZ 10 



2S.& 



■son 



St 



5- 



;lit of Mineral ' 



^|d^ate taken — t^ranis 



5 10 15 20 25 30 3£ 40 

 Diagram 8. See Table VIII. 



If the mineral phosphate contained merely tri- or di-calcium phos- 

 phate and was quite free from Ca(OH)a, CaO or CaCOj and also was in 

 excess, mere variations in the quantity taken would have had little or 

 no effect on the amount of citric acid present at the end of the period of 

 shaking. The presence of Ca(0H)3 naturally reduces the acid concentra- 

 tion with the result that, while 19-41 per cent, of the 5 grams mineral 

 phosphate was dissolved only 1-03 per cent, mineral phosphate was dis- 



10—2 



