DETEEMINATION OF ACIDITY AND ALKALINITY 



99 



point. Having added 9.5 cc. N NaOH we still have a 0.005 N HCl 

 solution, pH 2.3. With 10 cc. of the base added we have neutrality 

 or pH 7. On the further addition of the alkali free base is present in 

 solution and the indifferent NaCl. From this point we may calcu- 

 late the resulting pOH and therefore the pH. The lowest curve of 

 figure 12 is obtained in this manner. 



The curve is characterized by its initial flat or horizontal course 

 and then by the sudden and abrupt rise at the "end-point of the titra- 

 tion." In this region the addition of a few drops of the alkali causes 

 a steep increase of the pH from about 3.0 to 11.0. If we should ask 

 what relation the transformation point of the nitrophenol (pH 5.0 



13 



12 

 11 

 10 

 9 

 8 

 7 

 6 

 5 



3 

 pHz 



1 



13 

 12 

 11 

 10 

 9 

 8 

 7 

 6 

 5 

 V 

 3 

 PUz 



123^56739 10 11 12 13 

 can n Na OH 



Fig. 12. Titration curves of 10 

 cc. portions of A^ HCl, acetic acid 

 and phenol with A^ NaOH. 



1 2 3 V 5 6 



ccm n NaOH 



7 a 9 10 H 1Z 



Fig. 13. Titration curves of 5 cc. 

 and 10 cc. portions of A'^ HCl. 



to 6.0) bears to this curve, we shall see at once that this point falls 

 within the rapidly ascending portion of the curve. Thus p-nitro- 

 phenol gives us the correct end point of the titration. But it is also 

 to be observed that the use of this particular indicator did not in any 

 way mark the point pH 5.0 to 6.0. The same result could be obtained 

 with an indicator whose transformation point is at pH 7.0 (litmus) 

 or pH 8.0 (phenolphthalein). The amount of the alkali which in 

 this titration changes the pH of the solution from 5.0 to 9.0 is less 

 than one drop. From this we arrive at the rule: To titrate a strong 

 acid with a strong base one may choose an indicator whose transformation 

 point lies between pH 5.0 and pH 9.0. 



