718 ACTIVITY COEFFICIENT OF THE HYDROGEN ION 



Example of Titration. — {a) Solution Containing no Carbonate. — 10 cc, 

 of 2.5 per cent gelatin solution or the equivalent amount, if more 

 concentrated gelatin is used, are pipetted into a large test-tube, 

 1 drop of neutral red is added, and the solution is brought to approxi- 

 mately pH 7.0 by the addition of a few drops of concentrated al- 

 kali (carbonate-free) or strong HCl. The reaction is then corrected 

 accurately to pH 7.0 with 0.1 n NaOH (COa-free) or 0.1 n HCl, 

 using a standard of pH 7.0 in the comparator block. 5 cc. of the 

 form.alin solution are then added and the solution is titrated with 

 0.1 n NaOH to pH 8.4, using a standard tube with thymol blue for 

 comparison. The neutral red does not interfere with this titration 

 and if anything makes it more sharp. The number of cc. of 0.1 

 n alkali required to titrate from pH 7.0 to pH 8.4 after the addition 

 of the formalin is noted and is referred to as the formol titration. 

 This value, as Sorensen showed, is independent of the final pH taken, 

 provided this is above 8.2. It represents the number of cc. of 0.1 

 n acid present in the solution. 



{h) Carbonate Present. — The solution is brought to pH 2.5-3.0 with 

 a few drops of concentrated HCl and boiled for 10 to 20 seconds 

 over the free flame to remove the CO2. It is then titrated as de- 

 scribed under {a). 



In order to check the accuracy of the method several experiments 

 were made in which the results obtained by the formol titration 

 and by Van Slyke's method were compared. The results of a few 

 of these are given in Table I. The amino nitrogen was determined 

 as described by Van Slyke. The solution was shaken for 30 min- 

 utes. In the case of gelatin, the titration figures for gelatin alone, 

 without the addition of formalin are also given. These figures were 

 obtained from the titration curves of gelatin given by Loeb,^ by 

 correcting for the amount of acid or alkali necessary to bring the same 

 volume of water to the same pH. It is found that below pH 1.8 

 and above pH 11.0 at 25°C. the figures so obtained are constant 

 and independent of the pH; i.e., outside these limits, the gelatin 

 is all combined as a salt and the figures represent the normality of 

 the solution. 



5 Loeb, J., J. Gen. Physiol., 1920-21, iii, 85. 



