584 REVERSAL OF SIGN OF CHARGE 



about pH = 3.1 and then drops again. This drop is the concentra- 

 tion effect discussed in preceding papers and due to the ions with the 

 opposite sign of charge as that of the membrane, in this case CI and 

 NO3 (the latter being added with the nitric acid).^ 



Instead of using CaCl2 we can also use a m/256 solution of Na2S04 

 as a test for the sign of the electrification of water. We put m/256 

 Na2S04 into the collodion flask and dip the latter into H2O. The 

 outside water as well as the m/256 solution of Na2S04 is brought 

 to the same pH by adding HNO3 (or KOH). The SO4 ion attracts 

 positively charged water and the Na ion repels it, but the attractive 

 effect of the SO4 ion is greater than the repelHng effect of the Na ion. 

 Water will commence to be attracted by m/256 Na2S04 as soon as the 

 water is positively charged; when water is negatively charged it will 

 be repelled more powerfully by SO4 than it will be attracted by Na. 

 Hence the hydrogen ion concentration at which the water commences 

 to be positively charged will be indicated by a rise in the level of liquid 

 in the glass tube serving as a manometer. Fig. 1 shows that water 

 commences to be positively charged at a pH of about 4.0 or slightly 

 above where the turning point was also found when we used m/256 

 CaCl2 solution as a test. 



We can finally use solutioiis with monovalent anion and mono- 

 valent cation as a test solution; e.g., m/256 LiCl. In this case we get 

 an attraction for water both in alkaline and in acid solutions. In the 

 acid solution the negatively charged water is attracted by the Li ion 

 and in the alkaline solution the positively charged water is attracted 

 by the CI ion. Between the two effects there should be a point where 

 the water is neither positive nor negative and hence is not attracted 

 by either ion. Fig. 1 shows that this point lies again at a pH near 4.0. 



Fig. 2 gives the point of reversal in the presence of m/256 solutions 

 of NaH2P04, Na2 oxalate, and NaCl. The point of reversal Hes 

 between 4.4 and 4.0, but the steep rise of the curve commences at 

 pH 4.0. 



Experiments with a number of other salts and other acids were 

 made, all giving the same result; namely, that the membrane com- 

 mences to be distinctly positively charged as soon as the pH is 4.0 

 or slightly above, but below 4.7. At the isoelectric point, pH = 4.7, 

 no acid combines with the gelatin and no change of the sign of charge 



