ACIDITY 



321 



portional to the hydrogen-ion concentration. The current is 

 so very slight, owing to the high resistance across the glass (often 

 as much as 30 megohms, or 30,000,000 ohms) that it must be 

 measured by a supersensitive instrument or be amplified. For 

 the first of these methods, a quadrant electrometer is used, and, 

 for the second, vacuum-tube amplifiers increase the current to 

 the point where it can be measured on an ordinary galvanometer. 

 In more recent models, where the glass is made very thin, the 

 resistance is low enough to yield a potential that can be read 

 with an ordinary galvanometer and potentiometer without 

 amplication. The glass for the electrode must be a very pure 

 calcium-sodium-silicate glass. Lime, soda, and silica are melted 

 together to produce it. The slightest trace of lead or aluminum 

 is fatal. 



Errors. — Errors enough creep into pH technique, but some 

 can be systematically determined and usually allowed for. These 

 are the salt, protein, and temperature errors. 



Salt effects cause a displacement of the apparent pH, which 

 results in a shift in the color of the indicator. The following 

 table gives the correction in pH for certain salt concentrations : 



Indicator 



Methyl orange 

 Methyl orange. 

 Methyl orange, 

 Phenol red .... 

 Phenolpthalein 



Salt 



KCl 

 KCl 

 KCl 



NaCl 

 NaCl 



Salt 

 concentration 



0.10 A^ 

 0.50 A^ 

 1.00 A^ 

 0.50 A^ 

 0.50 A^ 



Correction 



-0.08 

 +0.02 

 +0.23 

 -0.15 

 -0.17 



The protein error is, in general, very small, unless the concen- 

 tration of the protein is exceedingly great, as the following table 

 shows: 



