THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 77 



cations other than H ions, the potential being influenced by the activity 

 and kind of such cations. Sodium and Uthium ions produce the most 

 marked effects; potassium and bivalent cations smaller effects. When 

 working under these conditions, it is advisable to standardize the electrode 

 with known buffer solutions of about the same composition and of pH 

 closely above and below the pH of the sample being tested. 



The standardization for linearity of response from pH 1 to 9 is a neces- 

 sary check on the operation of the glass electrode, since its results are 

 comparative, not absolute. The slope, — A£';i/ApH, should be not 

 merely constant at any temperature but also equal or closely equal to 

 0.000,198,3227". Obviously, a ''pH-meter" with its pH scale adjusted to 

 the theoretical slope for a given temperature cannot give correct readings at 

 all points from pH 1 to 9 if its glass electrode follows a significantly differ- 

 ent slope at the same temperature. For a brief discussion of the effects 

 of temperature, see Clark (1948). 



Cleaning of the glass surface, by immersion in a hot mixture of concentrated nitric 

 and sulfuric acids followed by soaking in water, may restore a sluggish or erratic 

 electrode to normal functioning. A somewhat drastic procedure that may be effective 

 is to dip the glass electrode for a second or two in dilute HF or in a 20 per cent solution 

 of ammonium bifluoride and then to wash it thoroughly in water. If the electrode 

 still behaves erratically, it should be discarded. For such an emergency, it is highly 

 advisable to have available a reserve electrode. This may obviate any mistaken 

 tendency to carry on with an electrode of doubtful reliability. 



The instructions accompanying the various glass-electrode "pH-meters" now on 

 the market are usually sufficient to aid the user in tracing out sources of trouble and 

 error in operation. A major source of trouble is electrical leakage due to accumulation 

 of films of moisture at critical parts of the circuit, and perhaps the most frequent sites 

 of such accumulation are the electrode support and lead, both of which are apt to be 

 spattered with water or salt solution during careless manipulation. 



The glass electrodes now available are fairly rugged and easilj^ adaptable to use 

 under a variety of conditions and on different types of biological material (e.g., liquid 

 and "solid" culture media). Measurements with an accuracy of 0.05 pH may be 

 made rapidly in poorly buffered, colored, or turbid solutions and in blood or serum. 

 The monograph by Dole (1941) discusses many of its uses. 



The Colorimetric Method 



The colorimetric method of measuring pH makes use of acid-base indi- 

 cators, which, within certain limits, vary in color with the pH of the 

 solution. Such indicators are compounds capable of existing in solution 

 as conjugate proton (H-ion) donor and proton acceptor, with one of the 

 conjugate pair differing in color from the other. The relation of these 

 two forms to pH is defined by the equation 



TT r-/ . 1 [proton acceptor] ,.. 



pH = pK' + log ,p,^ton donor] ^^^ 



