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Acids Content and pH
pH Values of Selected Foods
Beckman Explains pH and Potentiometry
Review of pH Excerpted from Beckman-Coulter
Logs
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The pH Scale -- Theory
The pH of a substance is a measure of its acidity just as a "degree" is a measure of temperature. For example, we can say that lemon juice is acid. This means no more than if we said water is hot. But if we speak of water having a temperture of 90C, then we have stated something specific. Similarly, if we say lemon juice has a pH of 2.3, we have stated an exact value of acidity.
Mathematically, pH is defined as the negative logarithm of the hydrogen ion acitivity:
"Activity" is defined as the effective concentration of species in solution; it is usually expressed in the units of moles/liter. The distinction between effective concentration and actual concentration decreases as we move toward more dilute solutions when ionic interactions become progressively less important. It should also be realized that the predominant form of the proton,
, in aqueous solution is the hydronium ion, 
, even though we find it convenient to speak of this as the "hydrogen ion."
In solution, the numerical values for hydrogen ion concentrations are extremely small numbers; for example, 1/10,000,000 moles/liter 
. To avoid use of these cumbersom numbers, the pH scale is especially helpful. Applying the definition of pH being minus the log of the hydrogen ion concentration to the examples just given, we speak of a neutral solution as having a pH of 7.0. An integral pH value then is simply the negative value of the power, p, to which 10 must be raised to equal the hydrogen ion activity.
The relationship between non-integral pH's and hydrogen ion activities is mathematically the same as in the simple example given above, except that log tables must be consulted for an interpretation of the decimal part of the pH. Consider a solution having a pH of 7.3. We should realize that this corresponds to a hydrogen ion activity of 
, but this latter number is probably even more confusing. Since 
is between 
and 
, the activity must be _____ x , where the missing number is found from the log table as follows: Subtract 7.3 from 8 (equals 0.7). Then find the antilog of the difference (equals 5.0 in this case). Therefore, a pH of 7.3 corresponds to a hydrogen ion activity of 
.
Excerpts used with permission from Beckman Instruments. 1973. Experiments in pH and Potentiometry. Technical Report 587. Beckman Instruments, Inc., 2500 Harbour Blvd, Fullerton, CA 92634
Updated: Monday, August 27, 2007.
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