Are variables like pH or log(EC50) interval or ratio?

 Many statistics books emphasize the difference between interval and ratio variables. 

• A interval variable is a measurement where the difference between two values is meaningful. The difference between a temperature of 100 degrees and 90 degrees is the same difference as between 90 degrees and 80 degrees.
• A ratio variable, has all the properties of an interval variable, and also has a clear definition of 0.0. When the variable equals 0.0, there is none of that variable. Variables like height, weight, enzyme activity are ratio variables. Temperature, expressed in F or C, is not a ratio variable. A temperature of 0.0 on either of those scales does not mean 'no temperature'. 

What about pH and EC50 (or logEC50) values? Acidity is measured by pH, which is the negative logarithm of the concentration of H⁺ ions. The mid-point of dose-response curves is quantified as the EC50, often expressed as its logarithm. (This midpoint is sometimes called the IC50 or ED50).

First let's look at the variables without a log transform, H⁺ and EC50. Both come pretty close to the definition of a ratio variable:

• Does zero mean none? Depends on how you look at it. Zero is zero in an abstract way. But a value of zero can never be achieved with those variables. No aqueous solution can have zero hydrogen ions, and a drug cannot have an EC50 of zero. 
• Does it make sense to compute differences between two values, or the mean and SD of a set of values? The calculations wouldn't be meaningless. But they aren't helpful either. 
• Does it make sense to compute ratios of two values? Sure! 

What about the variables after a log transform to pH and logEC50?

• Does zero mean none? Not at all, as you'd get a different zero if you changed the units used to measure concentration. A pH of zero does not mean no acidity! Or maximum acidity. It means that the concentration of H⁺ ions is 1 molar, an arbitrary value.
• Does it make sense to compute differences between two values, or the mean and SD of a set of values? Yes. 
• Does it make sense to compute ratios of two values? Not at all. Because zero is defined arbitrarily, ratios are entirely meaningless. A pH of 4 is double a pH of 2. A pH of 14 is double a pH of 7. The two doublings have nothing in common - computing that ratio is meaningless. 

Bottom line:

• Expressed on a concentration scale as  H⁺ and EC50, these variables come close to meeting the criteria of a ratio variable. But it is a bit of a stretch.
• Expressed on a log scale as pH and logEC50, these variables clearly are neither interval variables or ratio variables. They really are in a category of their own. 

This table summarizes how pH and EC50 (logged or not) compare to interval and ratio variables.