Question related to DoE for degradation of chemical compounds

[#1]

WARNING: noob question!

I want to use DoE to model the degradation of different compounds we monitor during stability studies. We know that there are three factors influencing degradation (all continuous) but want to extend our understanding to relatively extreme values of these factors. Eventually we want to use the experimental results to model degradation.

What I still have to understand is how time is factored in. Time naturally has an influence, but in all our stability studies only specific time points are sampled and we always use a fixed shelf-life for our product. 

Where do I start? Do I want to model the degradation slope(s) or do I want to model the residual fraction(s) of an active compound after a fixed time? Are there any JMP tutorials I could use? 

[#2]

Sorry returning in the middle of the holiday season, but I have a follow-up question.

Are there any precautions to be taken for leveling of factors that are non-linear, such as pH? pH is a logarithmic parameter, so what appears to be a linear change from say pH 4 to 5 is actually a shift by a factor of 10, i.e. one order of magnitude.

[#3]

We use p-functions of variables so that they are more useful. The order of magnitude change in hydroonium activity is not useful in many computations so we use pH instead. In those cases, it allows for a simpler relationship, often a linear relationship.

No, there really are no pre-cautions. Use whatever version (original or transformed) works best for you. We use pH, not [H+], all the time because it works.

[#4]

As Mark suggests, pH is already a rather useful transform, so nothing to add here.  I just want to reiterate, pH is not a factor (an X) in your study, but is a response variable (a Y).

[#5]

Thank you. But actually we are adjusting specific pH values to measure the response in something else. What am I getting wrong about referring to pH as a factor?

[#6]

Perhaps it is semantics. There is likely a hierarchy of cause-effect (Y=g(y)=f(x) where g and f represent functional relationships).  Your original discussion was about factors affecting pH.  You can't directly adjust pH ( Y, the concentration of H ions), but you may be able to adjust the factors that affect pH (X's, your hypotheses regarding time and temperature). Let's say we now want to investigate whether ph (and/or other factors) affect some other measure, for example, crop productivity.  In this case we might "create" or sample levels of pH in the soil and test the effects of pH on crop productivity .  In this case we consider pH to be an 'X' and crop productivity to be a 'Y'.

[#7]

Semantics - to some extent, yes. True, we can't know the true hydronium concentration. What we can do, is to estimate hydronium activity. In our case we adjust hydronium activity to a desired level and measure its effect on something else. I therefore refer to it as "factor". My original discussion, I'm pretty sure - not about this topic. HAPPY NEW YEAR!