Often times, biochemical behavior is dependent on pH. I have 8 chemical factors, that need to be evaluated at two different pHs. The problem is that I can' t mix pHs. For example, the pH factor has settings of either 6 or 8. This requires that I provide chemical solutions (buffers) that has been adjusted to pH 6 and pH 8. In each case the chemical is the same but is adjusted with acid or base resulting in the two differing pH values. If I provide them as two solutions and thus two factors, the designs invariably mix these two factors together which is undesirable. I there a way to include pH in a single design or do I need to make two designs, one at pH 6 and one design at pH 8.
If I allow them to be mixed then the mixture would be pH 7. However, the concentration of the buffer would be twice as high as it would be at pH 6 or pH 8, therefore the effect of pH would be confounded with the effect of buffer concentration.
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Great suggestion! I'll give it a try. I hadn't stopped to think of a continuous variable in categorical terms. Thanks for your help.
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