With so little information, it is difficult to give a definitive answer. Assuming that you have five treatments with at least three measures of each, the simplest analysis would be ANOVA subject to the assumptions being satisfied. Repeated measures or other design differences might complicate the analysis.

Hi Radwa,

My initial understanding of your study is effect of pH (3 independent l variables) on absorption of compound (dependent variable). Based on this, I'll suggest you use factorial ANOVA.

Remember, you can use SPSS General Linear Model (GLM) procedures to carry out a 3-way ANOVA in order to show the thought process behind a factorial ANOVA.

The primary purpose of a factorial ANOVA is to determine how the independent variables (factors) impact a response variable, and whether or not there is an interaction between the factors on the response variable.

In any discipline, studies investigating differences between groups will use post-hoc tests when the null hypothesis of an ANOVA model is rejected.

However, before you do this analysis, you need to check

the integrity of your data - the six assumptions that underpin a factorial ANOVA must hold in order to give a valid result.

To analyze the effect of pH on desorption considering three replicates for each of the five pH values, you can employ a mixed-effects model in R using the nlme package.