1. The problem statement, all variables and given/known data Hi. I know this will sound silly, but I really don't get it. Let's say that I want to determine phosphate in a given sample by using molybdate. . After plotting the calibration curve using the absorbance and concentration of the phophate standard, I calculate the concentration of phosphate in the sample like this : Csample = Asample - intercept/slope But... why does the concentration of the sample is equal to the concentration of phosphate? I know that when molybdate is added to phosphate a complex is formed... so, why does this happen? In fact I found this: Phosphate will readily react with ammonium molybdate in the presence of suitable reducing agents to form a blue coloured complex, the intensity of which is directly proportional to the concentration of phosphate in the solution... Why is it proportional to the concentration of phosphate?