I am trying to make a field pH testing kits for farmers using 0.3% Bromothymol Blue Neutral Aqueous solution. upon testing I found that the pH indicator actually decreasing the pH value of test sample by 0.5, generating false pH value. I also tried using alcoholic solution (0.3 % BTB in 60% Iso-Propenol), but nothing works. The pH variation is different at different acidity i.e. this solution decreases pH by 0.5 if test sample has 7.8 initial pH but if initial pH is 6.5 it will decrease by only 0.3. how to resolve the problem?