What are the problems with the effect of pH on enzymes practice?

The problems which the effect of pH on enzymes practice are that samples are only taken every 30 seconds, so there is only an approximate time for the reaction to complete, this problem could be addressed by taking samples every 10 seconds Secondly the person carrying out the experiment is looking for the time it takes for the iodine to not go blue-black which may not always be obvious as the colour change tends to be gradual and some wells might have blue-black mixed with orange, this problem could be addressed by asking several opinions on wether the colour change has occurred for the reaction to be complete