Hi,
Does anybody know of a way to measure phenol red concentration?
Thanks

Well, if you have phenol red in your hands (it's very cheap). Prepare a set of dilutions at the same pH that has the sample of which you want to know the concentration. Take it to the colorimeter and measure the absorbance of the dilutions, measure also your interest sample. With the know concentrations you can build a curve or line that show absorbance vs concentration and you can search the absorbance value of your unknown concentration sample to know its concentration.

Some clues:
Phenol red has low solubility in water.
It will be orange at neutral ph. When it goes acidic, under 6.5, it will turn yellow, when it goes alkaline, over 8-8.5 it will turn pinkish. If there is a high concentration it will remain mainly red regardless the pH.

If your solution is red then the concentration is probably high.

Of course you have to set the colorimeter to the appropiate wavelenght.

If you don't have phenol red around you can check somewhere where swimmingpool stuff is sold as it is used to meassure water ph, you may also try at pet shops.