pH paper strips.

Great stuff for small samples (a few ul pipetted) on a strip of paper that covers the needed pH range you wish to check. For example: EMD ColorpHast brand of non-bleeding pH Indicator Strips are available in a wide range of pH.

Universal Indicator Solution (pH). This is another option if you really wish to use a spectrophotometer to record the results. The solution can be purchased from most any chemical or catalog supply house and changes color depending on the pH of the solution it is in.

Thanks for your help, Michael and Bill. I need to measure the rapid decline of pH in the samples over time, so I think spectrophotometrically is the most accurate way to measure.