Dear Sir,

at first you must define the quality of your "phenol red" e.g.

Synonym: Phenolsulfonphthalein

Formula: C19H13NaO5S

Formula Wt.: 376.36

CAS: 34487-61-1

Characteristic: Water-soluble indicator

Notes: pH 6.8 yellow to 8.4 red

You MUST have a very dry and pure dye for the calculation of the concentration.

You MUST used quarz cuvette with a plug for the effect of the temperature.

You MUST read the following paper:

http://etd.aau.edu.et/dspace/bitstream/123456789/2338/1/95

Title: Spectroscopic pH Measurement

Using Phenol Red Dye

By

Tewodros Adaro 

You need a plug for your cuvette in order to have the same volume of water during a measure at room temperature and  40 ° C  p.e. .

What a kind of spectrometer are yyou using?

Please give your answer like :

The UV-vis

spectra of the solution containing phenol red dye in 0.1 M NaCl or 0.1 M SDS were

recorded between 300 nm and 1100 nm using a Shimadzu UV 1601 spectrophotometer.

Thank you.

JRG

Thank you very much for the response sir