Expert: George Maxwell - 8/18/2007

Question
QUESTION: If you are given a colored substance as a sample, how would you measure the pH of that sample?

ANSWER: Hi, and thank you for your question.

Some coloured substances, like copper sulfate solution, don't dye indicator papers (paper soaked in indicator solution), so you could use those instead of liquid universal indicator.

However, the most common method is to use a pH meter: this is really just a modified voltmeter that measures pH by indicating how good a sample is at conducting electricity.The meter reading is calibrated against standard samples, so you just put the probe in to the sample liquid and read the pH off the screen.

Even with colourless liquids, this method is used when you need more accuracy than indicator solution will allow.

Hope this helps,
Many thanks,

---------- FOLLOW-UP ----------

QUESTION: hi, thanks for answering my previous question. I am having trouble with the acid-base indicator phenophthalein. How come phenophthalein has an inherent error at a pH level of 8? How can you describe the error?

Answer
Hi, and thanks for your question.

Summary: it's uncertain because the indicator is at the turning point between colours, and slight variations it surroundings (other than pH) can affect the point at which the change occurs.  



An indicator, like phenolpthalein, works because the indicator molecule changes its shape at a certain pH (when there is a certain concentration of hydrogen ions surrounding the molecule).

When a molecule changes shape, the way it interacts with light passing through the indicator solution also changes - the solution changes colour.

There is a point at which the concentration of hydrogen ions (the pH) is just reaching the level where the indicator molecules in solution are starting to change shape, but haven't all changed. With Pheno. this occurs at pH 8.2, so around this pH, the point of colour change is uncertain and depends on things like the temperature and concentration of solutions as well as the pH.

Hope this helps, thanks again for the follow up.