pH, phenol red, 560nm wavelength, absorbance

[jlbarkley]

I work in industry water chemistry.  I am trying to figure out how effectively build a chart that would indicate the pH of water with phenol red using the absorbance value at 560nm wavelength.  I know the concentration of phenol red matters and it will change the absorbance of the pH.  So let's assume I am using 5ml of phenol red and 45ml of water.  I can figure out concentration later.

How would I even test the method?  How would I find an exact pH of 7, 7.1, 7.2, 7.3.....  etc. to even be able to figure out the absorbance?

I essentially want a chart that when I test 10 ml of water and test it on a Hach DR900 under the 560 wavelength I can read the meter output (absorbance), look at the chart, and know the exact pH.  There has to be some sort of calculation I can do for this.  I have searched everywhere.  I see graphs and curves for this exact thing, but it's all visual.  I can never find the exact data they're using so I can calculate it myself.

[exchemist]

2 hours ago, jlbarkley said:

I work in industry water chemistry.  I am trying to figure out how effectively build a chart that would indicate the pH of water with phenol red using the absorbance value at 560nm wavelength.  I know the concentration of phenol red matters and it will change the absorbance of the pH.  So let's assume I am using 5ml of phenol red and 45ml of water.  I can figure out concentration later.

How would I even test the method?  How would I find an exact pH of 7, 7.1, 7.2, 7.3.....  etc. to even be able to figure out the absorbance?

I essentially want a chart that when I test 10 ml of water and test it on a Hach DR900 under the 560 wavelength I can read the meter output (absorbance), look at the chart, and know the exact pH.  There has to be some sort of calculation I can do for this.  I have searched everywhere.  I see graphs and curves for this exact thing, but it's all visual.  I can never find the exact data they're using so I can calculate it myself.

Forgive the question, but why do you want to do it this way rather than using a pH meter? 

Edited January 20, 2023 by exchemist

[chenbeier]

Phenol red has a changing area from pH 6.4 to 8.2 yellow to Red. There is no exakt value for a pH. If you use photo meter there will be at 560 nm only one value. The absorbance is related to the concentration  of the indicator. But this is not what you want to know. You want to know the pH.

As already asked why not use pH Meter. For calibration your method you need anyway one.

Check the colour at different pH.

[John Cuthber]

On 1/20/2023 at 5:48 AM, jlbarkley said:

  How would I find an exact pH of 7, 7.1, 7.2, 7.3.....  etc. to even be able to figure out the absorbance?

Buffer solutions and/ or a pH meter.
But you will run into a problem.
You can set up a calibration and draw a graph of absorbance vs pH.
But it will depend on the concentration of the dye as well as on the pH.
It is difficult to be sure that you always have the same concentration of dye.
Most dyes are a bit unstable and a lot of them are not supplied as pure materials, but as crude products containing salt etc.
So, you would have to make a fresh calibration for each set of samples.
If you are going to do that- and use a pH meter to do it- you might as well just use the pH meter to determine the pH of your sample.

But there's another pitfall in this analysis.

pH meter probes always carry some buffer into the solution which you are trying to measure.
And that changes the pH of the sample.
To measure the pH of very dilute materials (like drinking water or even river water) you need specialist equipment.
I suggest that you contact the people who make pH meters and ask for their advice.