CYA Analysis by UV

[SBax]

I added CYA to new pool water but under-added on purpose to not overshoot.

I used the standard addition method to check CYA levels.

1) Make up 1000 ppm CYA standard
2) Add standard to 0.1 N NaOH for UV responsive tautomer form of CYA
3) Run UV response scan and note maximum wavelength abs (222 nm)
4) Make 4 samples from actual pool water. Add the required stock solution to get 5, 10, 20 ppm of additional CYA. One sample as is. Add NaOH to get 0.1 N in all samples.
5) Make 0.1 N NaOH blank.

Run the four samples and graph for slope-intercept.

Calculate x-intercept and re-graph. Add one more data point 0,0. R square is 0.9995.

Does this look right chem geek?
Thanks.

 

[chem geek]

This looks right if the second graph is "offset" or zeroed by the "pool sample as is" with 0 ppm CYA measurement, which is what it looks like you were doing. Is the purpose of this to see if you can measure CYA levels using a UV scan instead of a turbidimetric test?

If your UV scan has a broader range, specifically in the area of 290-380nm, then there are some experiments that would be GREAT for you to do since we've never really figured out the CYA absorption effect (extinction coefficient) in that range where hypochlorous acid and hypochlorite ion break down from sunlight. You'd need to scan at a range of wavelengths (every 10 nm should be plenty) at a known CYA concentration with pool water conditions (i.e. buffered pH near 7.5) relative to a sample with no CYA and with a known path length through the sample.

If you could then add chlorine at some known FC, then that would add the absorption of the chlorinated cyanurates to the mix. You could even see what happens with chlorine and no CYA (I have those extinction curves already, but these would be useful for comparison to your equipment).

This could help greatly in trying to model the chlorine breakdown rates as a function of CYA level. Finally, repeating the above at 7.0 and 8.0 would also be helpful, though not as important as the above.

Richard

 

[SBax]

I also had a co-worker verify my work and he says it looks good. Yes, the purpose is to avoid a home test kit because I work in a lab and can run a few tests during lunch time (pH, TA, and Cl).

I don't often work in the area where the uv is located so I can only test CYA occasionally. But, I'll try to run some in-depth uv testing if I can sometime later.

Thanks for your comments.