Comparing electronic pH measurement with that obtained by Bench Photometer

[New Reefer]

Hi everyone,

I have been using an industrial pH controller/meter on my tank for the past 5+ years, and have had no reason to doubt it's readings - I replace the electrode every 18 months, and calibrate the unit monthly with pH 7.01 and 10.01 buffers from Hanna Instruments.

I have recently purchased a Hanna Instruments Mod. C99 bench Photometer, mainly to test for phosphate, but would like to also make use of some of it's other capabilities, such as testing pH (as a check on the electronic meter).

After a fresh pH electrode calibration, my tank water was reading 8.1 on the digital (electronic) controller. The photometer, using the Hanna HI93710-01 Phenol Red reagent, gave an "over-range" error, thus indicating that the tank water was at or above a pH of 8.5

Now, a discrepancy of +0.4 pH units is quite large, and I would like to know the following:

- Which instrument is most likely to be accurate?
- If the fault is with the photometer, would this be caused by something in the salt water interfering with the test? if so, what?
- is there something I can do to check the accuracy of the photometer - perhaps make up a solution of a known pH within it's operational range of 6.5 - 8.4, and test both instruments against this (I am guessing that using one of the calibration buffers would not be a good idea, as being highly buffered and already containing a color indicator, it would not work with the phenol red...)

Any further advice on using the photometer will also be appreciated - perhaps I'm just being stupid and not doing the test correctly (although I am following the instruction manual to the "T")

Thanks for any responses.

Hennie

 

[New Reefer]

Just to add, all fish, corals, etc are doing well, and there is no sign of any problems in the tank, so I'm inclined to believe the digital controller, rather than the refractometer (although everything would still be healthy and looking good at a pH of 8.5 - 8.6, so I guess one could not really go by this either...

 

[toaster77]

I would trust your digital meter - especially if you have been calibrating and it reads the buffers 7.0 and 10.0 accurately from time to time.

The photometer/colorimetric assays for pH are less precise in general. They probably use indicator dyes and give a pH range at best.

 

[New Reefer]

<a href=showthread.php?s=&postid=14099042#post14099042 target=_blank>Originally posted</a> by toaster77
...The photometer/colorimetric assays for pH are less precise in general. They probably use indicator dyes and give a pH range at best.

Thanks for your reply

The instruction manual from Hanna Instruments states that it has a resolution of 0.1pH unit, with an accuracy of ±0.1 pH unit. The discrepancy I am getting is much more than this value.

This article: =http://pubs.acs.org/doi/abs/10.102...determination of seawater pH using phenol red suggests: "Phenol red is particularly well suited to pH determinations in cold sea water...

Potentiometric cells containing glass electrodes have, in general, proven satisfactory in measurements of solution pH. However, the behavior of potentiometric cells in seawater pH measurements often falls short of what is deemed acceptable... "

whilst a second article by RH Byrne, G Robert-Baldo, SW Thompson and CTA Chen, titled: "Seawater pH measurements: an at-sea comparison of spectrophotometric and potentiometric methods" state:

"Potentiometric and spectrophotomic pH measurements track one another quite well over a range of more than two pH units. the exceptional agreement obtained in this comparison when (pK2-1)<= pH <= pK2 suggests that sulfonephthaleins are particularly well suited to quantitative seawater pH measurements within their optimum indicating range." (this from: "Deep-sea Research Vol 35 No 8, pp 1405-1410, 1988")

Given the above two extracts, one could but wonder if the electronic/digital pH measurement really IS the most accurate. As a non-chemist, I really don't know, and would appreciate input & feedback from the "boffins" on this Forum.

Hennie

 

[Randy Holmes-Farley]

IMO, I would not use the photometer for pH.

As best I can tell, ALL pH measurements that are important are made using pH meters and electrodes, unless it is technically unfeasible or if one is really concerned with details about what pH scales really mean (hence there are different pH scales used in seawater, etc).

For our purposes, a meter is by far the best choice. I would not rely on or believe that Hanna if it disagreed with a properly calibrated pH meter.

 

[New Reefer]

Thanks, Randy - your advice is much appreciated, as usual

Would you mind replying to this question as well:

"- is there something I can do to check the accuracy of the photometer - perhaps make up a solution of a known pH within it's operational range of 6.5 - 8.4, and test both instruments against this (I am guessing that using one of the calibration buffers would not be a good idea, as being highly buffered and already containing a color indicator, it would not work with the phenol red...)"

Hennie

 

[Randy Holmes-Farley]

You can just put a pH electrode into it.

FWIW, dye error may well be a potential source of error. With a photometer, one must know exactly how your dye responds as a function of pH in the solution of interest. If the dye is set (calibrated/understood/etc.) for fresh water, then it will likely be off by some variable amount in seawater. I'm not sure if the Hanna folks understand this and/or take it into account properly. In short, the pKa of dyes shift as ionic strength changes, and it may also shift with specific ion interactions.

 

[New Reefer]

Thanks again, Randy.

Darn... There goes another bright idea of mine...

Quite a few people have used this (or similar) instrument to determine phosphate content - and that's the main reason I bought it - I hope that the die color changes between fresh water and sea water won't affect the accuracy of this particular test as well...

Hennie

 

[pjf]

What is the lowest concentration that your C99 can measure phosphate accurately at?

 

[New Reefer]

<a href=showthread.php?s=&postid=14104406#post14104406 target=_blank>Originally posted</a> by pjf
What is the lowest concentration that your C99 can measure phosphate accurately at?

Using the Phosphate Low-Range reagent it has a resolution of 0.01 mg/l, with a published accuracy of ±0.04mg/l ± 4% of reading.

 

[MCsaxmaster]

I've used spectrophotometric methods for seawater pH and total alkalinity determination (Byrne et al., Clayton and Byrne) and like them. I think that spectrophotometric pH measurments, with m-cresol purple, are probably preferable for absolute pH measurments in sea water for high precision work. However, if you're not using a good spectrophotometer and can't get the entire spectrum, I'd go with the pH meter. That's probably going to be doing the better job. Now, if you want to plunck down several grand for a good OOI spec + accessories, I can tell you what to get....

 

[New Reefer]

<a href=showthread.php?s=&postid=14105441#post14105441 target=_blank>Originally posted</a> by MCsaxmaster
I think that spectrophotometric pH measurments, with m-cresol purple, are probably preferable for absolute pH measurments in sea water for high precision work. However, if you're not using a good spectrophotometer and can't get the entire spectrum, I'd go with the pH meter.

Thanks for your input.

Unfortunately the Hanna instrument is pre-programmed for the phenol red die, and won't give meaningful readings if another die is used. As Dr. Randy said, it will even give false results using the phenol red if used in seawater, due to the slight change in color caused by something in the salt water.

I would love to have a "proper" photometer, but of course it's not financially viable for use as a hobby testing instrument. I bought the Hanna C99 second hand for a very good price, else that would also have been out of the financial range...

Hennie

 

[EnglishRebel]

<a href=showthread.php?s=&postid=14099956#post14099956 target=_blank>Originally posted</a> by Randy Holmes-Farley
You can just put a pH electrode into it.

FWIW, dye error may well be a potential source of error. With a photometer, one must know exactly how your dye responds as a function of pH in the solution of interest. If the dye is set (calibrated/understood/etc.) for fresh water, then it will likely be off by some variable amount in seawater. I'm not sure if the Hanna folks understand this and/or take it into account properly. In short, the pKa of dyes shift as ionic strength changes, and it may also shift with specific ion interactions.

Randy
Sorry if I ask a stupid question here but when you talk about "photometric" instruments being less accurate than electronic ones, are refractometers a type of "photometric" device? Is a refractometer the most accurate device that the home hobbyist can use (I'm ruling out anything over a couple hundred dollars here)?
Thanks

Hennie
Sorry to butt into your thread.

 

[therealfatman]

Good new portable conductivity meters cost more than $200. It is not unusual to pay that amount or more for a good used portable conductivity meter.

 

[Randy Holmes-Farley]

Sorry if I ask a stupid question here but when you talk about "photometric" instruments being less accurate than electronic ones, are refractometers a type of "photometric" device? Is a refractometer the most accurate device that the home hobbyist can use (I'm ruling out anything over a couple hundred dollars here)?

I was only referring to measuring pH, not anything else.

That said, a conductivity meter for salinity can be at least as accurate as a refractometer.

 

[pjf]

<a href=showthread.php?s=&postid=14104451#post14104451 target=_blank>Originally posted</a> by New Reefer
Using the Phosphate Low-Range reagent it has a resolution of 0.01 mg/l, with a published accuracy of ±0.04mg/l ± 4% of reading.

I'd like to get a photometer but I am puzzled by some of the specifications. If the reading is 0.01 mg/l, is the accuracy ±0.04mg/l or is it ± 4% of reading?

 

[Randy Holmes-Farley]

It is hard to say exactly how the stated specs translate into actual potential error at any given measured value. It may not be as high as +/- 0.04 when reading 0.01 ppm, but I would not assume it is as low as 0.01.

 

[Randy Holmes-Farley]

To expand a bit on the idea that dyes vary in pKa with salinity, here's a free pdf that shows the method of using m-cresol purple to measure seawater pH.

http://132.239.122.17/co2qc/handbook/sop07.pdf

My point in posting it is mostly to show the salinity dependence. They show an actual correction needed at 30-37 ppt, and the effect on the dye pKa (and hence the measurement) is quite large if you do not make that correction.

At 0 ppt, the pKa is 8.32

from

http://books.google.com/books?id=Ez...&hl=en&sa=X&oi=book_result&resnum=2&ct=result

at 30 ppt in the correcting equation it is about 7.6

and at 37 ppt it is about 7.5

So if a photometer method is made for freshwater and used in seawater, it could be off by 0.8 pH units using this dye.

 

[rexdenton]

ph meter

ph meter

I am guessing that the impedence would be lower than the pH meter, so trust the pH meter. While a pH meter is nothing more than a glorious voltmeter, a pH meter is distinguishes itslef from other garden variety voltmeters by being built with extremely high impedences. This is done because the pH probe itself can, and does impact the measured voltage when you test the solution (your tank). While I am not familiar with your other unit, it would seem to be the infereior choice for measuring pH. You may be able to find a spec sheet an compare the impedences of the two units to see if this assumption is correct.

 

[New Reefer]

Re: ph meter

Re: ph meter

<a href=showthread.php?s=&postid=14154350#post14154350 target=_blank>Originally posted</a> by rexdenton
I am guessing that the impedence would be lower than the pH meter, so trust the pH meter... ...You may be able to find a spec sheet an compare the impedences of the two units to see if this assumption is correct.

Umm, no, I don't think you have this quite right... the photometer shines a light through a glass cuvet (small bottle...) and measures the transmitted light, if I understand this correctly. It does nor make use of any electrical measurement in/of the water, so there is no impedance involved with this measurement (unless one would consider fingerprints and/or other dirty marks left on the cuvet as being "impedance"...)

To expand a bit on the idea that dyes vary in pKa with salinity, here's a free pdf that shows the method of using m-cresol purple to measure seawater pH.

Thanks, Randy - really interesting. Is there any way to determine the pKa change (and thus a correction factor) for the phenol red method used by the Hanna photometer?