pH Measurement Discrepancy

[Krulligo]

So I work in a biological research lab and have access to a laboratory grade probe-style pH meter (Mettler Toledo FE20). Out of curiosity, for the past few pH measurements that I did for my pool (chlorine) and my spa (bromine) using my Taylor K2006C, I also brought a sample in with me to measure using the lab pH meter. I would always calibrate the pH meter with fresh reagents for a curve between pH 7 and 10. Interestingly I am always getting a much lower pH measurement with the lab pH meter compared to the Taylor kit readings.

For example with todays test, using the Taylor kit, my pool measured at 7.5 and my spa measured at 7.6. However, using the lab meter, I was getting 6.95 for the pool and 7.05 for the spa.

I was just wondering why the discrepancy. I am hopeful that the Taylor measurements are correct and the kit is specifically designed for use with pool/spa water. However, the lab grade pH meter is a more sophisticated instrument that has an accuracy range of -/+ 0.01. My only thought is that the presence of chlorine or bromine obscures the lab meter results, but any other ideas?

 

[JoyfulNoise]

I’d suggest getting a new bottle of pH reagent. The difference on color between 7.5 and 7.0 is pretty dramatic - red/orange versus yellow.

Also, what is the FC of your water samples? How long are the water samples sitting around before you test them at work?

 

[Krulligo]

I’d suggest getting a new bottle of pH reagent. The difference on color between 7.5 and 7.0 is pretty dramatic - red/orange versus yellow.

Also, what is the FC of your water samples? How long are the water samples sitting around before you test them at work?

The Taylor K2006C is new (ordered last month) and although I cannot check right now, I think the pH reagents expire in 2024, so I don't think that is the issue.

So today I measured the pH using the Taylor kit this morning, and then I took the rest of the same water sample with me to work and measured it using the lab pH meter about 1.5 hours later. I did measure the pool chlorine and spa bromine with Taylor kit and pool was at 6.5 ppm chlorine and spa was at 4.5 ppm bromine. All I can really think of is the lab pH meter is not happy with having chlorine and bromine in the solution (but this is hard to believe). The one other thing I can think of is that the solution inside the probe might be a bit old (I don't know when it was last replaced) and is throwing off the pH results a bit. Unfortunately I dumped my testing water already, but next time I will try replacing the probe solution.

 

[Katodude]

My experience is that the reagents seem to match to my pH meter around 7-7.2. The orange is very easy to discern for me. However, at above 7.6 I cannot tell anything with the reagents. I trust my pH meter. I calibrate when it needs it and test it against standard solutions so I know it is accurate.

 

[Stump]

Use the Taylor kit to measure your tap water and then use the PH meter at work to compare.

 

[JoyfulNoise]

The Taylor K2006C is new (ordered last month) and although I cannot check right now, I think the pH reagents expire in 2024, so I don't think that is the issue.

So today I measured the pH using the Taylor kit this morning, and then I took the rest of the same water sample with me to work and measured it using the lab pH meter about 1.5 hours later. I did measure the pool chlorine and spa bromine with Taylor kit and pool was at 6.5 ppm chlorine and spa was at 4.5 ppm bromine. All I can really think of is the lab pH meter is not happy with having chlorine and bromine in the solution (but this is hard to believe). The one other thing I can think of is that the solution inside the probe might be a bit old (I don't know when it was last replaced) and is throwing off the pH results a bit. Unfortunately I dumped my testing water already, but next time I will try replacing the probe solution.

Not all pH probes are created equally and there are different types of probes for different applications. Pool water has an incredibly high ionic strength due to all the dissolved ionic solids as well as a very high ORP value. I’m not sure a pH meter designed for use in biological applications is properly suited for measuring pool water. If you were an industrial lab, like a cooling water plant or a water treatment facility, then the probes would be better suited.

And yes, old probes can become very sensitive especially if they are the type where the calomel or potassium chloride solutions need to be regularly drained and refilled. In the lab I worked in we always opted for sealed glass probes and simply disposed of and replaced them on a regular basis rather than fiddle around with solution changes as you could never ensure that it was being done properly.

 

[JoyfulNoise]

If you’d rather use a pH probe, the Apera60 is a very good and rugged pH probe that is designed to work with pools and has replaceable tips.

 

[HermanTX]

If you’d rather use a pH probe, the Apera60 is a very good and rugged pH probe that is designed to work with pools and has replaceable tips.

this is what I use and have been very pleased with it.

 

[Krulligo]

If you’d rather use a pH probe, the Apera60 is a very good and rugged pH probe that is designed to work with pools and has replaceable tips.

Thanks. Wasn't really interested in switching to a probe since I find the Taylor one easy to read. Was just a bit confused as to why the discrepancy and whether this can be explained somehow with pool water being incompatible with this instrument.

I did take a look at the fill solution in the probe and it did look like there was some precipitate starting to form so I have cleaned it out and replaced it with fresh fill solution. Will bring in some more water to test tomorrow.