Letter 37042

pH Meters  

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We have been having trouble with our desktop PH meter lately. I'm not sure if it is the heat affecting it or an electrical problem? We were having trouble with our old meter,so first off we bought a new probe,that did not seem to work,we thought we got a defective probe? So then we bought a brand new meter with a brand new probe. Were still having the same problem. We usually let the probe sit in DI when not in use,then we tried letting in soak in PH buffer solution closest to the value that we use. What it is doing is not holding the calibration,we calibrate it with a PH 4.00 buffer solution,we take one PH and it has to be re-calibrated it drops down to about 3.50 or somewhere in that area? Any suggestions??

Brian C. Gaylets
lock manufacturer - Scranton, Pennsylvania, U.S.A.

I'm with you, pH meters suck. At least the one's I've used. The thing I don't like about them is that you have to calibrate before each use. You should be calibrating to two buffers, a low and a high. That could be part of the problem, and keeping the tip wet with DI water is the way to go, not the buffer solution. Otherwise, maybe you should get some advise about your particular unit from the vender you bought it from. Good Luck.

Sheldon Taylor    supply chain electronics Wake Forest, No. Carolina

We need more information, but here are a few observations. First of all, when you are not using the probe, you should store it in pH 4 buffer, not deionized water. When you calibrate, what other buffers do you use besides 4? If you are only using 4, then you are not calibrating the meter. What types of solutions are you measuring and what is the typical pH? How often do you use it and for how many measurements?

Jon Barrows      aerospace jobshop Tulsa, Oklahoma

Don't store your pH probes in DI water. Try pH 4 or 7 buffer. Many of the pH meter manufacturers sell a pH probe storage solution that may be better yet.

And, never store probes in a high pH solution or buffer.

Dave Wichern - The Bronx, New York

Add 1-2 drops of Conc HCl in deionzed water and leave the electrode overnight.

It is preferable to keep the glass electrode in slightly acidic solution instead of deionized water.

T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India

MOST pH meters are great tools, but they need to be cared for properly. Darn few are where I have worked.
A plating forman whined about the short life of the probe and how he could never get the same pH as I did in the lab. The first reason, I found the probe in a cup if plating solution and it had been there for hours. This was a frequent happening. They tried to prove that you could control a woods nickel tank by pH, and faulted the equipment when the pH never changed (near zero). I have seen them left out on the counter to a point that they were dry. I have seen them reuse buffers for days on end. I have seen probes with so much slime that they could not possibly work. I ddo not like high purity DI water for storage. Mix it with pH 7 buffer or use tap water if it is good water. Best is to buy the storage solution for that precise probe. Sometimes you luck out. We had two units that I bought for $12.50 at a bid sale - unused with a gell probe that I thought was a joke. We used one on the waste treatment line for over three years (same probe). then we broke out the second one and it lasted for nearly as long. I work in a US government lab and I am apalled by the miss use of pH equipment. Some are PHD's.

James Watts
- Navarre, Florida

I would like to thank everyone for there help first off. Here is a little bit more info....hopefully this will help. On our meter we have a one point calabration we usually use that one(calibrated at PH 4.00) we do have 7.00 buffer solution and 10.00 buffer solution in the house. We mainly use the PH meter to check our Ni baths and Cr(trichrome) baths. Our lowest Ni bath is 2.00 and our highest one is 4.20 our Cr we run at 3.50. We check the Ph's first thing in the morning and then directly rite after lunch(12:30p.m. E.S.T.) Ocassionaly we do check alkaline cleaners with a much higher PH but that is few and far between because we maintain our cleaners with a weekly maintnence add. For the longest time we were storeing the probe in (straight) DI water,now we tried another approach with keeping it in PH 4.00 buffer solution,we change the rinse waters daily along with the buffer solution,we keep the probe clean(useing 10% Hcl and water) we do that once a week,and we keep the machine and the surrounding area very clean,just seems that none of this seems to be working for us.

Regards

Brian C. Gaylets
lock manufacturer - Scranton, Pennsylvania, U.S.A.

My setting is multi-staged wash lines for powder coating. I never let the floor have the pH meter for the reasons that Mr.Watts mentioned, I give them those pH strips that are used for swimming pools, (all the purist may frown now).
I still don't like calibrating the meter constantly(instructions say calibrate before each use), to me that means to re-calibrate it if it gets turned off or turns itself off automatically when not in use which could be several times in a (24 hr)day.
Wouldn't logic dictate that your not supposed to store the tip in a solution that has a contaminate that could influence it's function? So I'd stay with DI or RO water, or a solution sold with the meter for storage.

Sheldon Taylor    supply chain electronics Wake Forest, No. Carolina

You can not calibrate a pH meter with only one buffer. You must use at least two different buffers. Preferrable bracketing the range of your samples. In your case, it sounds like a pH 2.00 and a pH 4.00 will work well for most of your work. I think that you will find that your problem goes away when you calibrate properly.

Jon Barrows      aerospace jobshop Tulsa, Oklahoma

I see what you are saying,that you can not calibrate with only one buffer,but...my meter has a setting for a one point calibration, is it acceptable going that route? I have tried everyones suggestions and nothing seems to work for me, I am starting to wonder if I am picking up interference from another machine or something of that nature,i t does mention somthing about that in the instruction manual.


Regards

Brian C. Gaylets
lock manufacturer - Scranton,Pennsylvania, U.S.A.

1 point calibration is possible on meters that set the isopotential point electronically. This is not as correct as using a 7 buffer for that point on a two point calibration.

A wild guess!! You are wiping the liquid off of the probe after use. This can impart a huge voltage (static electricity) on a piece of equipment that measures millivolts. Try just rinsing it off with a squirt bottle of clean water and shake very gently.

James Watts
- Navarre, Florida

BRIAN !!!!!!!!!!

You have ph meter problem,if your ph meter's electrical code is plug in closer to high voltage like (480 volts) area .Your ph meter have trouble to read right,try to move your ph meter away from high voltage area.

Popat Patel     plating shop Roseville, Michigan

To properly calibrate you must use 2 buffers at least. The most recommended procedure is to adjust the CAL screw or knob using pH = 7 buffer. This is the so called iso-potential point. Soak thoroughly w/DI water and then immerse electrode in the second buffer, say, pH=4 for acidic solutions, when reading stabilizes, adjust the SLOPE screw or knob to pH=4 and the meter will be ready for use(If measuring alkaline solutions use pH=10 buffer instead of pH=4.). You can check immersing electrode in the pH=7 buffer again and see if the reading is correct. Otherwise meter/electrode are in bad condition or wrongly selected. Sometimes dirty/dry electrodes can be restored using the HCl solution overnight, but I wouldn't recommend this as a normal practice.
I don't know how can you calibrate using only one buffer. I am not going to get into theory of it, but measuring pH is done by correlating pH to a EMF developed in the sensor, so you must have at least two points to make this correlation. If your meter has only one point cal (no slope adjustment) then you probably should seek a better pH meter.

I doubt this is your scenario, but fluoroborate nickel baths may eventually destroy a glass sensor membrane. In fact, almost any fluoride represent a hazard to glass sensors. Special electrodes should be used in such case.

As they said before, magnetic or electromagnetic fields (and I would add, incorrect grounding of equipment) can definitively affect meter operation. I've seen this before. Older equipment is more sensitive to this effect. Avoid strong electomagnetic fields when measuring pH.

Check out the temperature. If you calibrate at 25°C and measure a solution of 60°C you must allow for thermal equilibrium and compensate for temperature difference before taking the actual reading. Most newer meters have a built in or separate additional temperature sensor for compensation. If you take the hot sensor back to the cold buffer solution your pH readings obviously will not be the same until the system reaches the thermal equilibrium (this may take a minute or so).

Good luck! :-)

Manuel Sández
- Hermosillo, Son. Mexico

Again thanks for all your help out there I appreciate it alot. I am actuallly a waste treatment guy, I do know alot about plating but there is alot I don't know! Mr. Watts I do not wipe the probe off, we use a freshwater rinse and then two DI rinses,they are changed daily along with the PH buffer solutions, I have been trying the three pont calibration 4-7-10 but I think my problem is an electrical issue.

Brian C. Gaylets
- Scranton, Pennsylvania

This is a pretty long shot...

A very uncommon problem associated with the use of p.H meters is due to the fact they measure very small potentials(mV). I have encountered one case where the movement of a certain solution produced a static charge to build up. This resulted in sudden changes of the readings like which you have described and was also not rectified by replacing the probe. The solution to this problem was through the use of a grounding/earthing rod introduced into the solution.. all of a sudden the p.h. meter gave a stable reading.

I know.. strange, but true..

Regards,

Peter Van de Luecht
- Melbourne, Vic, Australia.

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