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pH meter issues . . .

BigBella

BigBella

To those of you familiar with pH meters, I would like to pose an odd question. For some reason, I have recently had some difficulty in calibrating my HM Digital pH-200 meter with any of the testing solutions. When I attempted it, I could not manually calibrate the numbers by a long shot -- 4.01; 7.0; 10 -- even though I have tried different batches of the solutions, as well as different brands; have also figured in the temperature differences in terms of pH; and even the auto-calibration function is miles off.

The probe itself is undamaged and the tip had been replaced within the last year. To further confuse things, when I placed the probe in distilled water, it registered an an ideal pH 7.0 at 20˚ C -- exactly what to expect from distilled or deionized water . . .
 
electrode going funky. might have some mineral deposits on them. I actually remember doing rinses in distilled water and sanding things in the old days
 
kulamauiman said:
electrode going funky. might have some mineral deposits on them. I actually remember doing rinses in distilled water and sanding things in the old days
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Mach, I thought so too; but even with a powerful loupe, there is absolutely no trace of any deposit to be seen; and I religiously rinsed the probe in distilled or deionized water after each use . . .
 
battery going bad? internal hardware issue with corrosion?


in the distilled water try take a reading swirling it around
 
kulamauiman said:
battery going bad? internal hardware issue with corrosion?


in the distilled water try take a reading swirling it around
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Battery indication is fine; so too the display, so I don't expect internal corrosion. When swirling in distilled water, it drops into the pH 5.4-5.6 range, then settles to 7.0 or 7.01 after a minute or so . . .
 
Not to derail the thread, but I was going to get a cheapo pH meter to test the pH of my nep soil and I didn't want to get a bum one... What is the best brand?
 
Pineapple said:
Not to derail the thread, but I was going to get a cheapo pH meter to test the pH of my nep soil and I didn't want to get a bum one... What is the best brand?
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It is difficult to get a reliable pH reading from compost, since the probes basically measure hydrogen ion concentration in solutions. You're far better off getting a ballpark figure of the pH from the leachate in your drainage trays.

There are many on the market, but I would suggest one that allows push-button calibration over those that require a screwdriver . . .
 
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Back in the college lab days the probe on the pH meters we used had to be kept wet at all times in a storage solution when not in use. Letting them dry out meant ruining the probe. No idea with current units.
 
Not a Number said:
Back in the college lab days the probe on the pH meters we used had to be kept wet at all times in a storage solution when not in use. Letting them dry out meant ruining the probe. No idea with current units.
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Warren: it is still the very same situation. After rinsing, the probe is stored with a cap saturated with a pH 4.01 storage solution. it arrived from HM moist and hasn't dried in two years -- including the replacement tip . . .
 
  • #10
Is it possible that the pH standards are contaminated? I use a pH meter everyday at work and my experience with purified water is that it is not only a little acidic, the readings doesn't stabilize too well. The lack of ions to bind with interferes with stabilizing. We change out or storage solution every week. The probe, when not in use, is kept a small tube of that solution.
 
  • #11
jimscott said:
Is it possible that the pH standards are contaminated? I use a pH meter everyday at work and my experience with purified water is that it is not only a little acidic, the readings doesn't stabilize too well. The lack of ions to bind with interferes with stabilizing. We change out or storage solution every week. The probe, when not in use, is kept a small tube of that solution.
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It seems unlikely that the standards were an issue, since different lots and brands have been used in the last couple of weeks; and when I had mentioned that freshly-opened distilled water registered pH 7.0, it did fluctuate a tiny amount , as you had described -- in terms of .02 back and forth -- but settled in at about seven. The probe is also stored in a 4.01 solution, recommended by the manufacturer . . .
 
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  • #12
Each pH meter brand has a different setup and working operation. Some "should not use with distilled water" and some store without solution. Read careful your manual and follow it. IMO, don't measure pH 7.0 using distilled water because the meter measures the ion H+ in solution and there's (almost) none ion H+ in distilled water. Use the calibrate solution pH 7.0.
 
  • #14
flasker said:
Each pH meter brand has a different setup and working operation. Some "should not use with distilled water" and some store without solution. Read careful your manual and follow it. IMO, don't measure pH 7.0 using distilled water because the meter measures the ion H+ in solution and there's (almost) none ion H+ in distilled water. Use the calibrate solution pH 7.0.
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I have used pH meters for decades and the manuals have always been followed; there was never an issue with using distilled water with my brand; and I am fully aware of the pH issue with distilled water and its rapid "contamination" by C02 -- and took that into consideration. The water was freshly steam-distilled. I only used it because it was a known quantity in terms of pH and was trying to determine what was wrong with the meter. The probe is also new, within a year . . .

mobile said:
See the link below, particularly the pH-value chapter, with regards to pH of deionised water. The fact that your meter is reading deionised water as pH 7.0 doesn't necessarily mean it is measuring correctly.
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I realize what you're saying; but my first thought was that the batch of testing media was simply off; and why it was a sudden problem was also a bit of a mystery, since the probe is waterproof and the o-rings perfectly intact and lubricated. Also, I was generally using steam distilled water, though had some deionized on hand -- which also registered pH 7.0 . . .
 
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  • #15
The meter is probably toast. Did you try a new battery?
 
  • #16
Not a Number said:
The meter is probably toast. Did you try a new battery?
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Yep, you're probably right, though it is still within warranty -- and the company is in SoCal.

The batteries were the first thing I replaced.



Thanks one and all for the assistance and suggestions; but it's time to give it its last rights and this topic is as dead as disco . . .
 
  • #17
What's your pH readings when you measure pH solutions 4.0, 7.0 and 10.0 ? If the readings are quite different then your pH meter could be wrong.

As long as you don't pump CO2 under pressure into water, water absorb CO2 slowly. Tap water (pH ~ 7.5 - 8.0, basic) absorbs fast and more CO2 than distilled water. Measure pH of a cup of tap water, measure again after an hour you won't see any different (I bet).
 
  • #18
flasker said:
What's your pH readings when you measure pH solutions 4.0, 7.0 and 10.0 ? If the readings are quite different then your pH meter could be wrong.

As long as you don't pump CO2 under pressure into water, water absorb CO2 slowly. Tap water (pH ~ 7.5 - 8.0, basic) absorbs fast and more CO2 than distilled water. Measure pH of a cup of tap water, measure again after an hour you won't see any different (I bet).
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Within the course of a few hours, a beaker of freshly-distilled water can easily drop from pH 7.0 to as low as 5.8; and I had mentioned the issue with the solutions earlier in the thread.

It's all academic; I put in an order for a new meter . . .
 
  • #19
BigBella said:
I realize what you're saying; but my first thought was that the batch of testing media was simply off; and why it was a sudden problem was also a bit of a mystery, since the probe is waterproof and the o-rings perfectly intact and lubricated. Also, I was generally using steam distilled water, though had some deionized on hand -- which also registered pH 7.0 . . .
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Sorry, I wasn't trying to 'teach grandma to suck eggs'... might me useful information for other though, who naturally assume that demineralised water must be pH 7.0
 
  • #20
mobile said:
Sorry, I wasn't trying to 'teach grandma to suck eggs'... might me useful information for other though, who naturally assume that demineralised water must be pH 7.0
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No, that wasn't my thought at all; and thanks for the input. The manufacturer, HM Digital, is going to take care of it. Apparently there were some "issues" with a batch of their replacement probes . . .
 
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