PH prob!!!

[eyedoc]

I have had PH values of 7.7-8.0 and suspecting a PH probe calibration issue I calibrated with PH 7 and 9 standards.
I used two different 7.0 standards and they don't agree!!!!
The PH meter is a Milwaukee SMS122.
The PH 9.0 standard seems OK.

I calibrated with Coralife PH 7.0 standard.

After this I used Pinpoint brand 7.0 it reads 7.4!

Repeating several times yields this result. The 7.0 standards don't agree.

This is a huge difference between standards. One means my tank PH is fine, the other means I have a acidity problem.

What do I do to solve this difference?
Are these brands known for problems with accuracy?
Thanks

[Randy Holmes-Farley]

What do I do to solve this difference?

Buy a third and hope it agrees with one of them. :lol:

Actually, I'd calibrate with one (Pinpoint), and then check the probe in borax from the grocery store:

http://web.archive.org/web/20021015005420/www.animalnetwork.com/fish2/aqfm/2000/feb/bio/default.asp

If it reads OK, then go with that one.

[eyedoc]

Actually , I hope the Pinpoint PH 7 standard is the one that is wrong.
If the Coralife is right then the tank is OK.

The Borax seems to be PH 9.0 and I thought my 9.0 standard is OK.

Could you comment on the reliability of the Pinpoint and Coralife standards?
What is the gold standard and I'll get it.
Thanks

[Randy Holmes-Farley]

No, I do not know anything in detail about the reliability of either standard.

Actually, now that I think about it, using the borax isn't the best way to distinguish the pH 7 differences. I'd get a new pH 7 standard from a third company. Maybe also a pH 10.

[eyedoc]

Would a PH 10 be better than the 9 I have now?

[Randy Holmes-Farley]

Not if the pH 9 is accurate and your meter is able to be calibrated with pH 9. But if you have a 10 and a 9, and they agree, that is further assurance that things are working OK.

You might find this article useful:

Measuring pH with a Meter
http://www.advancedaquarist.com/issues/feb2004/chem.htm

[eyedoc]

I just got off the phone with Milwaukee tech support (it is their meter).
He said different standards are not the same in terms of buffer concentration and other factors.
I need to only use their brand of standard. He was not surprised my two standards don't agree.
He will send me a few free packets of his standard.
I always thought all standards were interchangable.

He could just be trying to sell his brand, but he was very convincing. He seened to really know his material and was very helpful.


He also said that for his meter you only want to use PH7 and not the second higher or lower standard. He said you want to tell the meter what PH to be most accurate and using the 4 or 10 distracts from your expected target. This is not consistant with what I read.

[Randy Holmes-Farley]

They are interchangeable. :lol:

[eyedoc]

Here is an update.
I'm trying to find out if I really have a low PH problem and my standard PH solutions don't agree with each other. The Milwaukee rep said I must use Milwaukee brand standards for their probe (which you and the local dealer do not agree with). So I got standards that match the brand of the meters and brought in a second meter.

I just checked my early morning PH (lights off).
I used a Pinpoint PH meter which I calibrated today with Pinpoint brand PH 7 and 10 standards. It reads 8.0

I also used my Milwaukee meter which I calibrated today with Milwaukee brand PH 7 and 10 standards. It reads 7.8

Both probes calibrated perfectly with both their 7 and 10 solutions and I was able to go back and forth between 7 and 10 several times confirming good calibration. Everything was done at the same room temp.

I think 8.0 and 7.8 is a significant difference given how carefully I did this.
Also, this difference is clinically significant since 7.8 is low and 8.0 is OK.
As far as I can tell all inverts look good.
Ca is 480-500, alk is 11.2 DKH. I have a dual stage Ca reactor.
I just shut off the reactor given the high Ca.
Thanks for your input.

[MSHUR]

hm...my ph also drop down sinch i start using rowaphos..maybe its coincedent....mine aslo reads 8.0 in the morning....
ca-380
alk 8.7
mike

[eyedoc]

My situation is unrelated to yours.

I just don't know what my PH is given the different readings I'm getting.

[MSHUR]

i caliblrate mine with pinpoint solution 7 and 10..
maybe u should contacs milwaukee to finad out problem

[Randy Holmes-Farley]

I don't know why they would give different results unless they are not exactly the same pH (either intentionally or by mistake or contamination).

Do they say they are the same pH? Buffers are often not exactly 7.000 and 10.000.

[eyedoc]

Yes they say PH 7 and 10.

[Randy Holmes-Farley]

Well, I presume then that one or more is not exactly pH 7.00 and 10.00.

[eyedoc]

That sounds logical.
I would think the PH meters should be working.

I could try calibrating them both with the same solutions to confirm the meters work, they should then give the same reading.

But how would I determine which solution is wrong?

[Randy Holmes-Farley]

Oh, I thought the above reading was with the meters calibrated with the same solutions. Calibrating witht he same solutions sounds like a good test of the meters themselves (or their ability to use the particular standards that you bought that may not reallly be exactly 7 and 10).

It is not easy to tell which calibration fluid might be off. If you calibrate with one and get the right reading for the borax. Or you can get a new set of pH 7 and 10 from a third brand.

Or you can just split the difference and think of the pH as about 7.9. :D