Acceptance Criteria For pH Meters and other Laboratory Equipment

6thsense

Involved In Discussions
#1
An auditor gave us an improvement opportunity for Including the Acceptance Criteria for our pH meters. I am looking for help regarding how some people go about this with regards to pH meters and other Laboratory Equipment.

Thank You Covers
 
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Qara123

Involved In Discussions
#3
An auditor gave us an improvement opportunity for Including the Acceptance Criteria for our pH meters. I am looking for help regarding how some people go about this with regards to pH meters and other Laboratory Equipment.

Thank You Covers
By acceptance criteria, are you referring to acceptable results when performing a pH meter calibration/verification? If so, the pH meter maker should have the specification listed somewhere, probably in the manual.

Also, if you plan on using buffer standards to calibrate/verify your pH meter, I recommend buying the buffer in sachels (small packets) rather than an entire bottle because the shelf-life of the buffer is no longer valid after you open the bottle.

Q
 
A

arios

#4
The bottle of the buffer solution may already display a tolerance range for PH. I would suggest you to use that
 
A

amanbhai

#5
Certificed reference materials/ buffer solution has their values and tolerance specified for like 4 pH, 7 pH & 10 pH values.
You can use them while calibrating your pH meters.
 

Doug Tropf

Quite Involved in Discussions
#6
"the shelf-life of the buffer is no longer valid after you open the bottle"

Can you explain how the shelf-life is invalidated by opening the bottle? Our bottles of buffer solution have an expiration date printed on the label but it has nothing to do with when the bottle is opened.
 
A

arios

#7
"the shelf-life of the buffer is no longer valid after you open the bottle"

Can you explain how the shelf-life is invalidated by opening the bottle? Our bottles of buffer solution have an expiration date printed on the label but it has nothing to do with when the bottle is opened.
I agree with Doug that such comment does sound clear.

If the bottle expires when it gets open, then it means that the buffer solution can only be used the same day. That does not make much sense :confused:
 

Marc

Fully vaccinated are you?
Leader
Admin
#8
The date on the bottle is the shelf life of the unopened bottle. Once you open it contamination can creep in and make the date on the bottle useless. Or, in cases where the bottle contains something that is sensitive to air, you have a different life time of the contents once it has been opened and air has crept in. Just opening the bottle does not mean you have to use it all on that day, or that it is only good for a day or anything like that.

The closest analogy I can think of is food. Let's say you buy a piece of meat which is wrapped and in the a grocery display case which has a "consume by" date. The meat was probably packed in a relatively sterile environment (especially if it is "corporation processed food") in accordance with local, state and federal laws (at least in the US) or other requirements (such as FDA rules and regulations). You bring the meat home and open the package and transfer it to another container. Once you have opened the package you have exposed it to an un-sterile environment and it may "go bad" prior to the "consume by" date. It doesn't mean you have to eat the meat that day. It means that the meat may go bad prior to the expiration ("consume by") date.

The question here is, assuming no contamination such as by someone using the actual bottle such as sticking the probe in the bottle, which I have seen people do, how stable is a buffer solution once the bottle has been opened and exposed to air. This I can not answer. I don't remember too much about buffer solutions and their stability so I really can't say how long after the bottle is opened the solution is good for.

Interesting topic. Anyone here knowledgeable in buffer solution stability?
 

BradM

Leader
Admin
#9
Well... I certainly am no expert here or anything, but just some thoughts:

General: We typically calibrate the pH meter for millivolts, although they are rarely off. I think the tolerance is like 1 millivolt.

We calibrate the temperature compensation module (if one is installed), if the solution being measured is at a different temperature than the buffer solutions. pH is highly affected by temperature.

Accuracy: I'm not sure if we have a specified accuracy, per se. You are basically adjusting the meter to the solutions, so not sure where an acceptance criteria would come into play. Generally they keep track of the slope, which should fall somewhere between 95-105. Outside of that, are there is something wrong (probe/solution/etc.).

Solutions: I really cannot speak as to the exact expiration and such of buffer solution. However, we had a large number of problems with meters not adjusting, and called Orion. After about the sixth time they told us the same thing,:) we finally did it. We had about a tenth of the problems after. What was it you ask?:D

Get very small bottles of buffer solution, and have more orders of them made. So instead of having larger bottles that stayed in the lab longer, we got smaller bottles and bought larger lots. So a given lot of solution did not stay around very long.

It cannot be stressed enough how important it is to constantly rotate your pH solution. Buy smaller containers and purchase it more often. It's a little more expensive, but how much is good pH measurement worth?;):agree1:
*****
We get a call that states something like this:
My pH meter won't accept the solutions. Please come and calibrate it.
On these calls, 999/1000 times we end up replacing the probe or get new solution, and it works. :tg: Those probes (general glass ones with solution in them) do have a shelf-life. The other 1/1000 is usually some meter setup issue.

As to the OP's original question, as long as traceable solutions are purchased, then refer to the material listed with the solutions. That will give you the accuracy of the solutions. Past that, I'm not sure. A "fresh" pH meter that was just set up has a very short shelf life before needing to be verified again. That shelf life is even shorter if the meter is being used at extremes (highly caustic-highly acidic-neutral, etc.).

If you are really interested in the quality of your pH measurement, I would suggest conducting an uncertainty analysis on the system.

Here are the specifications for an Orion meter (as an example):

http://www.thermoscientific.com/eco...&groupType=PRODUCT&searchType=0&storeId=11152

You could then get the analysis for you particular lot, do some R&R, etc. to develop the collective uncertainty.:)
 

v9991

Trusted Information Resource
#10
To add onto what Brad has already covered...Pharmacopeia specifies an acceptance criteria of pH +/- 0.02 (essentially what is done is after adjusting the controls/slope against known buffers; at the end... third unknown buffer is measured and verified for above acceptance criteria)

Hope that helps.
 
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