Pinpoint PH monitor calibration question

kenyacat

New member
We calibrated this the best we could, but have a couple questions. It took about 8 tries going back and forth between the 7 and 10 to calibrate it. Is that normal to have that much trouble?

Sometimes it would take a good minute for the numbers to stop moving. I think we got it though.

Any suggestions on how to do this in the future? I guess you need to do it once a month. I am leaving it in the water 24/7.
 

Billybeau1

Premium Member
It is not unusual to have to go back and forth that many times.

I keep a glass of fresh ro/di to rinse the probe in between 7 and 10.

6 to 8 weeks should be good. It has been for me. :)
 

kenyacat

New member
I was rinsing after each. It didn't say to rinse after 10 before I went back to 7. It says.
- 7
- rinse
- 10
-7
-rinse
-10

Was I rinsing too much? Does it make a difference? I just couldn't see how to keep the 7 solution clean without rinsing the 10 off.
 

Randy Holmes-Farley

Reef Chemist
Premium Member
You do need to rinse between any fluids to not bring one into the other. It also takes time to equilibrate. 2-3 minutes is normal, 5 minutes is not unreasonable. I often walk away and do something else for a bit. If you want long enough at 7, then wait long enough at 10, you should be done, and should then read 7 and 10 correctly (after waiting a bit).
 

Randy Holmes-Farley

Reef Chemist
Premium Member
You're welcome

These articles have much more:

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

A Comparison of pH Calibration Buffers
http://reefkeeping.com/issues/2005-02/rhf/index.htm

from the first one:

Calibrating and Using a pH Meter
The most important aspect of using a pH meter is correctly calibrating it. Each meter will have a slightly different way of calibrating it. There are a number of general rules that are very useful, however:

1. Any analytical method, including measurement of pH, is best calibrated with standards that span the range expected. Most aquarists calibrate pH meters using two solutions of known pH. A meter that only allows one is a very poor choice. Using more is fine if your meter allows more than 2. When using 2 solutions to calibrate a pH electrode for use in a marine aquarium, one should optimally be below 8.0 (typically 7) and one should be above 8.5 (typically 10, but 9 is also sometimes used). If you are measuring pH in something other than aquarium water then there may be special tricks which are detailed below.
Using pH 4 and 7 is often done, but can be a less optimal choice because the range expected to be measured for reef aquarium water (about pH 7.8 - 8.6) is outside of the calibration range. In some cases the error is small enough that this is acceptable. In others, it may be a problem.

The tables below show the maximum errors that are attained from various errors in the standard buffer solutions themselves (with problems with the standards being the only source of error considered; in reality, there can be additional errors in real measurements). These tables were obtained by simply looking at how much the calibration solutions might vary (first column), and seeing how much the actual measured value can be off if both standards vary to the stated maximum error and in directions that result in the maximum measurement error (which turns out to be varying in opposite directions when using pH 4 and 7, and varying in the same direction when using pH 7 and 10 standards).

It is clear that with similar errors in the standard solutions, the errors in the measurements at pH 8-10 are smaller when calibrating at pH 7 and 10 than at pH 4 and 7. Whether these differences are important depend on the application and expectations of the aquarist.

Additionally, if you are measuring pH in a fluid of a lower pH (such as inside of a CaCO3/CO2 reactor, then calibrating at pH 4 and 7 is more sensible than calibrating at pH 7 and 10.



2. Make sure the calibration standards are either new or at least adequate for the purpose. New standards, and especially new standards in one-time-use foil packets are the best.
I have several bottles of pH fluid that I have been using for years, however. Occasionally I use a fresh bottle or packet to calibrate my meter. At that time I check the pH values of all of these older bottles, and note the pH on the bottle. I can then use that bottle for future pH calibrations BECAUSE my meter allows me to calibrate with standards at any pH (such as pH 7.03 and 8.85). If your meter does not allow you to input the pH values that precisely, then you will not be able to use this trick.

3. Rinse the pH electrode in pure fresh water before putting it into any calibration standard, and between each standard.

4. It takes some time for a pH meter to get a correct reading. So let the meter equilibrate to each standard long enough that the value stabilizes (say, within +/- 0.01 pH unit for 30 seconds or longer). Some meters beep or otherwise tell you when you are suitably equilibrated.

5. Stirring the solution can help the pH probe equilibrate to the solution, but it also encourages CO2 to enter the fluid. This CO2 can lower the pH of high pH standards, such as pH 8 and greater. I stir mine for about 30 seconds (often with the pH probe itself, though I've also broken them this way) and then let it sit to get a reading.

6. The temperature of the standards is important for two reasons. One is that standards actually change pH as a function of temperature. The other is the pH electrodes change their response as a function of temperature (described above). The change in standard solution pH as a function of temperature cannot be automatically adjusted for by inputting temperature into the meter, or via its ATC. It is an attribute of the exact chemistry of the buffer used. Some have pH that rises as temperature rises, and some fall as temperature rises. Others rise with temperature in some temperature ranges and fall with temperature in other temperature ranges. You should be aware of the exact pH at the temperature that you are using it. Buffers will often have such pH values as a function of temperature printed on the bottle. For example, a standard phosphate buffer has a pH of 7.000 at 25 ºC, but 7.04 at 15 ºC (a small difference). At the same time, a carbonate buffer with a pH of 10.01 at 25 ºC has a pH of 10.12 at 15 ºC (a larger difference).

7. After you calibrate the meter. Go back and make sure that it reads the calibrating solutions correctly (to within whatever error you are willing to accept) to be sure that you did it correctly.

8. Sometimes calibration solutions themselves can be off. In that case, you can verify proper operation by testing the meter in other standard solutions. One such solution is borate, at about pH 9.2. Craig Bingman described that useful test in a previous article.

9. If you are doing certain kinds of pH measurements, direct comparison to a known standard may be more useful than using the absolute numbers that the pH meter reads. For example, if you are assessing the strength of limewater via pH. In that case, make a standard of known saturated limewater (from, for example, a teaspoon of calcium hydroxide in a cup of pure fresh water). That solution will have a pH of about 12.45 at 25 ºC, but regardless of what you get, you can use the number as the standard and see how far off from it your actual limewater is (if it is 0.1 pH unit lower, then your limewater is about 79% saturated; 0.2 pH units lower and it is 63% saturated; 0.3 pH units lower and it is about 50% saturated; 0.4 pH units lower and it is 40% saturated, etc.). In this case, exact temperature equivalence between the samples is important. A difference of only 3 ºC means a pH difference of 0.1 pH unit for saturated limewater.
 
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