sanking said:
Kirk,
I am using two pH measuring instruments at this time, both by Oaklon. One is the pH Testr 1, the other their pH Wand. Both provide very consistent readings from as low as pH 1.5 to as high as pH 15.0, but they are off (relative to each othe) by about pH 0.3 in the pH 8.0 to 12.0 range. I don't know which of the meters is most accurate, but at least they are consistent in readings within the range noted.
I would be very appreciative if you would send me the address/email/website of a company that can provide control samples/standards to verify pH measurements. I have use in the past pH strip indicators but these have almost always proven to be less accurate than my instruments.
Sandy
Sandy, while I don't have any experience with Oakton pH meters, I found some instructions for the Tester 1 online - it looks like it can be calibrated at 1 point and it has no Automatic Temperature Control (ATC). (I didn't find any info on the pH Wand in the time I looked.) The Tester 1 a very minimal design for a pH meter and it may not be accurate enough for what you are hoping to do. There is no way any pH meter is going to be accurate over a wide range with only one calibration point and no ATC. The manual for your pH Tester 1 says that it is accurate to +/- 0.2 pH. If you like the Oakton line, it looks like the pH Tester 2 (among other meters) will give you 3 point calibrations and ATC. Either way, these will be much better than using the multistrip pH papers!
But anyway, for what you have now, I suggest that you get a pH 10 standard, and using that standard, make a reading of it. I recommend that the solution be stirring gently while making pH readings. Your meter should have an adjustment screw with which you can make any changes to the meter if there is an offset from the nominal value of the buffer. Remember that pH changes with temperature, and the buffer bottle may have a table on it that will list the pH of that buffer at various temperatures - you can interpolate the actual pH of the solution by measuring the temp and then using the table. So once you have done that, you could use a pH 7, 8, 9, 11 or 12.5 pH buffer to check the linearity of your meter of the range of interest. You may find that your meter is fairly accurate over the narrow range of interest. And by narrow, I mean maybe 1.5-2 pH on either side. (Even though your meter manual says it can make readings from -1 to 15, that really just reflects the error and difficulty associated with making pH measurements at the ends of the pH scale, which in water ranges from pH 0 to 14. While your meter may read numbers outside of that range, they are undefined (i.e. non-sense) and they just mean you've exceeded the actual practical range of the meter. Also, try to have the solution you want to measure as close in temperature to the temp of the calibration standard.
AS far as where to get pH standards, check with you favorite chemistry supply place. Here's my local favorite - they will ship:
http://www.nurnberg.com/ Looking at their online catalog, they list 4,7, and 10 pH buffers, and I am pretty sure they can get more values if you want. As far as brands, I have had good luck with Fisher, Ricca, and Orion buffers. Remember that pH buffers do not last forever, so buy small amounts, like 500 mls (about $15-20 each). Find some small resealable bottles of a few ounces and pour out some buffer into them for daily use, and reseal them to keep them from changing when not in use. You don't need some sort of special control samples for this, just look "pH buffers".
Other things to think about about are that the electrode will age and become sluggish and less accurate with age. You will want to be storing the electrode in a storage solution as recommended by your manufacturer to increase the life of the electrode.
Sandy, since you are at Clemson, swing by the Chemistry department and someone there could let you check your meter. If been to there in the past for mass spectrometry training several years ago and it's pretty nice! They may also have some recommendations on local places to get some buffers.
Also, remember that unless you are taking extra care with higher quality meters with ATC and better designed electrodes, you probably cannot put a lot of faith in making really precise pH measurements, especially when comparing numbers over time. For environmental laboratory work, I put a lot of faith in the whole number portion of a pH measurement, pretty good faith in the first decimal place, not much faith in the second decimal, and as far as I'm concerned, for that type of testing, the 3rd decimal place could be used as a random number generator.
So for using your meter, you may want to consider readings right around the pH buffer used for calibration to be +/- 0.1, one or two units away may be +/- 0.2, 3 or more units away will probably be greater in error. It all depends on the slope of the response of your electrode, which changes with time and age. If you have a meter that you can calibrate with more than one point, you may expect to be within 0.1 or better pH units between the two calibration points. THis is because the meter can measure the slope of the pH electrode response and then use that in the calibration. But even then, the only way to be certain is to get a buffer to measure in the range of interest to be certain.
As an example of how hard it is to measure water samples for pH precisely, in a recent national water pollution proficiany test, with a pH test sample that had an assigned pH value of 8.50, the labs in the test had to get a measurement between 8.25 to 8.75 to get an acceptable result. That +/- 0.25 pH acceptance range was not just a made up number, it was determined by doing a statistical analysis of the results submitted by the labs and it reflects the calibration and performance of all the equipment used in the study.
It's good that your meters are consistent, but with a little work with calibration, you should be able to get them more accurate for small ranges near the calibration point.
If you have any other questions, let me know.
Kirk -
www.keyesphoto.com
