Before you spend a lot of time chasing the numbers, take some time to test the probe's function using
sodium metaborate, even though you've already calibrated the meter.
Calibration is good, but it is always a good idea to do a separate test with the sodium metaborate (borax) to see if your meter is testing well during normal continuous usage. I read a lot of posts where folks say that they have calibrated their pH meters and followed the directions exactly, followed all the temp bath suggestions, corrected for every possible error, yet they consistently see pH values that are skewed either high or low in their aquaria based on even the new calibration. When questioned on their procedure, almost every person I have spoken with had used standards that had either expired, or been in their LFS’s shops long enough to have expired. I read with interest Craig Bingman’s posts about a similar problem with (name deleted)brand of pH
calibration solution where the pH 10 standard was off by almost a full pH unit. Craig had purchased a few packets of this particular brand, used a lab bench pH electrode, calibrated with fresh research-quality pH standards and tested these calibration fluids. After much board discussion, Craig proposed that folks use a freshly prepared buffering solution of sodium metaborate that will always come to the same value under home conditions. This lead to the publishing of his article in Aquarium Frontiers on the use of 20 Mule Team Borax to prepare a home standard solution. I would suggest that you use this procedure to make sure that your pH probe and meter are in actuality measuring what they are supposed to. I have issues with the Hanna brand of pH probe, but that is a separate issue.
Essentially, the process of testing your probe's ability/accuracy involves making a standard solution of borax in RO/DI water and testing the resulting solution. This solution differs from NIST standard solutions in that NIST solution standards rely on a combined solution using controlled measured amounts of an acid and its conjugate base. Usually you would need to mix precise quantities of two different compounds to make a pH standard solution in a precisely measured volume. These will have a set shelf life, and will deteriorate more rapidly if exposed to less than ideal conditions. Rather than try and make these lab standards (you could, if you had the precision lab ware and balances needed to measure end volume and the acid and conjugate base weights), we can use the properties of sodium metaborate in water to form these equimolar acid/conjugate bases for us (Boric acid and hydrated borate ions). This is one of the advantages of using borax as a standard.
Quote:
pH of Sodium Metaborate Standard Solution at Various Temperatures
(3.80 grams Na2BO7 10H2O/liter)
Degrees Celsius . . . Degrees Fahrenheit . . . . . . pH
0 . . . . . . . . . . . . . . 32.0 . . . . . . . . . . . . . 9.464
5 . . . . . . . . . . . . . . 41.0 . . . . . . . . . . . . . 9.395
10 . . . . . . . . . . . . . 50.0 . . . . . . . . . . . . . 9.332
15 . . . . . . . . . . . . . 59.0 . . . . . . . . . . . . . 9.276
16 . . . . . . . . . . . . . 60.8 . . . . . . . . . . . . . 9.266
17 . . . . . . . . . . . . . 62.6 . . . . . . . . . . . . . 9.256
18 . . . . . . . . . . . . . 64.4 . . . . . . . . . . . . . 9.245
19 . . . . . . . . . . . . . 66.2 . . . . . . . . . . . . . 9.235
20 . . . . . . . . . . . . . 68.0 . . . . . . . . . . . . . 9.225
21 . . . . . . . . . . . . . 69.8 . . . . . . . . . . . . . 9.216
22 . . . . . . . . . . . . . 71.6 . . . . . . . . . . . . . 9.207
23 . . . . . . . . . . . . . 73.4 . . . . . . . . . . . . . 9.198
24 . . . . . . . . . . . . . 75.2 . . . . . . . . . . . . . 9.192
25 . . . . . . . . . . . . . 77.0 . . . . . . . . . . . . . 9.180
26 . . . . . . . . . . . . . 78.8 . . . . . . . . . . . . . 9.172
27 . . . . . . . . . . . . . 80.6 . . . . . . . . . . . . . 9.167
28 . . . . . . . . . . . . . 82.4 . . . . . . . . . . . . . 9.155
29 . . . . . . . . . . . . . 84.2 . . . . . . . . . . . . . 9.147
30 . . . . . . . . . . . . . 86.0 . . . . . . . . . . . . . 9.139
35 . . . . . . . . . . . . . 95.0 . . . . . . . . . . . . . 9.081
40 . . . . . . . . . . . . . 104.0 . . . . . . . . . . . . 9.068
Characteristics of Sodium Metaborate pH Standard solution
Characteristic . . . . . . . . . . . . . . . . . . . . . . . . Value
Composition(gm Na2B4O7 10H2O/1000 ml) . . . . . . 3.80
molality (m) . . . . . . . . . . . . . . . . . . . . . . . . . . 0.01
Molarity (M). . . . . . . . . . . . . . . . . . . . . . . . . . 0.009971
pH at 25 degrees Celsius . . . . . . . . . . . . . . . . . 9.180
change in pH after dilution
to half original concentrationin water .. . . . . . . . +0.01
Temperature coef.
dpH/dt,unit degrees Celsius-1 . . . . . . . . . . . . . -0.0082
data taken from C.Bingman Aquarium Frontiers article
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These tables give the properties of differing temperatures on the pH of borax (sodium metaborate) in purified water and the resulting pH, as well as the characteristics that allow us to demonstrate the relatively small change that varying the concentration of the solution has on pH when compared to the standard solution. To make the textbook standard, you dissolve 3.8 grams of borax per liter water using a lab balance and a volumetric flask. Fortunately 1/2 level teaspoon of solid borax weighs just about two grams, AND the pH of borax solutions is only weakly dependent on the concentration of borax in the solution (SEE CHART), SOOOOOooooo, we don’t need to be tremendously precise in our measurement of the borax to still produce a relatively accurate pH standard. We can make a useful product here without the use of a lab balance.
Use a pint of water (473ml) and add a half teaspoon (1/2 level tsp.) of 20 Mule Team Borax to make a resultant pH standard solution with 2 gm of sodium metaborate per 473 ml or 4.3 grams of sodium metborate/litre. Mix well. Although using this method to make the standard will only result in an accuracy of about plus or minus 10% of the lab standard sodium metaborate pH calibration solution, there is such a weak dependence of pH on concentration of the solute buffer here that it is close enough for our purposes. To use this solution to check your probe, bring a freshly made batch of borax solution to aquarium temp and immerse the probe into the solution, allowing 5 minutes for final probe measurement. The meter should show a value of 9.19
+ 0.02 pH units. If your value with the meter is not within range, cleaning nd recalibration of the probe are indicated. If after cleaning and recalibrating the probe with known fresh good standards your meter/probe still cannot pass this test, then it is time to either have the meter tested against a known good meter probe combination, and failing this comparison, then replacing either the probe or the entire pH meter setup. Unfortunately Hanna pH meters at hobbyist levels are notorious for drift from calibration in as little as 2 weeks of continuous use.
Chronically high pH values in marine aquaria are more often measurement issues than anything else, although they can occur as a result of inappropriate use of buffering dditives, especially high pKa products like sodium carbonate (present in varying amounts in many reef buffer builders). Use the borax to check your meter's ability to measure the correct value, and proceed from that point. If the meter is fine, folks here on the board can suggest several possible causes and some resolutions for a fix. Let us know when you've tested the meter with the borax. I saw 20 Mule Team Borax at Target in the cleaning supplies for about $3.00, its in a green box now.
hth.
