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Found this post on Tristate reef.

Ph Probe 20% Borax test

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 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

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