If you know the pH of your test samples is really high (above 13.00 pH) or really low (below 1.00 pH), the measuring process could be challenging.

All of Apera Instruments’ meters are able to test in the range of 0 to 14 pH, some advanced models even have a range of -2.00 to 19.99 pH. However, these ranges are only representing the instrument’s theoretical capability. In practice, the range which the meter can actually reach is limited by the pH electrode you use. Almost all pH electrodes are not specialized in the extreme ranges below 1.00 pH or above 13.00 pH. At these extreme pH levels, the measurement error will be significantly enlarged, and in many cases, it will cause a shortened service life for the electrode when used, because the strong acids or bases will damage the sensitive pH glass membrane.

As such, when you must test strong acids or bases within those extreme ranges, we recommend diluting the solute to a 1% concentration. An easy way to do this is to mix 10 mL (make sure the volume is accurate) of the given strong acid/base to 1000 mL of deionized water.

*Please refer to the Appendix List for what is defined as strong acid/base. Acids/bases outside this list are not considered as “strong acid/base”, even though their pH could be very low or very high, hence are NOT recommended for dilution test.

With the strong acid/base properly diluted, place the meter into the solution and allow the meter to achieve a stable smiling face indicator on the display. Whatever reading you get, it will be different than if you tested directly in the substance you wished to test. So you must now correct for dilution. A pH of 6 is 10 times more acidic than 7, but a pH of 5 is 100 times more acidic than 7 because the pH scale increases and decreases logarithmically by factors of 10 for every whole number.

So if you dilute 10 mL of acid/base with 1000 mL of deionized water (1:100 ratio) then you can apply that logic to your measurement. If a ratio of 100 is the difference in the volume of acid/base to deionized water, then they are different by 2 whole pH.

For Acids:

So whatever you get as your test result in that situation, you simply subtract 2. For example, if your dilution test gives you a pH reading of 4.00 pH, then the actual pH is 2.00 pH.

For Bases:

So whatever you get as your test result in that situation, you simply add 2. For example, if your dilution test gives you a pH reading of 10.00 pH, then the actual pH is 12.00 pH.

With your dilution test calculated, you may record your findings and safely ensure the longevity of your pH electrodes.

Backed by proprietary Swiss sensor technologies and materials, Apera’s LabSen 841 pH Electrode is specifically designed for strong basic solutions (up to 14.00 pH), and LabSen 831 is for strong acidic solutions (down to 0.00 pH). With these specialized glass-body electrodes equipped to your pH meter, tons of work can be saved when testing highly acidic or basic solutions. But still, if you estimate the pH could be higher than 14 or lower than 0, we still recommend the dilution test method even with these specialized electrodes.

Appendix

Strong Acids

HCl - hydrochloric acid

HNO3 - nitric acid

H2SO4 - sulfuric acid

HBr - hydrobromic acid

HI - hydroiodic acid

HClO4 - perchloric acid

Strong Bases

LiOH - lithium hydroxide

NaOH - sodium hydroxide

KOH - potassium hydroxide

Sr(OH)2 - strontium hydroxide