pH - Keep it Wet!!!
pH meters work by measuring minute particles, called ions, with two glass electrodes, one of which has a permeable junction. This permeable junction allows a small amount of gel solution to pass through to complete a liquid bridge. If this junction is allowed to dry out, the liquid bridge is prevented from being formed and the instrument will not perform accurately. Therefore, unlike most instruments, the glass electrodes need to be kept wet with a storage fluid for the junction not to get blocked.
The pH meter has a finite life due to the gradual aging of the sensor and will only last for approximately 360 readings (or a year for one reading a day).
COMMON CAUSE | FAULTS | REMEDIES/REASONS |
· Slow reading · Drift reading | Unit is left without protective cap with storage fluid, or not used for long time | Dry pH glass causes slowness, soak probe in pH 7 buffer for 24 hours - or - Reference junction dried up – no remedy |
· Faint display · Low readings · Cannot calibrate · No display | Battery weak or flat after frequent usage | Change batteries |
· Readings stay at 6pH to 7pH · Cannot calibrate · Erratic readings | Extreme shock or impact applied at glass probe | Broken or cracked glass probe – no remedy |
· Erratic readings · Inaccurate reading · Cannot calibrate · Display “E” or “1” | Water leaked into the electronics of the unit | Unit is unlikely to be repairable. |
· Slow reading · Inaccurate reading | Measure in oily solutions. Did not rinse after testing | Contaminated pH glass –rinse with 50% acetone/water solution then soak in pH 7 buffer overnight. Reference junction clogged – no remedy |
· Inaccurate reading | No proper rinsing between measuring solution | To avoid solution carry over, always rinse in de-ionized or distilled water then rinse in the next measuring solution follow by the actual measuring solution. |
· Erratic/unstable readings | Using tester in high magnetic field or high power area | Use in less interference area |
Frequently Asked Questions
1. Q: How often do I need to calibrate my pH tester or meter?
A: Depending on the accuracy required, once a day if several measurements are required during the day; or at least once weekly if you use 2 or 3 times per week.
2. Q: Why does the pH tester or meter Display "1" or show erratic reading when power on?
A: When the electrode is held in the air and not dipped or in contact with a solution, it will display erratic readings. This is because the reference and glass electrode are in close contact, therefore the electronics cannot show a fixed reading. Dip the electrode into any solution and a reading will appear.
3. Q: Why does the pH tester or meter take sometime to get a stable reading?
A: Electrode was dry or not properly conditioned. Reference junction is possibly clogged up.
Remedy: Pre-soak the electrode for 1 hour in tap water before using.
4. Q: How long will a pH tester last?
A: All pH glass electrode will age with time and temperature.
Most glass electrodes have a storage life of about 1 year. A life-span of about 3 to 6 months for frequent/continuous use.
5. Q: What other problems are associated with measurement?
A: Temperature is important as liquid properties are affected. Therefore, a correct temperature of 25 °C for Buffer Solutions should be maintained during calibration. However correction of values may be made if the temperature of the solution is known, by comparing with the table below.
Different temperatures of test samples will give inconsistent results.
| | pH Value |
Temp. ºC | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
5 | .30 | .24 | .18 | .12 | .06 | 0 | .06 | .12 | .18 | .24 | .30 |
15 | .15 | .12 | .09 | .06 | .03 | 0 | .03 | .06 | .09 | .12 | .15 |
25 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
35 | .15 | .12 | .09 | .06 | .03 | 0 | .03 | .06 | .09 | .12 | .15 |
45 | .30 | .24 | .18 | .12 | .06 | 0 | .06 | .12 | .18 | .24 | .30 |
55 | .45 | .36 | .27 | .18 | .09 | 0 | .09 | .18 | .27 | .36 | .45 |
65 | .60 | .48 | .36 | .24 | .12 | 0 | .12 | .24 | .36 | .48 | .60 |
75 | .75 | .60 | .45 | .30 | .15 | 0 | .15 | .30 | .45 | .60 | .75 |
85 | .90 | .72 | .54 | .36 | .18 | 0 | .18 | .36 | .54 | .72 | .90 |
