The difference between the electrical potential can be converted to a measure of pH, which measures the amount of free hydrogen ions (H+) in the test sample.
Other methods can measure the pH more simply but less accurately, by using litmus paper (turns color depending on the pH of the test solution) or liquid indicators, which also turn the test sample a different color depending on how much free hydrogen ion is present. This last method is also called a liquid pH meter or pH indicator solution testing.
Using a Liquid pH Meter
A liquid pH meter uses indicator solutions to determine the approximate pH of a sample solution. These indicators are weak organic acids of bases with a pH as low as 4.0 (for acidic indicators) or 10.0 (for alkaline indicators).
Liquid indicators are only effective when the test solution is within the pH range for the indicator. The free hydrogen ions (H+) or hydroxide ion (OH-) within its pH range will bind to those in the test solution and change the solution’s color. Selecting the correct test solution is difficult and often results in a narrow range of pH that can be tested. For wider ranges, more test solutions are necessary.
Liquid pH meters are often used for acid-base titration when a solution needs to be close to a specific pH, often a neutral pH of 7.0. By titrating a solution into the test sample and watching the color change, the scientist can determine when the solution is at a neutral pH with reasonable accuracy. This is the most useful activity offered by a liquid pH meter as it can be quick, easy, and inexpensive to perform when acid-base precision is not required.
A pH Meter for Viscous Liquids
When testing a viscous liquid such as paint or cosmetics, measuring pH becomes a bigger challenge. A standard pH meter will take a much longer time to read the pH as the viscosity of the sample slows the hydrogen ion movement and makes it hard for a steady reading to be detected.
Depending on the purpose of the pH measurement, a liquid pH meter may be a good answer to measuring the pH of a viscous sample. Titration can still be done along with the addition of an indicator solution. However, the difficulties of using a liquid pH meter for viscous liquids are not minimal. The viscous liquid must be stirred constantly and mixed well, the time it takes for a steady color to appear from an indicator solution is much longer than for a typical liquid, and the viscous liquid must initially be colorless so that the appropriate color change caused by the liquid indicator can be detected. So, while some viscous liquids may be amenable to a liquid pH meter, many will not work with a liquid pH meter requiring more complicated manual procedures.
In fact, the requirement of a colorless starting sample for using a liquid pH meter greatly limit the utility of this approach.
Automated Liquid Handling
Whether the samples are watery or viscous, the best way to measure pH in any life science or pharmaceutical lab is not to use electrodes, litmus papers, or a liquid pH meter. The ideal answer is automation. By using automated liquid handling instruments, plates of varying sizes (e.g., 96, 48, or 24 well) can be processed and tested automatically without the need for human intervention.
Regardless of the procedure to be performed, automated laboratory equipment will do it faster, more reliably, and at a lower cost.This includes measuring pH. A liquid pH meter is no longer needed as automation takes care of the entire measurement process, from calibration through recording the result for each of 96 samples in a 96 well plate and cleaning and drying the pH probe between samples/wells. Since it can take a minute or so for the pH probe to stabilize, one advantage of using automated laboratory equipment to perform this testing is to relieve the tedium of sitting and watching and waiting for the reading to stabilize.
This reduces error, increases reliability and accuracy, and frees up technicians to perform other more valuable tasks. And automation can not only measure pH in typical solutions but in viscous liquids, too.
Contact the leader in life science and pharmaceutical automation today! With almost 40 years of experience, Hudson Robotics can answer all of your questions about automated pH meters, liquid pH meters, and total laboratory automation.
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