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These pages provide a quick start guide to those who are either new to pH, or just need a quick refresh of the essential points. pH measurement requires basic procedures to be followed, such as calibration using buffer solutions to the correct value. The following guide will provide you with the basics for accurate and reproducible pH measurement.
Why is pH so important?
pH is one of the most common laboratory measurements because many chemical processes are dependent on pH. The speed and rate of chemical reactions can often be significantly altered by changing the pH of the solution. The solubility of many chemicals in solution and their bio-availability is dependent on pH. The physiological chemistry of living organisms also has very specific pH boundaries.
What is pH?
The term pH derives from a combination of p for the word power and H for the symbol of the element Hydrogen. pH is the negative log of the activity of hydrogen ions.
pH = log10aH+
pH represents the activity of hydrogen ions in a solution, at a given temperature. The term activity is used because pH reflects the amount of available hydrogen ions, not concentration of hydrogen ions.
pH is a measurement
In water or aqueous solution, the following equilibrium exists between the water (H20), the acid (H+) and the alkali (OH-)
H20 = H+ + OH-
The pH scale for aqueous solutions ranges from 0 to 14 pH units with pH7 being neutral.
How can we measure pH?
Although everyone is familiar with Litmus paper, the only quantitative way to measure pH is by using potentiometric electrodes. These electrodes monitor the change in potential (voltage) caused by differing concentration of Hydrogen ions (H+). Electrodes are excellent for measuring pH for a number of reasons:
- Wide linear range
- Good precision
- Unaffected by sample colour or turbidity
- Low cost
- Fast response
- Real - Time, in-situ measurements
Selecting an electrode
Before selecting a pH electrode, it is important to understand the different types of electrodes available, there is no such thing as a universal pH electrode. Your choice of pH electrode depends on several criteria, including your sample/application, cost, usage etc. For more information about the correct electrode for your application please visit the pH Application pages.
pH electrodes
The majority of pH electrodes available are combination electrodes. They combine the reference and pH sensing elements in a single electrode. Separate pH and reference electrodes can be used, but this tends to be for the highest precision research measurements. Naturally, combination pH electrodes are much more practical to use.
Construction of a pH electrode
The electrochemical circuit between the reference and pH sensing parts of an electrode is connected using a liquid junction (key - j), that allows a slow flow of a suitable conducting liquid.
A - pH sensor
B - reference half cell
C - Combination pH electrode (A+B)
D - Seal
E - Internal filling solution
F - Internal reference electrode
G - External filling solution
H - pH sensitive glass membrane
I - Internal liquid junction
J - External liquid junction
K - Fill hole
pH buffers
pH buffers are used as standards to calibrate your pH electrode. There are a large number of buffers available of different pH values and of varying quality, so it is important to know what you should be using. The calibration buffers used should bracket the expected pH of your sample. The list of buffers below outlines those that are recognised by various regulatory bodies.
pH 1.68 at 25ºC
pH 4.01 at 25ºC
pH 6.86 at 25ºC
pH 7.00 at 25ºC
pH 9.18 at 25ºC
pH 10.01 at 25ºC
Buffers are also available as primary buffers (those that are made with direct traceability to the components that make up the buffer) and secondary buffers.
pH buffers are dependent on temperature. Orion pH meters contain all the pH-temperature profiles for the buffers listed above, so you don't need to adjust anything yourself. However, if you are not using Orion buffers, your pH meter may incorrectly adjust your calibration!
Final word on pH electrodes
Choosing the correct pH electrode for your application is the most important step before making any pH measurement. The choice and variety may seem baffling, hence if you are bothered about your pH results and the ongoing costs, it pays to talk to an expert.
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