What is pH?
pH is a unit of measurement which is used to express the degree of acidity of a product or it can also be defined as negative logarithm of Hydrogen ion concentration i.e
pH= -log[H+]
The pH scale runs from 0 which means very acidic to 14 which means very basic.
The scale is actually referring to the concentration of hydrogen (H+) ions in the product being tested. The more H+ ions that are present, the more acidic the product will be. The scale is logarithmic, meaning that each pH unit has 10 times more H+ ions than the one above it.pH can be measured using either pH indicators (like phenolphthalein) in form of solution or pH strips or using potentiometric method. Strips are very useful when all you need is 0.2-0.5 pH unit accuracy.
Tool for measuring pH with higher precision is the pH meter. Its working depends on the voltage that is caused by the H+ ions present in the product. A typical pH meter consists of a glass electrode connected to an electronic meter that measures and displays the pH value.
Parts of a pH meter:
pH meter consists of two or three parts that depends on the model(purchased)
- Main Body Component; it houses the microchip used to process the measurements made by meter and the meter’s display.
- A Probe commonly called an Electrode; it is the part where the measurement actually takes place, this part is consumable, sensitive and most expensive part of meter and should be handled with care.
- Automatic Temperature Compensation (not present in all pH meters); some pH meters are equipped with the ability to measure the temperature of the solution being measured .This feature is called “Automatic Temperature Compensation.”Some ATC pH meters have the temperature sensor built into the electrode, and some have an independent thermometer probe which plugs into its own port on the back of the body of the meter.
How pH Meter works?
A pH meter is a precise voltmeter that measures the potential difference, in thousandths of a volt (mV), between the reference electrode and the measuring pH electrode. It’s scaled in such a way that it displays not the measured potential, but converts it to a display of pH. The pH meter calculates a value by measuring the voltage differences between the pH electrode (responsive to hydronium ion concentration) and the reference electrode (which provides a constant voltage). The meter must be calibrated in order to compensate for the difference in voltage output from different electrodes. It is recommended that at least two buffer solutions are used to calibrate the machine.(pH values 4,7 and 10).
According to Nernst equation (Nernst equation is a mathematical description of an ideal pH electrode behavior) a standard pH electrode generates a voltage of about 59 mV per pH, and at pH 7 (neutral pH) the electrode produces 0 volts. Acids produce positive and bases produce negative voltages.
Calibration of a pH meter
This is required to calibrate the pH meter against a known set of values (e.g. pH= 4, 7 &10) so that it can yield accurate readings. For best results, calibrate pH with a buffer that is within 3 pH units of the test sample. Procedure:
1. Rinse the pH electrode in tap water.
2. Insert pH electrode in a fresh pH buffer solution.
3. Measure the temperature of the buffer solution.
4. Adjust the TEMP screw to the corresponding temperature value.
5. Observe the pH reading and adjust the “Cal” screw to pH 7 if required.
6. Remove pH electrode and rinse in distilled water.
7. Insert in a pH 4 if measuring an acid solution, or pH 10 bufsfer if measuring an alkaline solution.
8. Adjust the slope screw until the measurement value corresponds with the pH 4 or pH 10 buffer
What are the Types of pH meter?
There are three different types of pH meters:
- portable,
- bench and
- in-line.
• Portable devices are useful when there is a need to perform measurements outside the laboratory. Portable pH tester should be ergonomic, rugged and versatile.
• Bench tester is usually more precise and has more options. They are typically used in a laboratory where an interface for data communication and advance measurement features are important.
• In-line pH transmitters for process control are designed to with stand harsh environments.
Safety guidelines for pH meter
i) When it is not in use, it should be turned off and the electrode tip should be covered with the plastic cap.
ii) If the meter has not been used for some time, it should be soaked for several days in distilled water prior to use, with the power off. For effective soaking, the protective cap must be removed.
iii) The meter should ever not be placed in water deep enough to submerge any part of the upper body of the meter. this will ruin the electronics.
iv) After soaking, remove the pH meter from the water, dry the probe tip, and cap the probe until one is ready to use the meter again.
v) The meter may be laid on a paper towel on the bench top when not in use. However, under any circumstances the meter should not be inverted (turned probe-up); this may cause water inside the probe tip to run up into the meter electronics.
vi) Before using the meter to determine the pH of your analyte, it may be necessary to standardize/caliberate it–that is, to immerse it in a buffer of known pH and adjust the meter to display the correct pH.
