PH meter

A pH meter is a device that measures the pH of a solution by measuring the voltage between two electrodes submerged in the solution.

Definition of pH:

pH is a measure of hydrogen ion concentration. It is the scientific way of measuring of the acidity or alkalinity of a solution. Aqueous solutions at 25°C with a pH less than seven are acidic, while those with a pH greater than seven are basic or alkaline. A pH level of is 7.0 at 25°C is defined as 'neutral' because the concentration of  H₃O⁺ equals the concentration of OH^- in pure water.

pH = -log [H+]

pH meter calibration

pH electrode itself or sensor sends a signal to the digital meter based on the acidity, alkalinity or ionic potential (ORP) of the solution. Normally signal is a very weak and measured in millivolts. The meter display pH value with reference to the developed EMF in electrode. However the signal can and will change over time for all electrodes. This is a good analogy: Think of the electrode as a chemical battery. Over time the battery no longer develops the same voltage and the attached flashlight bulb light weakens. Eventually the light fails totally. The same is true for an electrode. Because the signal for the same pH value changes, the meter needs some way to assign the proper pH value to it. This is the reason why pH meters need periodic calibration. Each time the meter is calibrated, the meter assigns a new set of values to the incoming signals. For instance, a new pH electrode when placed in a known standard solution (commonly referred to as a “buffer”) with a value of pH 7.01 may have an output signal of -5.1mV. The meter after calibration will show on its display a reading of 7.01 (+/- the accuracy of the meter’s electronics). Let us assume that after a month the same electrode now sends a signal of -5.5mV to the meter for the same buffer. Without recalibration, the meter will show the pH incorrectly. The error of 0.4mV is the drift of the signal. In this case the error is small so the offset from the true value will not be very much (0.01pH). However, as the electrode ages, the error will increase and the rate of the change will increase as well. That is why electrodes need more frequent calibration as they age. At the end of their useful life, the electrode may exhibit an offset of -30mV which is equivalent to 0.5 pH units or more depending upon where on the pH scale the reading is occurring. To the user, all of this happens automatically each time the meter is calibrated. (No math required!) However, when the drift exceeds about -30 to -35mV, the electrode becomes unstable. The user will find that the pH value on the display changes for no reason with the same solution. And, the meter more than likely will not calibrate at all.

Two or more point calibration

Most pH meters should be calibrated at a minimum of two points. The most common points are values of pH 7.01 and 4.01 or 7.01 and 10.01 if you are going to be reading alkaline values. The more advanced meters will let the user calibrate at three, four or five pH values. These meters will also let the user select which buffers are to be used.

pH meter Calibration Steps. 

The calibration steps most of the machines are follows, but it may vary according to the manufacturer.

1. Rinse the pH electrode with Distilled water.
2. Dry the outside of the electrode but blot the bottom of the sensitive glass with a paper towel. This is done to avoid abrasion of the glass measuring end and to prevent dilution of subsequent use.
3. Place the electrode into a pH 7.01 buffer solution making sure that the junction (located on the bottom or side of the electrode) is wet. For most electrodes this means an immersion of only 1 to 2 inches.
4. For more precise lab work, use a magnetic stir plate.
5. Turn the meter on and select the calibration mode.
6. The meter will normally default to asking for a pH 7.01 buffer. If not use the up and down keys to select pH 7.
7. The meter’s display will ask to “CONFIRM ” when the value in the display has stopped changing. If the electrode is in good condition, the value will jump to near the buffer value very rapidly. As the electrode deteriorates or if dirty/clogged, this process will take longer.
8.  Press the proper key to CONFERM or other key as stated in your manual.
9. The meter will now ask the user to place in another buffer (usually pH 4.01). The user can accept this value or change it to the desired second calibration point.
10. Rinse the electrode as before in distilled water and dry as in step 2.
11. Place the electrode in the buffer selected.
12. Wait for the meter to accept the second value and press the confirm key when prompted.
13. Repeat as needed for additional buffers if needed.
14. If the meter indicates “Wrong buffer” it means that the value selected does not match the immersion solution or the electrode’s diverges too far from an accepted value. To remedy, a new electrode must be used or the electrode must be cleaned prior to use.
15. If the meter indicates “Wong electrode” it could mean that the attached electrode is of the wrong type (i.e.- ORP rather than pH).
16. Once the calibration at two or more points has been completed, the meter will automatically return to the normal measuring mode. It will show the temperature compensated value of the solution. Some small variation may occur depending upon the inherent accuracy of the meter and the variation of temperature. If the probe and the solution are at different temperatures, readings will continue to change slightly as the two reach equilibrium. 

pH meter

Preventive maintenance once in a Every six months

1. Examine the exterior of the equipment and evaluate its general physical condition. 
2. Verify the cleanliness of the covers and their adjustments.
3. Test the connection cable and its system of connections.Check that they are in good condition and clean.
4. Examine the equipment controls. Verify that these are in good condition and activated without difficulty.
5. Verify that the meter is in good condition. To do this, the instrument must be disconnected from the electric feed line. Adjust the indicator needle to zero (0) using the adjustment screw generally found below the pivot of the indicator needle. If the equipment has an indicator screen, check that it is functioning normally.
6. Confirm that the on indicator (bulb or diode) operates normally.
7. Verify the state of the electrode carrying arm. Examine the electrode attachment and assembly mechanism to prevent the electrode from becoming loose. Check that the height adjustment operates correctly.
8. Check the batteries (if applicable); change them if necessary.
9. Test its function by measuring the pH of a known solution.
10. Inspect the ground connection and check for escaping current

Download pH meter manual