pH Meter: Lecture 8 PDF

pH meter Lecture 8 PURPOSE OF THE EQUIPMENT The pH meter is commonly used in any field of science related to aqueous solutions. It is used in areas such as agriculture, water treatment and purification, in industrial processes such as petrochemicals, paper manufacture, foods, pharmaceuticals, res...

pH meter Lecture 8 PURPOSE OF THE EQUIPMENT The pH meter is commonly used in any field of science related to aqueous solutions. It is used in areas such as agriculture, water treatment and purification, in industrial processes such as petrochemicals, paper manufacture, foods, pharmaceuticals, research and development, metal mechanics, etc. In the health laboratory, its applications are related to the control of culture mediums and to the measurement of the alkalinity or acidity of broths and buffers. In specialized laboratories, diagnostic equipment microelectrodes are used to measure the pH of liquid blood components. The plasma pH allows the patient’s health to be evaluated. It normally measures between 7.35 and 7.45. This value relates to the patient’s metabolism in which a multitude of reactions occurs where acids and bases are normally kept in balance. Acids constantly liberate hydrogen ions [H+] and the organism neutralizes or balances acidity by liberating bicarbonate ions [HCO –]. The acid-base ratio in the organism is maintained by the kidneys, (organs in which any excesses present are eliminated). The plasma pH is one of the characteristics that vary with factors such as age or state of health of the patient. OPERATION PRINCIPLES The pH meter measures the concentration of hydrogen ions [H+] using an ion-sensitive electrode. Under ideal conditions, this electrode should respond in the presence of only one type of ion. In reality, there are always interactions or interferences with other types of ions present in the solution. A pH electrode is generally a combined electrode, in which a reference electrode and an internal glass electrode are integrated into a combined probe. The lower part of the probe ends in a round bulb of thin glass where the tip of the internal electrode is found. The body of the probe contains saturated potassium chloride (KCl) and a solution 0.1 M of hydrogen chloride (HCl). The tip of the reference electrode’s cathode is inside the body of the probe. On the outside and end of the inner tube is the anodized end. The reference electrode is usually made of the same type of material as the internal electrode. Both tubes, interior and exterior, contain a reference solution. Only the outer tube has contact with the measured solution through a porous cap which acts as a saline bridge. This device acts like a galvanized cell. The reference electrode is the internal tube of the pH meter probe, which cannot lose ions through interactions with the surrounding environment. Therefore as a reference, it remains static (unchangeable) during the measuring process. The external tube of the probe contains the medium which is allowed to mix with the external environment. As a result, this tube must be filled periodically with a potassium chloride solution (KCI) to restore the capacity of the electrode which would otherwise be inhibited by a loss of ions and evaporation. The glass bulb on the lower part of the pH electrode acts as a measuring element and is covered with a layer of hydrated gel on its exterior and interior. Metallic sodium cations [Na+] are diffused in the hydrated gel outside of the glass and in the solution, while the hydrogen ions [H+] are diffused in the gel. This gel makes the pH electrode ion-selective: Hydrogen ions [H+] cannot pass through the glass membrane of the pH pH METER COMPONENTS A pH meter generally has the following components: 1. The body of the instrument contains the circuits, controls, connectors, display screens and measuring scales. The following are among some of its most important components: a) An ON and OFF switch. Not all pH meters have an on-and-off switch. Some simply have a cord with a plug which allows it to be connected to a suitable electrical outlet. b) Temperature control. This control allows adjustments according to the temperature of the solution measured. c) Calibration controls. Depending on the design, pH meters possess one or two calibration buttons or dials. Normally these are identified by Cal 1 and Cal 2. If the pH meter is calibrated using only one solution, the Cal 1 button is used; making sure that Cal 2 is set at 100%. If the pH meter allows two- point calibrations, two known pH solutions covering the range of pH to be measured are used. In this case, the two controls are used (Cal 1 and Cal 2). In special cases, a three-point calibration must be done (using three known pH solutions). d) Mode selector. The functions generally included in this control are: I. Standby mode (0). In this position, the electrodes are protected from electrical currents. It is the position used for maintaining the equipment while stored. II. Ph mode. In this position, the equipment can take Ph measurements after performing the required calibration procedures. III. Millivolt mode (Mv). In this position, the equipment is capable of performing millivoltage readings. IV. ATC mode. The automatic temperature control mode is used when the Ph is measured in solutions for which the temperature varies. This function requires the use of a special probe. Not all Ph meters have this control. 2. A combined electrode or probe. This device must be stored in distilled water and stay connected to the measuring instrument. A combination electrode has a reference electrode (also known as a Calomel electrode) and an internal electrode, integrated into the same body. Its design varies depending on the manufacturer. GENERAL CALIBRATION PROCEDURE pH analyzers must be calibrated before use to guarantee the quality and accuracy of the readings following these procedures: 1. One point calibration. This is carried out for normal working conditions and for normal use. It uses one known pH reference solution. 2. Two point calibration. This is done prior to performing very precise measurements. It uses two known pH reference solutions. It is also done if the instrument is used sporadically and its maintenance is not carried out frequently Description of the process Frequency: Daily 1. Calibrate the pH meter using one known pH solution (one point calibration). 1.1 Connect the equipment to an electrical outlet with a suitable voltage. 1.2 Adjust the temperature selector to the environmental temperature. 1.3 Adjust the meter. 1.4 Remove the electrodes from the storage container. The electrodes must always be stored in a suitable solution. Some can be maintained in distilled water, others must be kept in a different solution as their manufacturers recommend1. If for some reason, the electrode becomes dry, it is necessary to soak it for at least 24 hours before use. 1.5 Rinse the electrode with distilled water in an empty beaker. 1.6 Dry the electrode with material able to absorb residual liquid on its surface, without impregnating the electrode. To avoid possible contamination, the electrodes must be rinsed between different solutions. 2. Place electrodes in the calibration solution. 2.1 Submerge the electrode in the standardization solution in such a manner that its lower extremity does not touch the bottom of the beaker. This decreases the risk of breaking the electrode. If the test requires that the solution be kept in motion using the magnetic agitator, special care must be taken so that the agitation rod does not hit the electrode as this could break it. Buffer solution Is used as a calibration solution, because its pH is known and therefore will still be maintained Even if a little contamination occurs. In general, a solution of pH = 7 is used for this purpose1. 3. Turn the functions selector from the Standby position to pH position. 3.1 This action connects the electrode to the pH measuring scale in the pH meter. 3.2 Adjust the meter to read the pH of the calibration solution using the button marked Cal 1. This enables the meter to read the pH of the calibration solution. For example: For a solution at pH = 7, the needle can oscillate slightly in units of 0.1 pH; on average, the reading should be 7. The reading of the meter (reading scale) should be done perpendicularly, to avoid or eliminate parallel-type errors (reading errors produced by the shadow of the meter’s needle, visible on the mirror of the reading scale). The pH meter is then ready (calibrated), to carry out the correct pH readings. 3.3. Put the functions selector in the Standby position 4. Measuring the pH of a solution. 4.1 Remove the electrode from the calibration solution. 4.2 Rinse the electrode with distilled water and dry it. 4.3 Place the electrode in the solution of unknown pH. 4.4 Turn the functions selector from the Standby position to the pH position. 4.5 Read the pH of the solution on the meter’s scale or the screen. Register the reading obtained on the control sheet. 4.6 Turn the functions selector again to the Standby position. If it is necessary to measure the pH of more than one solution, repeat the previously described procedures, rinsing the probe with distilled water and drying with clean, lint-free paper between readings. When the pH has to be measured in numerous solutions, the pH meter must be calibrated frequently, following the steps previously described. 5. Turn off the pH meter. 5.1 Remove the electrode from the last solution analyzed. 5.2 Rinse the electrode in distilled water and dry it with a drying material that will not penetrate it. 5.3 Place the electrode in its storage container. 5.4 Verify that the functions selector is in the Standby position. 5.5 Activate the off switch or disconnect the feed cable, if it lacks this control. 5.6 Clean the work area. General maintenance procedures for the pH meter’s body Frequency: Every six months 1. Examine the exterior of the equipment and evaluate its general physical condition. Verify the cleanliness of the covers and their adjustments. 2. Test the connection cable and its system of connections. Check that they are in good condition and clean. 3. Examine the equipment controls. Verify that these are in good condition and activated without difficulty. 4. 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. 5. Confirm that the on indicator (bulb or diode) operates normally. BASIC MAINTENANCE OF THE ELECTRODE Frequency: Every four months The measuring or detector electrode requires periodic maintenance of the conducting solution to obtain precise readings. The recommended steps for replacing the electrolyte solution are the following: 1. Remove the detector electrode from the storage buffer solution. 2. Rinse the detector electrode abundantly with distilled water. 3. Remove the upper cover of the detector electrode. 4. Fill the conduit surrounding the internal electrode with a saturated potassium chloride (KCI) solution. Use the syringe or applicator supplied with the KCI solution. Verify that the tip of the syringe does not touch the inside of the electrode. 5. Close the electrode with its cover. Rinse the electrode in distilled water. 6. Keep the electrode in a storage buffer solution while not in use. Cleaning of the electrode The type of cleaning required for electrodes depends of the type of contaminant affecting it. The most common procedures are summarized next: 1. General cleaning. Soak the pH electrode in a 0.1 M HCl solution or 0.1 M HNO3, for 20 minutes. Rinse with water. 2. Removal of deposits and bacteria. Soak the pH electrode in a diluted domestic bleach solution (e.g. 1%), for 10 minutes. Rinse abundantly with water. 3. Cleaning oil and grease. Rinse the pH electrode with a mild detergent or with methyl alcohol. Rinse with water. 4. Cleaning of protein deposits. Soak the pH electrode in 1% pepsin and 0.1 M HCl for 5 minutes. Rinse with water. After carrying out each cleaning operation, rinse with deionised water and refill the reference electrode before use. Other precautionary measures 1. Do not strike the electrode. Given that the structure is generally made of glass and very fragile, it is necessary to manipulate it very carefully, preventing it from being knocked off.  TROUBLESHOOTING TABLE PROBLEM PROBABLE CAUSE SOLUTION The pH meter displays There are air bubbles in the electrode. Soak the electrode to eliminate the bubbles. Replace the batteries inconsistent readings. The electrode is dirty Clean the electrode and recalibrate. The electrode is not immersed. Verify that the sample covers the tip of the electrode perfectly. The electrode is broken Replace the electrode The electrode’s response is slow. The electrode is dirty or greasy. Clean the electrode and recalibrate. The screen shows an error message. Incorrect operating mode selected Verify the operation mode selected. Select a valid operation. The screen shows a calibration or error There is a calibration error. Recalibrate the Ph meter message. The calibration of the buffer value is Verify the buffer values used. erroneous. The electrode is dirty Clean and calibrate the electrode. The pH meter is on, but there is no signal The batteries are badly installed. Verify the polarity of the batteries. on the screen.* The batteries are worn out. Replace the batteries The battery indicator is flashing.* The batteries are worn out. Replace the batteries