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pH - theorypH measurement – additional information1. GeneralpH probe is a unit, measuring the solution acidity or alkalinity degree. It is measured on scale of 0 to 14. The term pH is derived from "p", the mathematical symbol for the negative logarithm, and "H", the chemical symbol of Hydrogen. The formal definition of pH is the negative logarithm of the Hydrogen ion activity. 2. pH Electrode For pH measurement Lactoscan needs a combination electrode, compatible with most pH electrodes that have BNC connectors and zero potential (the pH where the mill volt output of the electrode equals 0) near 7 pH. 2.1. Electrode part The electrode is the most important part of the pH measurement. The electrode glass membrane is fragile and must be handled with care. To protect the glass membrane and to maintain activation, a protective rubber cap containing a suitable storage solution covers the glass membrane. 2.2. Electrode care & Electrode maintenance pH Electrodes are susceptible to dirt and contamination and need to be clean regularly depending on the extent and condition of use. At no time should one touch or rub the glass bulb as this causes the build-up of electrostatic charge. 2.3. Storage For best results, always keep the pH bulb wet. An optimal storage solution for combination electrode is pH 4 buffer with 225 grams of KC1 per liter. Table salt, NaCl, can be used if KC1 is not really available. Other pH buffers or tap water are also acceptable storage media, but avoid storage in de-ionized water. The protective rubber cap filled with the buffer solution provides ideal storage for long periods. 2.4. After Use After measurement is completed, follow the sequence below for storage. a) Wash the electrode and reference junction in de-ionized water. b) Close the refilling hole by returning its rubber sleeve or stopper cap.(Necessary for only refillable electrode). c) Store the electrode as mentioned above (see section Storage). 2.5. Electrolyte Replacement (for refillable electrode only). The reference electrolyte needs to be refilled when the electrode has been used for a long period, or when the internal electrolyte has dried up. To accomplish this, follow the procedure described below. a) Remove the protective rubber cap or sleeve. b) Remove the protective rubber sleeve to expose the filling port of the electrode. Remove the old reference electrolyte with a syringe. c) Fill the new reference electrolyte. New electrolyte preparation: Open the KCl container. Add in de-ionised water until it reaches the level of 20 ml. Close the container and shake it to dissolve the KCl. Add in fresh electrolyte until it reaches the level of the refilling port. The reference electrolyte used should be 3M(Mol) KC1. Replace the rubber sleeve. Re-use the electrode. Rinse the liquid junction with de-ionized water. Note: If these steps fail to restore normal electrode response, you may attempt to rejuvenate it (See: Electrode Rejuvenation). 2.6. Electrode cleaning Electrodes which are mechanically intact can often be restored to normal performance by one or combination of the following procedures. a) Salt deposits: Dissolve the deposit by immersing the electrode in tap water for ten to fifteen minutes. Then thoroughly rinse with de-ionized water. Wash the electrode pH bulb in a little detergent and water. Rinse electrode tip in with de-ionized water b) Oil/Grease films: Wash electrode pH bulb in a little detergent and water. Rinse electrode tip with de-ionized water. c) Clogged Reference Junction: pH electrodes have junction, which allows the internal fill solution of the measuring electrode to leak out into the solution being measured. The junction can become clogged by contaminations in the solution. If a clogged junction is suspected it is best to clear the junction. Heat up the diluted KC1 solution to 60-80°C. Place the sensing part of the pH electrode into the heated KC1 solution for approximately 10 minutes. Allow the electrode to cool while immersed in some unheated KCl solution. d) Protein Deposits Prepare 1% pepsin solution in 0.1 M HCI. Allow the electrode to stand in this solution for five to ten minutes. Rinse the electrode with de-ionized water. 2.7. Electrode activation Generally, if the procedure of storage and maintenance had been closely followed, the electrode can be used immediately. However, should the electrode response become sluggish, it may be possible that the bulb has dehydrated. The bulb can be dehydrated by immersing the electrode in an ideal storage solution (e.g. buffer pH 4 solution) for 1-2 hours. If this fails, the electrode may require reactivation. If the above procedure does not reactivate the electrode to acceptable status, try rejuvenation the electrode by following the procedure outlined below. 2.8 Rejuvenation Procedure a) Dip and stir the electrode in freon or alcohol for 5 minutes. b) Leave the electrode in tap water for 15 minutes. c) Dip and stir the electrode in concentrated acid (HCI, H2S4) for 5 minutes. d) Repeat Step b - leave the electrode in tap water for 15 minutes. e) Dip and stir in strong base (NaOH) for 5 minutes. f) Repeat Step b - leave the electrode in tap water for 15 minutes. g) Test with standard calibration solution. Finally, test with standard calibration buffer solution to see if the electrode yields acceptable results. You may repeat steps “c” to “f” again for better response (maximum 3 times). If the response does not improve, then the electrode has completed its useful life. Replace with a new electrode. 3. Electrode Lifespan pH electrodes have a finite lifespan due to their inherent properties. How long a pH electrode will last will depend on how it is cared and the solution it is used to measure. Even if an electrode is not used it still ages. Electrode demise can usually be characterized by a sluggish response, erratic readings or a reading, which will not change. When this occurs an electrode can no longer be calibrated. pH electrodes are fragile and have a limited lifespan. How long an electrode will last is determined by how well the is maintained and the pH application. The harsher the system, the shorter the lifespan. For this reason it is always a good idea to have a back-up electrode on hand to avoid any system down time. 4. Buffer Solutions Buffers are solutions that have constant pH values and the ability to resist changes in that pH level. They are used to calibrate pH measurement system. 5. pH Electrode Calibration pH Electrodes are like batteries; they run down with time and use. As an electrode ages, its glass changes resistance. For this reason, electrodes need to be calibrated on a regular basis. Calibration in pH buffer solution corrects for this change. Calibration is an important part of electrode maintenance. This assures not only that the electrode is behaving properly but that the system is operating correctly. Usually pH meters require calibration at 3 specific pH values. One calibration is usually performed at pH 7, second and third are typically performed at pH 4 and pH 10. Attention: It is best to select a buffer as close as possible to the actual pH value of the sample to be measured. Use standard calibration buffers that the temperature and the sample solution are the same. Use the operation manual for the corresponding pH meter. For Sensorex pH electrodes, originally supplied with Lactoscan read the following information: Temperature compensations The output of pH electrodes varies with temperature in manner, predicted by theory. When needed, Sensorex can supply electrode holders with build-in automatic temperature compensators. The need of automatic compensation depends on the temperature variation, the pH value being measured. At pH of about 7 there is no error due to temperature and, of course, at a constant temperature there is no error. As shown in the following table, the pH error due to temperature is a function of both the temperature and the pH value being measured. At a pH of about 7 there is no error due to temperature and, of course, at a constant temperature there is no error. The more the temperature changes from the ambient calibration temperature and the more the pH departs from 7 the greater is the pH error. pH temperature error table
For pH probes type ÝÑÊ-10601 – special for raw milk, please, refer to the following tables: Table 1: Isopotential point coordinates for pH probe type ÝÑÊ-10601 Ei=18 mV pH-6.70
Table 2: Isopotential point coordinates for pH probe type ÝÑÊ-10601 Ei=0 mV pH 4
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