Molecular Biology Problem Solver 10. Sách được nhiều nhà khoa học có uy tín, nhiều kinh nghiệm trong nghiên cứu thực nghiệm trình bày những vấn đề thường hay phát sinh trong phòng thí nghiệm. Do vậy mà sách không trình bày các protocol hay quy trình như các sách khác, thay vào đó các tác giả sẽ trình bày các vấn đề nhằm giúp giúp đọc giả: Tự nâng cao khả năng chẩn đoán nguyên nhân khi gặp các vấn để về kỹ thuật, quy trình, hóa chất, thuốc thử trong quá trình thực nghiệm trong phòng thí. | Figure 1 Mechanism of pH meter function. Reproduced with permission from Thermo Orion Inc. Calibration In order for the meter and electrode system to determine the pH of a sample it must compare the sample to known values or standards. The standards are specially formulated buffers that have been carefully studied to determine the effect of temperature on their pH. These buffers are used to generate a calibration curve plotting the known pH value versus the measured millivolts to determine the pH of a sample. The millivolt measurement of the sample is plotted on the curve and the pH value is read from the curve. The pH calibration curve is essentially linear. An average slope is determined at the end of the calibration and reported as percent with 100 being the theoretical value at that temperature or as a millivolts per decade of pH where is theoretical at 25 C Figure . This slope will change with temperature but it will automatically be corrected by the meter if the correct temperature is entered. Almost all laboratory pH electrodes are designed so that they read zero millivolts at pH 7. This zero point is called the isopotential point. Calibration curves at different temperatures and therefore with different slopes all pass through this point. Buffers are sold with the temperature-corrected pH values on their labels or they can be found in the research literature. How Does the Meter Measure the Sample pH The meter calculates the sample pH by measuring the mV of the sample then using the Nernst equation to solve for pH. The Nernst equation is used to describe electrode behavior How to Properly Use and Maintain Laboratory Equipment 81 Electrode Potential mV Figure Effect of slope on pH determination. Reproduced with permission from Thermo Orion Inc. Emeasured E0 5 log Uh where 5 is the slope and Emeasured is the electrical potential measured at the sensing electrode and E0 is the electrical potential measured at the reference electrode and the
