Accumet XL 20 User Manual

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Accumet XL 20 User Manual Manual pdf 185 page image

174Appendix: pH TheoryXL15, 20, 25, 50 and 60 meterspH MeasurementThe pH value of a sample can be determined in several ways. These include the use of organicdyes which change color in certain pH ranges. The dyes can be added directly to the solution orimpregnated onto paper which may be dipped into the solution. At best, these "colorimetric"methods yield approximate pH values, often with an accuracy of plus or minus 0.5 pH units.The preferred and most accurate way to measure a pH value is the potentiometric measurement,using a pH electrode, a reference electrode, and a pH meter. This method is based on the factthat certain electrodes, immersed in solution, produce a millivolt potential (i.e. voltage ) that isrelated to the hydronium ion concentration or pH of a solution in a precise way.The relationship between the electrode's voltage and the solution pH is defined by the Nernstequation:2.3RTEmeas = E* – nF (pH)In this equation, Emeas is the voltage output of the electrodes, E* is the total of all other voltagesin the system including the reference voltage, R is the Gas Law constant, T is the temperature in°Kelvin, n is the charge on the hydronium ion (+1), and F is the Faraday constant.pH MeterThe pH meter is a sensitive voltmeter capable of accurately measuring small voltage differencesbetween the pH electrode and the reference electrode. This voltage difference is amplified, andshown as a pH reading. Almost all modern meters are microprocessor controlled, andprogrammed to use electrode voltages, efficiency, and temperature to calculate an accurate pHfor the sample. Since the voltage output from different electrodes will vary, it is essential tocalibrate a pH meter. The ability to calibrate or to standardize a pH meter permits the meter tomatch the pH reading on the meter with the known pH value of a buffer solution. For bestaccuracy, it is recommended that at least two buffer solutions be used to calibrate a pH meter.Buffer solutions are available as ready-to-use solutions, concentrated solutions, capsules, orprepackaged salts.The following table lists three of the most widely used NIST buffers along with their pH values atdifferent temperatures.Temperature (C) 4.01 6.86 9.180 4.003 6.984 9.46410 3.998 6.923 9.33220 4.002 6.881 9.22525 4.008 6.865 9.18030 4.015 6.853 9.13940 4.035 6.838 9.06850 4.060 6.833 9.011( )