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pH BUFFERS pH Buffer Solutions are an Indispensable tool for maintaining an accurate pH Measurement. A pH Measurement Loop requires regular Calibration of the electrodes to compensate for their aging and deterioration, therefore it is essential to have a reliable standard with which to do the calibration. This is the task of the pH buffers. They are the reference points to which the measurement electrodes are calibrated. Consequently, if these buffers are not accurate themselves, the calibration serves no useful purpose. By definition, a buffer is a solution which is designed to maintain its related value even with the intrusion of contaminants. The level of contaminants a buffer can withstand before becoming inaccurate has a great deal to do with how close to equilibrium the solution is in the first place. A good buffer solution is a mixture of a weak acid and the salt of a weak acid, mixed in a 1:1 ration (equilibrium). The pH of such a solution if calculated by the following formula: pH = pKa + Log(Salt)(Acid) If the Salt /Acid Ration is 1, then we have: pH = pKa, because log = 0. For NBS Buffers (Now N.I.S.T., National
Institute of Standards and Technology)
Therefore, the pH Values for the best buffer solutions are 4.01pH, 6.865 pH, and 9.18 pH, respectively. These are the equilibrium values at which the buffer can accept the maximum level of contaminants before becoming inaccurate. This is referred to as the "Buffer Capacity". Why the do most customers use technical or traceable buffers with values of 4.00 pH, 7.00 pH and 10.00 pH? It is a carry-over from the days of analog instruments. It was obviously much easier to calibrate an analog meter to 7.00 than to 6.865, therefore a market for modified or NBS traceable buffers was born at the expense of buffer capacity and accuracy. Is this convenience still necessary? Not in today's world of modern pH Instrumentation. With LCD indicators and automatic buffer recognition via microprocessors, ease of calibration has been built into the instrument. So, there is no longer a need to create crutches to compensate for the weakness of analog devises.
Traceable and Standardized What does "traceable to NBS" mean? Not much really! It is a lot like saying that orange soda is traceable to oranges. It is an "adjusted" NBS buffer, therefore having reduced "buffer capacity". A buffer solutions that is "standardized against NBS" is one that uses NBS values as a reference while "adjusting" the solution to the desired value. It is just another type of "traceable" buffer.
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