why can measured and also calculated pH differ

Discussions around HPLC, CE, TLC, SFC, and also various other "liquid phase" separation techniques.




You are watching: Why was your measured ph level of the buffer solutions different from the calculated values?

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Bill TindallPosts: 144Joined: Mon Aug 30, 2004 10:12 pm
This write-up is in response to a question buried in a previous short article on pH.To paraexpression the question: When I consist of a buffer according to calculated quantities of buffer components, the measured pH differs from the calculated pH. Why?This question does not have a straightforward answer. I spfinish an hour of a Pittcon short course on pH giving sufficient background on the concept and practice of pH measurement to completely answer this question. Within the boundaries of my keying stamina right here is a partial answer:1. The globally embraced definition of pH is characterized as a result of a measurement process (an operational definition) and only in very limited cases is pH equal to –log (hydrogen ion activity). Hence, accuracy in a lot of pH measurements is a meaningmuch less concept; only precision is appropriate. Unfortunately, few publications and also classes get past the outdated and restricted meaning, –log (hydrogen ion activity), so the remainder of this discussion may be difficult to accept.2. The glass electrode response includes a junction potential response and the desired pH response. This junction potential component is dependent on the solution components including any kind of charged species (ions, colloides, zwitter ions and polyelectrolytes) and also nonaqueous solvent components. The magnitude of this response have the right to be between zero and several pH unit equivalents.3. If the buffer is dilute, aqueous and also in the mid selection of pH, and the glass electrode was calibrated through NIST pH standards, then the measured pH of the buffer in question will be exceptionally close to –log (hydrogen ion activity). Under these special problems, the junction potential errors of the calibration traditional and also sample buffer fundamentally cancel. When these particular criteria are not met, then the measured pH response differs from –log (hydrogen ion activity).4. If a pH is calculated utilizing a specific thermodynamic pKa (the value most likely uncovered in referral tables) and also activity coefficients of all buffer components are appropriately accounted for, then the calculated pH will equal –log (hydrogen ion activity). These calculations are tedious. Fortunately, there is a buffer regime accessible that does these calculations. 5. While a precision of better than 0.01 unit is feasible, the typical precision of making a pH dimensions is closer to 0.1 unit. From the indevelopment in 3, 4 and also 5 we view ample opportunity for a calculated pH to differ from a measured pH. Does it matter? Many pH dimensions just have to be reproducible; accuracy is irappropriate or also meaningless in the context of these operational pH dimensions. Because measured pH values usually have no basic significance, when reporting pH worths it is necessary to describe how the values were obtained so that others can reproduce them. When this summary is lacking, others recreating the work deserve to (usually will) make differing presumptions or measuring operations, which have the right to bring about different results. We have actually viewed this trouble many kind of times on this Forum. Suppose one reports that a reaction or separation is finest accomplished by preparing a buffer by adding XX grams of component A and YY grams of component B. There is no ambiguity in reproducing this buffer and also it have the right to be redeveloped via good precision.

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In many type of handy instances the true –log (hydrogen ion activity) of this buffer is unrecognized, can"t be measured, nor does it issue. An operational pH of this buffer can be measured and also reported, but is this value more valuable than the buffer recipe?