Why is observed ph different from actual ph

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This post is in response to a question buried in a previous post on pH. To paraphrase the question: When I make up a buffer according to calculated amounts of buffer components, the measured pH differs from the calculated pH. This question does not have a simple answer. I spend an hour of a Pittcon short course on pH providing enough background on the theory and practice of pH measurement to fully answer this question. Within the limits of my typing stamina here is a partial answer: 1.

Hence, accuracy in most pH measurements is a meaningless concept; only precision is relevant. The glass electrode response includes a junction potential response as well as the desired pH response.

This junction potential component is dependent on the solution components including any charged species ions, colloides, zwitter ions and polyelectrolytes as well as nonaqueous solvent components.

The magnitude of this response can be between zero and several pH unit equivalents. Under these special conditions, the junction potential errors of the calibration standard and sample buffer essentially cancel.

These calculations are tedious. Table 1: Hydrogen ion and hydroxide ion activities on the pH scale A change on the pH scale of 1. We encounter items of varying pH every day, as can be seen in Figure 1 below. How To Choose a pH Electrode. Yaseen 30 Oct why we use -log in PH? Add a comment Name. Email For verification purposes only. Your comment will appear after it is reviewed. Subscribe to Our Blog. Diffusion voltages at the junction are a common measurement error, so the junction plays a major role in the precision of measurements.

To keep these disruptive potentials small, the junction must guarantee a relatively large and consistent outflow of reference electrolyte. However, the junction must only be slightly permeable to prevent electrolyte from escaping too quickly, which is especially important with electrodes utilizing liquid electrolyte.

Different junction types have different outflow rates of electrolyte. In addition to the permeability of the junction, its electrical resistance should be as low as possible and it must be chemically inert. A ceramic junction uses the porosity of unglazed ceramic. Diffusion potentials are easily created in measurement solutions with greater ionic strength, as the concentration gradient at the junction is very large.

In lower ionic strength solutions, the resistance of the test material may be too high for exact measurements. Both effects are amplified by the low outflow rate, so ceramic junctions are less suitable in such cases. Due to the high risk of blockage of its pores, it is also not suitable for solutions containing suspended particles. The platinum junction consists of fine, twisted platinum filaments between which the electrolyte flows out along precisely defined channels.

The platinum junction features a very constant outflow and does not easily become blocked. The platinum junction is more sensitive to mechanical stress and is also less than optimal for strongly oxidizing or reducing solutions due to the occurrence of disruptive potentials. However, the platinum junction can be used almost universally. The ground-joint junction works with the thin gap of unlubricated ground glass as an outflow opening for the electrolyte.

It features a very low electrical resistance 0. The ground-joint junction is suitable for measurements in contaminated solutions, as it is easy to clean. Due to the high outflow rate, it is suitable for both high and low-ion solutions. The Science pHT-G is what we typically recommend when there are a lot of suspended particles in the sample.

Additional junctions can be used to prevent contamination of the reference system. With this design, the reference electrode is immersed in electrolyte solution within an additional chamber. This additional chamber acts as an extra barrier against contamination while additional junctions are used to ensure the reference system still has contact with the measurement solution.

The reference system can still become contaminated by the measurement solution, but the solution must first diffuse through the additional junction s. There is not a pH electrode currently available that can be used for every possible application, as there are different requirements for different applications.

Ask a Question. A variable in the Nernst equation is temperature, so the response i. Therefore, pH measurements should be completed with an accurate measurement of temperature.

The pH values of buffer solutions are temperature dependent and the response can vary from manufacturer to manufacturer. As a general rule, basic buffer solutions exhibit stronger temperature effects than acidic ones. Modern pH meters automatically adjust for the respective temperature profile once the buffer set being used has been correctly set. Buffers are aqueous solutions whose pH remains virtually unaltered by the addition of small quantities of acids or bases.

Buffer solutions are capable of binding hydrogen ions with the addition of acids and releasing hydrogen ions with the addition of bases.

Buffer solutions are often colored to clearly differentiate them from one another during calibration. The composition of buffer solutions varies depending on the manufacturer. When selecting a buffer set, care must be taken to ensure they are made according to the formula established by the National Institute of Standards and Technology NIST. These buffers have pH values of 4. Alternatively, NIST traceable buffers are also sufficient for use when calibrating.

Since both of these can change over time, frequent calibration is necessary. The theoretical zero point is, not surprisingly, 0 mV. This is true because the reference electrode is typically in a solution of electrolyte that has a pH of 7.

If the reference and the sensing electrode are both in a solution with the same pH, there should theoretically be no difference in their potentials, resulting in a display of 0 mV on the pH meter. A new electrode will have an asymmetry potential that is typically only a few mV if it has been carefully prepared. The zero point is helpful when determining the operating condition of the electrode. If the asymmetry point begins to drift too far from zero, the electrode may need to be cleaned, serviced, or replaced.

The asymmetry point will change as the electrode ages, so regular calibration is recommended and will be required more frequently as the electrode ages to compensate for these changes.