Potentiometry - Online Tutor, Practice Problems & Exam Prep

Potentiometry utilizes electrodes to measure voltage, revealing essential chemical information about solutions. The non-standard cell potential is calculated as the potential of the indicator electrode minus that of the reference electrode, plus the junction potential, which can be minimized using potassium chloride in the salt bridge. The Nernst equation relates electrode potential to analyte concentrations, allowing for detailed analysis of the analyte's composition through electrochemical cells, where half-reactions occur at the anode and cathode, facilitating the understanding of oxidation-reduction processes.

Potentiometry - Line Notation

Hey everyone, so here we're going to say that potentiometry uses electrodes to measure voltages that also provide vital chemical information on their solutions. We're going to say the difference in potential between the 2 electrodes provides an analysis for the composition of the analyte. And we're going to see if the measurement of the voltage at an electrode relative to the reference is done in the absence of current flow. So no current or electricity is being run through this. It's going to say, we're going to say here that our non-standard cell potential equals the potential of our indicator electrode minus the potential of our reference electrode plus the potential of our junction potential. Now, the junction potential arises from both ends of our salt bridge.

So here, we're going to say this is a result of a type of imbalance in the ions from one side to another. It could also be an imbalance or difference in ionic size for the salts, the neutral salts used within our salt bridge. But we can say that by fine-tuning the types of ions and the size of ions being used, we can make our potential or sub-potential for our junction potential negligible, meaning that it gets close to 0. And if it's close to 0 we can ignore it.

Now, we're going to say that when it comes to the salt bridge, to do this, we typically use potassium chloride. So we have our chloride ions here and our potassium ions here. Remember chloride ions, our negative ions, would flow towards the anode side, and our potassium ions would flow towards the cathode side. The reason that this is the ideal combination of ions is that they're pretty close in size and because of that they're the best types of ions used within our salt bridge. Doing this would help us get to our cell potential for our junction potential being close to 0.

Now, here if we take a look at this we have our typical electrochemical cell. In this electrochemical cell, we see that we have our silver chloride, this is our saturated solution of potassium chloride solution. We could say here, that we can create a solid, and here the solid would actually be AgCl, Ag, or AgCl. That will be our precipitate that forms when silver and chloride are allowed to connect together. We're going to say here that this side is our reference electrode, and because it's the anode side we can say that these 2 are connected together.

So our anode here is represented by our reference electrode. On this side here we have our indicator electrode, which is connected to our cathode. Now, here remember our reference electrode, this is the part of the cell that is held constant, so it's not changing in terms of concentration. The indicator electrode, this is the part of the cell that contains the analyte being analyzed, so the portion of the solution that we want to focus all our attention on determining its composition. Now, the half-reactions are given as we have silver chloride solid and here, we're going to say that we have an electron.

This electron is being given to the chloride ion. And by doing this, our silver goes from being plus 1 to being neutral. So here we have silver solid being created, a precipitate. We know that this is the anode from this notation of minus because here our minus sign is indicative of the anode. And then here we have Fe³⁺ absorbs an electron to become Fe²⁺.

This is indicative of a cathode. Notice here we don't form a precipitate, which is why within our image we don't have an iron electrode, instead, we have a platinum electrode. This platinum electrode serves as an inert electrode within this reaction. Now, with this information, we can say that the Nernst equation provides a mathematical relationship between the electrode potential and an analyte's reduced and oxidized forms. Now, here we're going to say the difference in potential is based on only one of the half solid concentrations because remember our reference electrode is held constant.

So here we would have this side represented for the cathode because here we're dealing with our positive electrode which is the cathode. Here we're dealing with the negative electrode which is our anode, and we're going to say here that remember that your non-standard cell potential equals in this case: E_positive − E_negative , cathode minus anode, which in this case since we're dealing with indicator and reference electrodes will be, sub potential of our indicator electrode minus the sub-potential of our reference electrode. So when it comes to this whole process, we're basically making sure that one of our half reactions is done with the reference electrode, but we know its value based on literature. And all we're doing is examining the analyte, the one that has a varying concentration. From this, we can obtain a plethora of information that gives us a lot of detail in terms of the composition of that particular analyte.

And again, this can be based on different concentrations, as well as the cell potentials that are being created.

Here’s what students ask on this topic:

What is potentiometry and how does it work?

Potentiometry is an analytical technique that uses electrodes to measure the voltage of a solution, providing essential chemical information about its composition. It involves two electrodes: a reference electrode, which has a constant potential, and an indicator electrode, which responds to the analyte's concentration. The difference in potential between these electrodes is measured in the absence of current flow. The non-standard cell potential is calculated using the equation:

$E{\mathrm{cell}}_{\mathrm{non-standard}}=E_{\mathrm{indicator}}-E_{\mathrm{reference}}+E_{\mathrm{junction}}$

By fine-tuning the ions in the salt bridge, typically using potassium chloride, the junction potential can be minimized, allowing for accurate analysis of the analyte's composition.

What is the role of the salt bridge in potentiometry?

The salt bridge in potentiometry serves to maintain electrical neutrality within the electrochemical cell by allowing the flow of ions between the two half-cells. It typically contains a solution of potassium chloride (KCl), which provides chloride ions (Cl^-) and potassium ions (K⁺). These ions migrate to the anode and cathode, respectively, to balance the charge. The salt bridge helps minimize the junction potential, which arises from the imbalance of ions at the interface of the two solutions. By using ions of similar size, such as K⁺ and Cl^-, the junction potential can be made negligible, ensuring accurate measurement of the cell potential.

How is the Nernst equation used in potentiometry?

The Nernst equation is used in potentiometry to relate the electrode potential to the concentrations of the reduced and oxidized forms of the analyte. The equation is given by:

$E=E_0-\frac{\mathrm{RT}}{\mathrm{nF}}ln\left(\frac{\mathrm{[Ox]}}{\mathrm{[Red]}}\right)$

where $E$ is the electrode potential, $E_0$ is the standard electrode potential, $R$ is the gas constant, $T$ is the temperature, $n$ is the number of electrons transferred, and $F$ is the Faraday constant. This equation allows for the determination of the analyte's concentration by measuring the electrode potential.

What are the components of an electrochemical cell used in potentiometry?

An electrochemical cell used in potentiometry consists of several key components:

• Reference Electrode: This electrode has a constant potential and is typically made of silver/silver chloride (Ag/AgCl) or calomel (Hg/Hg₂Cl₂).
• Indicator Electrode: This electrode responds to the analyte's concentration and can be made of various materials, such as platinum or glass.
• Salt Bridge: This component contains a solution of potassium chloride (KCl) to maintain electrical neutrality and minimize the junction potential.
• Electrolyte Solution: The solution in which the analyte is dissolved and the electrochemical reactions occur.

These components work together to measure the voltage difference, which provides information about the analyte's composition.

How does the junction potential affect potentiometric measurements?

The junction potential arises at the interface of the two solutions in the salt bridge and can affect the accuracy of potentiometric measurements. It is caused by the imbalance of ion concentrations and differences in ionic mobility. If not minimized, the junction potential can introduce errors in the measured cell potential. To reduce its impact, potassium chloride (KCl) is commonly used in the salt bridge because its ions (K⁺ and Cl^-) have similar sizes and mobilities, making the junction potential negligible. By fine-tuning the types and sizes of ions in the salt bridge, the junction potential can be minimized, ensuring more accurate and reliable potentiometric measurements.