Problem 17 - Analytical Chemistry

Question

The potential of a hydrogen electrode in an acid solution is \(-0.465 \mathrm{~V}\) when measured against an SCE reference electrode. What would the potential be measured against a normal calomel electrode \((1 M \mathrm{KCl}) ?\)

Step-by-Step Solution

Answer

The potential is \(-0.426 \text{ V}\) against a normal calomel electrode.

Step 1: Identify Key Information

We are given the potential of a hydrogen electrode against a Saturated Calomel Electrode (SCE) as \(-0.465 \text{ V}\). We need to find the potential when measured against a normal calomel electrode (NCE) with \(1 \text{ M} \text{ KCl}\).

Step 2: Understand Reference Electrode Potentials

SCE has a standard potential of \(+0.241 \text{ V}\) relative to the Standard Hydrogen Electrode (SHE). A Normal Calomel Electrode (NCE) has a slightly different potential of \(+0.280 \text{ V}\) due to the different concentration of KCl.

Step 3: Calculate the Potential Difference

Calculate the difference in potential between the SCE and the NCE. The difference is given by: \[ 0.280 \text{ V} - 0.241 \text{ V} = 0.039 \text{ V} \]

Step 4: Adjust the Given Potential

The potential measured against the NCE is the given potential against the SCE added to the potential difference between SCE and NCE:\[ -0.465 \text{ V} + 0.039 \text{ V} = -0.426 \text{ V} \]

Step 5: Conclude the Calculation

Thus, when measured against the Normal Calomel Electrode, the potential of the hydrogen electrode in the acid solution would be \(-0.426 \text{ V}\).

Key Concepts

Saturated Calomel Electrode (SCE)

The Saturated Calomel Electrode (SCE) is a type of reference electrode used widely in electrochemical measurements. It is known for its stability and reproducibility. The SCE is made by inserting a mercury/mercury chloride paste into a solution saturated with potassium chloride (KCl). This creates a stable reference point for measuring electrode potentials.
The SCE has a standard electrode potential of approximately +0.241 V relative to the Standard Hydrogen Electrode (SHE). This value remains relatively constant, making the SCE a reliable choice for many laboratory applications. Because of its saturation with KCl, this electrode helps to minimize the changes in potential due to concentration variations. In experiments, this feature ensures that comparisons between different electrode potentials are accurate and consistent.

Normal Calomel Electrode (NCE)

The Normal Calomel Electrode (NCE) is another type of calomel electrode, similar to the SCE, but with a fundamental difference in the concentration of the potassium chloride solution. Instead of being saturated, the KCl concentration in an NCE is typically around 1 M.
This difference in concentration results in a slightly higher standard electrode potential, roughly +0.280 V, when compared to the SCE. The difference in potential between the NCE and the SCE is important for specific measurements, and it must be accounted for in calculations, such as the potential of an unknown electrode measured against these reference electrodes.
Utilizing an NCE is valuable in situations where different ionic strengths are required, offering flexibility in experimental conditions while still maintaining a high level of precision in measurements.

Standard Hydrogen Electrode (SHE)

The Standard Hydrogen Electrode (SHE) is considered the primary reference electrode in electrochemistry. It is the standard against which all other electrode potentials are measured. The SHE consists of a platinum electrode in contact with hydrogen gas at 1 atm pressure and an acidic solution with a hydrogen ion concentration of 1 M.
The SHE is assigned a potential of exactly 0.000 V at all temperatures, which acts as the baseline for the electrochemical series. As a result, it serves as a fundamental reference in determining the electrochemical behavior of various systems.
While the SHE provides the most fundamental reference point, it can be challenging to use in practice due to the requirement for precise conditions (hydrogen at 1 atm and a constant temperature). Therefore, other electrodes, such as the SCE and NCE, are often used in practice as they offer ease of use and greater practicality while still providing comparability to the SHE.