A chemist performs controlled-potential coulometry to measure Cu²⁺ concentration, but the result is 15% higher than expected from a parallel ICP-MS measurement. What is the most likely cause?
AThe Faraday constant used in the calculation was incorrect
BA side reaction such as water electrolysis consumed charge without converting Cu²⁺, inflating the measured Q
CThe molar mass of copper was not correctly accounted for in the Faraday's law calculation
DThe controlled potential was too low, causing incomplete electrolysis of the analyte
Coulometry requires 100% current efficiency — every coulomb of charge must convert analyte, not drive side reactions. If water electrolysis or oxygen reduction consumes some charge, Q is artificially high, and the calculated analyte amount is overestimated. This is the critical limitation: the method is absolute only when side reactions are suppressed. Option D would cause underestimation (incomplete conversion), not overestimation. Options A and C are calculation errors unrelated to the chemistry.
Question 2 Multiple Choice
Why is coulometry considered an 'absolute' analytical method?
AIt is more accurate than all other electroanalytical methods
BIt requires no calibration against external standards because the charge-to-moles relationship is defined by fundamental constants
CIt can be applied absolutely to any analyte without modification
DThe electrode potential is held absolutely constant throughout the measurement
The term 'absolute' means the method relies on fundamental constants rather than comparisons to known samples. Faraday's law (Q = nFN) connects measurable charge to moles of analyte through F (96,485 C/mol of electrons) and n (electrons per analyte molecule), both of which are known exactly. No calibration curve or standard solution is needed. This makes coulometry a primary reference method — it can verify other analytical methods rather than relying on them.
Question 3 True / False
In a coulometric titration, a chemist can calculate exactly how much titrant was delivered without measuring any volume.
TTrue
FFalse
Answer: True
This is one of coulometric titration's key practical advantages. The titrant is electrogenerated in situ, and the amount produced equals Q/(nF) — charge divided by the product of electrons per molecule and the Faraday constant. No volumetric glassware, no standardized solutions, no volume measurement is required. This eliminates several sources of volumetric error and makes coulometric titrations highly precise for trace analysis. The Karl Fischer water determination is the most commercially important example of this principle.
Question 4 True / False
Since coulometry requires no external calibration standards, it achieves high accuracy even when dissolved oxygen or other electroactive species are present in the solution alongside the analyte.
TTrue
FFalse
Answer: False
Coulometry is an absolute method only when current efficiency is 100% — all charge must go toward converting the intended analyte. Dissolved oxygen, water, and other electroactive impurities can consume charge through side reactions, causing the measured Q to exceed what was used for the analyte. The result is a systematic positive error in the calculated analyte amount. Ensuring current efficiency requires careful potential control, degassing the solution, and selecting supporting electrolytes that minimize competing reactions.
Question 5 Short Answer
Why does coulometry not require external calibration standards, and what experimental condition must be met for this advantage to hold?
Think about your answer, then reveal below.
Model answer: Coulometry relies on Faraday's law: Q = nFN, where F is a fundamental constant and n is known from the chemistry. The relationship between charge and moles is fixed by nature, not by comparison to a standard. The condition that must be met is 100% current efficiency — every electron must participate in the intended reaction, with no charge diverted to side reactions. If current efficiency falls below 100%, the measured charge overstates what converted the analyte, and the absolute relationship between Q and N no longer holds.
The elegance of coulometry is that it 'counts' molecules using a quantity (electric charge) that can be measured with extreme precision using current-time integration. The Faraday constant acts as an exact conversion factor. But this only works if there's a one-to-one correspondence between charge and analyte molecules — any charge 'leaking' into side reactions breaks the accounting. This is why ensuring current efficiency is the central technical challenge in coulometric methods, requiring careful potential control and solution preparation.