You measure the same seawater sample with a sodium ISE twice: once after adding TISAB and once without. The reading without TISAB is higher. What is the most likely explanation?
ATISAB introduces a systematic negative error by binding sodium ions
BWithout TISAB, samples and standards have different ionic strengths, making activity coefficients unequal and causing concentration to be overestimated
CThe ISE membrane is damaged by high salt concentrations and must be equilibrated with TISAB first
DSeawater's ionic strength is too low to activate the ISE membrane properly without TISAB
ISEs respond to ion activity, not concentration. Activity = activity coefficient × concentration. Without TISAB, the seawater sample has much higher ionic strength than the calibration standards (which are typically prepared in low-ionic-strength solutions), so the sample's activity coefficient is lower than the standards'. The electrode reads a similar potential to a lower-concentration standard, causing the calculated concentration to be wrong — not necessarily higher. The key principle: TISAB swamps variable ionic strength by adding a high concentration of an inert salt, making all samples and standards have the same ionic strength and therefore the same activity coefficient. Option A is wrong: TISAB contains complexing agents for interfering ions (like CDTA for fluoride) but not for the target ion.
Question 2 Multiple Choice
A potassium ISE has a selectivity coefficient of K(K,Na) = 0.01 for sodium. A sample contains 1 mM K⁺ and 100 mM Na⁺. How significant is the sodium interference?
ANegligible — sodium is a different element and ISEs are perfectly selective
BSignificant — the effective sodium contribution equals 100 mM × 0.01 = 1 mM, which equals the potassium concentration and introduces ~100% error
CModerate — sodium at 100 mM contributes 0.1 mM equivalent of potassium, or ~10% error
DSignificant — sodium at 100 mM overwhelms the ISE regardless of the selectivity coefficient
The Nikolsky-Eisenman equation tells us that an interfering ion j with selectivity coefficient K(i,j) contributes an apparent concentration of K(i,j) × [j] to the measured signal. Here: 0.01 × 100 mM = 1 mM apparent potassium from sodium alone. Since the actual potassium is only 1 mM, the interference doubles the measured signal — a 100% error. This example illustrates why knowing the selectivity coefficient and your sample matrix is essential: an apparently good selectivity coefficient (10⁻²) can still cause catastrophic error when interfering ions are present at 100-fold excess.
Question 3 True / False
An ISE with a Nernstian slope measures the concentration of an ion directly.
TTrue
FFalse
Answer: False
A Nernstian slope means the electrode responds ideally to changes in ion activity, not concentration. Activity = γ × c, where γ is the activity coefficient that depends on ionic strength. In dilute solutions or when ionic strength is carefully controlled (e.g., via TISAB), activity and concentration are numerically close. But in real complex samples, they can differ substantially. The Nernstian slope confirms the membrane is functioning correctly, but converting potential to concentration still requires either (1) matching ionic strength between standards and samples, or (2) knowing the activity coefficient. This is the most persistent misconception about ISEs.
Question 4 True / False
A fluoride ISE with a measured slope of 45 mV/decade (instead of the theoretical 59.2 mV/decade) is likely to give accurate results if the samples are measured immediately.
TTrue
FFalse
Answer: False
A sub-Nernstian slope indicates membrane degradation, contamination, or poor equilibration. The slope matters not just for one measurement but for the entire calibration: if you use the 45 mV/decade slope to convert potentials to concentrations, your results will be systematically wrong — the electrode compresses the dynamic range relative to ideal behavior. Accuracy requires a slope close to theoretical. In practice, a slope below about 54 mV/decade for a monovalent ion is considered unacceptable and the membrane should be reconditioned or replaced. Speed of measurement does not compensate for a bad slope.
Question 5 Short Answer
Why do ISEs require total ionic strength adjustment buffer (TISAB), and what problem does it solve?
Think about your answer, then reveal below.
Model answer: ISEs measure ion activity, not concentration, and activity depends on ionic strength via the activity coefficient (a = γ·c). Different samples have different ionic strengths, so their activity coefficients differ even at the same concentration. TISAB solves this by adding a high-concentration inert electrolyte that overwhelms the variable background ionic strength of all samples and standards, making the ionic strength (and therefore γ) effectively constant across all measurements. This converts the activity measurement into a reliable proxy for concentration.
Without TISAB, a sample with high ionic strength would show lower activity (lower γ) than a low-ionic-strength standard at the same concentration, leading to underestimation. TISAB eliminates this source of error. For fluoride ISEs specifically, TISAB also contains CDTA (a chelating agent) to complex iron and aluminum that would otherwise bind fluoride and reduce the free [F⁻] available to the electrode, plus acetic acid/acetate buffer to control pH (since OH⁻ can interfere with the fluoride electrode at high pH).