At the morning calibration at 37 °C, an analyzer's ion-selective electrode module rejects the sodium electrode, which shows a slope of 46 mV per tenfold change in activity; the acceptable range is 52 to 68 mV. The potassium slope is 59 mV and the chloride slope -57 mV, both acceptable. For two days, sodium readings had been slow to stabilize and sodium QC had drifted, while potassium and chloride QC stayed near their means. The sodium electrode has been in use for 8 months; the shared reference electrode and the calibrator solutions were replaced last week. The most likely cause is:

A protein-coated or aged sodium electrode

A blocked junction in the reference electrode

Contaminated or evaporated calibrator solutions

A low temperature in the measuring chamber

During calibration at 37 °C, an analyzer's sodium electrode shows a slope of 60 mV per tenfold change in activity, and its ionized calcium electrode a slope of 30 mV. The technologist should conclude that:

Both are acceptable, since Ca2+ is divalent

The sodium electrode is oversensitive

The calcium electrode is failing

Both need recalibration at 25 °C

How does the Clark electrode in a blood gas analyzer measure pO2?

By the current from O2 reduced at a cathode

By the voltage across a glass pH membrane

By the change in the sample's conductivity

By the time taken to generate silver ions

A child's sweat chloride is measured by coulometric titration. The chloride concentration is calculated from the:

Time needed to react all chloride with generated silver ions

Turbidity of the silver chloride formed

Potential across a chloride ion-selective membrane

Color change of a mercuric indicator

Osmometry, electrophoresis and electrochemistry

8 ASCP MLS practice questions on Osmometry, electrophoresis and electrochemistry, part of Laboratory Operations. The first 8 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
At the morning calibration at 37 °C, an analyzer's ion-selective electrode module rejects the sodium electrode, which shows a slope of 46 mV per tenfold change in activity; the acceptable range is 52 to 68 mV. The potassium slope is 59 mV and the chloride slope -57 mV, both acceptable. For two days, sodium readings had been slow to stabilize and sodium QC had drifted, while potassium and chloride QC stayed near their means. The sodium electrode has been in use for 8 months; the shared reference electrode and the calibrator solutions were replaced last week. The most likely cause is:
1. AA protein-coated or aged sodium electrode
2. BA blocked junction in the reference electrode
3. CContaminated or evaporated calibrator solutions
4. DA low temperature in the measuring chamber
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Answer: A · A protein-coated or aged sodium electrode
The Nernst equation predicts a slope of about 61.5 mV per tenfold change in activity for a monovalent ion at 37 °C, so 46 mV means the sodium electrode responds poorly. Because only sodium is affected, the fault must lie in the part that sodium does not share with the potassium and chloride electrodes: the sodium-selective membrane. Protein buildup and age reduce a membrane's response, which gives both the slow stabilization and the low slope seen here. The electrode should be cleaned with the manufacturer's deproteinizing solution and recalibrated, and replaced if the slope stays low.
- B: The reference electrode is shared, so a fault there would also affect potassium and chloride.
- C: The calibrators are shared by all the electrodes and would not affect sodium alone.
- D: Temperature changes the slope of every electrode, and only slightly: about 59 mV at 25 °C.
Question 2 · Hard
During calibration at 37 °C, an analyzer's sodium electrode shows a slope of 60 mV per tenfold change in activity, and its ionized calcium electrode a slope of 30 mV. The technologist should conclude that:
1. AThe sodium electrode is oversensitive
2. BBoth are acceptable, since Ca2+ is divalent
3. CThe calcium electrode is failing
4. DBoth need recalibration at 25 °C
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Answer: B · Both are acceptable, since Ca2+ is divalent
The Nernst slope is 2.303RT/zF, about 61.5 mV per tenfold change at 37 °C for a monovalent ion (z = 1). For a divalent ion such as calcium (z = 2) it is half that, about 30.8 mV. Both electrodes are therefore close to their theoretical slopes. A slope falling below an electrode's own expected value, not a calcium slope half that of sodium, is what signals membrane ageing.
- A: 60 mV is slightly below, not above, the theoretical 61.5 mV for a monovalent ion at 37 °C.
- C: A slope of about 30 mV is the expected Nernst slope for a divalent ion.
- D: Changing the temperature would change both slopes and is not needed here.
Question 3 · Medium
How does the Clark electrode in a blood gas analyzer measure pO2?
1. ABy the current from O2 reduced at a cathode
2. BBy the voltage across a glass pH membrane
3. CBy the change in the sample's conductivity
4. DBy the time taken to generate silver ions
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Answer: A · By the current from O2 reduced at a cathode
The Clark electrode is amperometric: a fixed voltage is applied to a platinum cathode behind an oxygen-permeable membrane. Oxygen that diffuses through is reduced at the cathode, and the current produced is proportional to the pO2. A silver/silver chloride anode completes the circuit. A damaged or protein-coated membrane gives drifting results, so membranes are changed on a schedule.
- B: A potential across a glass membrane is the pH and pCO2 principle.
- C: Conductivity is used to estimate hematocrit, not pO2.
- D: Timed generation of silver ions is coulometric chloride titration.
Question 4 · Medium
A child's sweat chloride is measured by coulometric titration. The chloride concentration is calculated from the:
1. ATurbidity of the silver chloride formed
2. BPotential across a chloride ion-selective membrane
3. CColor change of a mercuric indicator
4. DTime needed to react all chloride with generated silver ions
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Answer: D · Time needed to react all chloride with generated silver ions
A constant current releases silver ions from a silver electrode, and they combine with chloride to form insoluble silver chloride. When all the chloride is used up, free silver ions appear, indicator electrodes detect the rise in current, and a timer stops. By Faraday's law, the elapsed time is proportional to the amount of chloride. Coulometric titration is a recommended method for quantitative sweat chloride in cystic fibrosis testing.
- A: Silver chloride turbidity is not what the titrator measures.
- B: Measuring potential across a membrane is direct potentiometry with an ISE.
- C: A mercuric indicator is used in mercurimetric titration, a different method.
Question 5 · Medium
A patient's serum freezes at -0.540 °C. If 1 osmol of solute per kg of water lowers the freezing point by 1.86 °C, what is the serum osmolality?
1. A1,004 mOsm/kg
2. B290 mOsm/kg
3. C145 mOsm/kg
4. D540 mOsm/kg
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Answer: B · 290 mOsm/kg
Osmolality = freezing point depression ÷ 1.86 °C per osmol/kg = 0.540 ÷ 1.86 = 0.290 osmol/kg, or 290 mOsm/kg. Freezing point depression depends only on the number of dissolved particles, not on their size or charge. A freezing point osmometer supercools the sample, starts freezing, and reads the plateau temperature. Normal serum osmolality is about 275 to 295 mOsm/kg.
- A: 1,004 mOsm/kg multiplies by 1.86 instead of dividing.
- C: 145 mOsm/kg halves the result, wrongly treating each osmole as two ions.
- D: 540 mOsm/kg reads the depression in thousandths of a degree as if it were the osmolality.
Question 6 · Easy
On agarose at pH 8.6, gamma globulins may move slightly toward the cathode, behind the point of application. This is caused by:
1. AMolecular sieving
2. BElectroendosmosis
3. CIsoelectric focusing
4. DWicking
Answer and explanation in LearnFree account
Question 7 · Hard
Three proteins of similar size have isoelectric points of 4.8 (A), 6.8 (B) and 9.2 (C). In zone electrophoresis at pH 8.6, ignoring electroendosmosis, how do they move?
1. AAll three to the anode, with C moving farthest
2. BA and B to the anode, A farther; C to the cathode
3. CA and B to the cathode; C to the anode
4. DAll three to the anode, with A moving farthest
Answer and explanation in LearnFree account
Question 8 · Easy
In a potentiometric measurement, the reference electrode, such as a silver/silver chloride electrode, provides:
1. AThe current that drives the reaction
2. BA constant potential for comparison
3. CA membrane selective for the ion
4. DCorrection for sample temperature
Answer and explanation in LearnFree account

Osmometry, electrophoresis and electrochemistry

During calibration at 37 °C, an analyzer's sodium electrode shows a slope of 60 mV per tenfold change in activity, and its ionized calcium electrode a slope of 30 mV. The technologist should conclude that:

How does the Clark electrode in a blood gas analyzer measure pO2?

A child's sweat chloride is measured by coulometric titration. The chloride concentration is calculated from the:

A patient's serum freezes at -0.540 °C. If 1 osmol of solute per kg of water lowers the freezing point by 1.86 °C, what is the serum osmolality?

On agarose at pH 8.6, gamma globulins may move slightly toward the cathode, behind the point of application. This is caused by:

Three proteins of similar size have isoelectric points of 4.8 (A), 6.8 (B) and 9.2 (C). In zone electrophoresis at pH 8.6, ignoring electroendosmosis, how do they move?

In a potentiometric measurement, the reference electrode, such as a silver/silver chloride electrode, provides: