The sodium electrode slope on an ISE analyser, using the same calibrator lot, has fallen over two weeks: 58, 54, 49 and now 41 mV per decade (acceptable 52-62). The response has become slow. What is the best action?

Clean or replace the sodium electrode membrane

Open a new lot of quality control material

Raise the sample aspiration volume

Report results with a correction factor

The flame of a routine laboratory flame photometer is usually fed with:

LPG or propane with compressed air

Colorimetry, instruments and laboratory calculations

50 Lab technician recruitment practice questions on Colorimetry, instruments and laboratory calculations, part of Clinical Biochemistry. The first 25 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 · Medium
A 0.5 M solution of phosphoric acid (H3PO4), when fully neutralised, has a normality of:
1. A0.5 N
2. B1.0 N
3. C0.17 N
4. D1.5 N
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Answer: D · 1.5 N
Normality equals molarity multiplied by the number of replaceable hydrogen ions. Phosphoric acid has three, so 0.5 M x 3 = 1.5 N when all three react. In a titration that stops after the first or second hydrogen, the normality would be lower, so the reaction must be stated. For hydrochloric acid, with one hydrogen, normality equals molarity.
- A: 0.5 N treats phosphoric acid as if it had one replaceable hydrogen.
- B: 1.0 N counts only two of the three hydrogen ions.
- C: 0.17 N divides by three instead of multiplying.
Question 2 · Hard
The sodium electrode slope on an ISE analyser, using the same calibrator lot, has fallen over two weeks: 58, 54, 49 and now 41 mV per decade (acceptable 52-62). The response has become slow. What is the best action?
1. AOpen a new lot of quality control material
2. BRaise the sample aspiration volume
3. CClean or replace the sodium electrode membrane
4. DReport results with a correction factor
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Answer: C · Clean or replace the sodium electrode membrane
A healthy sodium electrode gives close to the Nernst slope of about 59 mV per decade. A steady fall with the same calibrators, along with a slow response, points to a membrane coated with protein or simply worn out. The electrode is cleaned with the maker's protein-removing solution and reconditioned, or replaced, and the analyser recalibrated. Patient results are not reported while the slope is out of range.
- A: A new QC lot does not fix the electrode; the calibration slope itself has failed.
- B: Sample volume does not change the electrode's mV response per decade.
- D: Results from a failing electrode must not be patched with a factor.
Question 3 · Medium
A glucose stock standard contains 1,000 mg/dL. How much stock is diluted to 100 mL to make a working standard of 100 mg/dL?
1. A90 mL
2. B10 mL
3. C1 mL
4. D100 mL
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Answer: B · 10 mL
Use C1V1 = C2V2: 1,000 x V1 = 100 x 100, so V1 = 10 mL. The 10 mL of stock is pipetted into a 100 mL volumetric flask and made up to the mark with the diluent, often benzoic acid solution as preservative. This gives a tenfold dilution. Working standards are made fresh at intervals because dilute glucose solutions can grow bacteria.
- A: 90 mL is the volume of diluent added, not of stock.
- C: 1 mL in 100 mL gives only 10 mg/dL.
- D: 100 mL of stock leaves the strength at 1,000 mg/dL.
Question 4 · Medium
To get a straight-line standard graph through the origin for a method that obeys Beer's law, what is plotted against concentration?
1. ALog of concentration
2. BPercent transmittance
3. CAbsorbance
4. DTransmitted light intensity
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Answer: C · Absorbance
Absorbance is directly proportional to concentration, so absorbance against concentration gives a straight line through zero. Transmittance falls exponentially as concentration rises, so a %T plot is a curve; it becomes straight only on semi-log paper. A straight line lets a single standard be used for calculation within the linear range. Points that bend away at the top show the upper limit of linearity.
- A: Concentration is already on the x-axis; its log is not plotted.
- B: Percent transmittance falls exponentially and gives a curve on ordinary graph paper.
- D: Transmitted light falls exponentially with concentration and gives a curve.
Question 5 · Easy
The flame of a routine laboratory flame photometer is usually fed with:
1. ANatural gas with argon
2. BLPG or propane with compressed air
3. CAcetylene with nitrous oxide
4. DHydrogen with pure oxygen
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Answer: B · LPG or propane with compressed air
Routine flame photometers burn liquefied petroleum gas or propane with air from a small compressor. This flame, at about 1,900 °C, is hot enough to excite sodium and potassium but excites few other atoms, which keeps the spectrum simple. Much hotter flames such as nitrous oxide-acetylene are used in atomic absorption for refractory elements. A steady gas and air supply is essential for stable readings.
- A: Argon is the plasma gas of ICP instruments, not a flame photometer fuel.
- C: Nitrous oxide-acetylene is a very hot flame used in atomic absorption.
- D: Hydrogen-oxygen flames are not used in routine flame photometers.
Question 6 · Easy
Flame photometry for sodium and potassium is based on:
1. ACurrent produced at an electrode
2. BLight absorbed by ground-state atoms
3. CLight scattered by particles
4. DLight emitted by excited atoms
Answer and explanation in LearnFree account
Question 7 · Medium
An acetate buffer is made with sodium acetate and acetic acid in a ratio of 10:1 (pKa 4.76). What is its pH?
1. A3.76
2. B6.76
3. C5.76
4. D4.76
Answer and explanation in LearnFree account
Question 8 · Easy
pH is defined as the:
1. ANegative log of hydrogen ion concentration
2. BNegative log of hydroxyl ion concentration
3. CRatio of acid to base in a solution
4. DPositive log of hydrogen ion concentration
Answer and explanation in LearnFree account
Question 9 · Medium
What is the pH of a 0.001 M sodium hydroxide solution at 25 °C?
1. A13
2. B3
3. C11
4. D10
Answer and explanation in LearnFree account
Question 10 · Medium
A patient with multiple myeloma has total protein 11 g/dL. Sodium is 128 mmol/L on the main analyser (indirect ISE) and 139 mmol/L on the blood gas analyser (direct ISE). Which statement is correct?
1. AIndirect ISE gives the true plasma-water sodium
2. BBoth values are wrong; flame photometry is needed
3. CDirect ISE gives the true plasma-water sodium
4. DThe difference is due to haemolysis of the sample
Answer and explanation in LearnFree account
Question 11 · Easy
The violet colour of the biuret reaction for serum total protein is read with which filter?
1. AViolet (about 400 nm)
2. BRed (about 680 nm)
3. CGreen (about 540 nm)
4. DBlue (about 450 nm)
Answer and explanation in LearnFree account
Question 12 · Medium
What effect does stray light reaching the detector of a spectrophotometer have on readings?
1. AFalsely low absorbance at high concentrations
2. BA constant positive error at every level
3. CFalsely high absorbance at low concentrations
4. DNo effect if a reagent blank is used
Answer and explanation in LearnFree account
Question 13 · Easy
Before a centrifuge is started, the loaded tubes should be:
1. ALeft uncapped so that air can escape
2. BFilled to the brim to stop spilling
3. CAll placed together on one side
4. DOf equal weight, placed opposite each other
Answer and explanation in LearnFree account
Question 14 · Easy
In a filter photoelectric colorimeter, which part selects the band of wavelengths that passes through the sample?
1. AThe photocell
2. BThe cuvette
3. CThe tungsten lamp
4. DThe coloured glass filter
Answer and explanation in LearnFree account
Question 15 · Hard
To make a buffer, 0.1 M NaOH (50 mL) is added to 0.2 M acetic acid (50 mL; pKa 4.76). The resulting pH is:
1. A4.76
2. B2.87
3. C7.00
4. D5.76
Answer and explanation in LearnFree account
Question 16 · Easy
Which wavelength does the sodium filter of a flame photometer pass?
1. A767 nm
2. B622 nm
3. C671 nm
4. D589 nm
Answer and explanation in LearnFree account
Question 17 · Medium
A blood tube breaks during a run in a centrifuge without sealed buckets. What should the technician do first?
1. AOpen the lid at once and remove the glass
2. BSpray disinfectant through the lid while it spins
3. CSwitch off and keep the lid closed for 30 minutes
4. DRestart the run to finish the other samples
Answer and explanation in LearnFree account
Question 18 · Medium
All of the following are parts of a photoelectric colorimeter EXCEPT:
1. AA tungsten filament lamp
2. BA hollow cathode lamp
3. CA coloured glass filter
4. DA photovoltaic cell
Answer and explanation in LearnFree account
Question 19 · Easy
The standard cuvette used in routine colorimetry has a light path of:
1. A1 mm
2. B5 cm
3. C1 cm
4. D10 cm
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Question 20 · Hard
A grossly lipaemic sample is tested by an end-point colorimetric glucose method zeroed with the reagent blank. Test absorbance 0.55; serum blank (serum with reagent lacking the enzyme) 0.15; 100 mg/dL standard 0.40. The glucose is:
1. A175 mg/dL
2. B38 mg/dL
3. C100 mg/dL
4. D138 mg/dL
Answer and explanation in LearnFree account
Question 21 · Medium
An ion-selective electrode responds directly to which property of the ion in the sample?
1. AIts total concentration
2. BIts protein-bound fraction
3. CIts mass per litre
4. DIts activity (free ion)
Answer and explanation in LearnFree account
Question 22 · Hard
The psychiatry ward asks for serum lithium on bipolar patients (therapeutic range 0.6-1.2 mmol/L). The flame photometer uses a lithium internal standard for sodium and potassium. What change is needed to measure lithium?
1. AUse caesium as the internal standard
2. BRead lithium at the sodium wavelength
3. CAdd lanthanum chloride as releasing agent
4. DKeep the lithium diluent as usual
Answer and explanation in LearnFree account
Question 23 · Medium
During a spectrophotometer performance check, the wavelength accuracy is verified with:
1. APotassium dichromate solution
2. BA holmium oxide filter
3. CSodium nitrite solution
4. DA distilled water blank
Answer and explanation in LearnFree account
Question 24 · Easy
Which pair of chemicals in solution forms a buffer?
1. ASodium hydroxide and sodium chloride
2. BHydrochloric acid and sodium chloride
3. CAcetic acid and sodium acetate
4. DSulphuric acid and sodium sulphate
Answer and explanation in LearnFree account
Question 25 · Medium
A patient's arterial pH falls from 7.40 to 7.10. How has the hydrogen ion concentration changed?
1. AIt has risen tenfold
2. BIt has about halved
3. CIt has risen by 4%
4. DIt has about doubled
Answer and explanation in LearnFree account

25 more Colorimetry, instruments and laboratory calculations questions

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Colorimetry, instruments and laboratory calculations

A 0.5 M solution of phosphoric acid (H3PO4), when fully neutralised, has a normality of:

The sodium electrode slope on an ISE analyser, using the same calibrator lot, has fallen over two weeks: 58, 54, 49 and now 41 mV per decade (acceptable 52-62). The response has become slow. What is the best action?

A glucose stock standard contains 1,000 mg/dL. How much stock is diluted to 100 mL to make a working standard of 100 mg/dL?

To get a straight-line standard graph through the origin for a method that obeys Beer's law, what is plotted against concentration?

The flame of a routine laboratory flame photometer is usually fed with:

Flame photometry for sodium and potassium is based on:

An acetate buffer is made with sodium acetate and acetic acid in a ratio of 10:1 (pKa 4.76). What is its pH?

pH is defined as the:

What is the pH of a 0.001 M sodium hydroxide solution at 25 °C?

A patient with multiple myeloma has total protein 11 g/dL. Sodium is 128 mmol/L on the main analyser (indirect ISE) and 139 mmol/L on the blood gas analyser (direct ISE). Which statement is correct?

The violet colour of the biuret reaction for serum total protein is read with which filter?

What effect does stray light reaching the detector of a spectrophotometer have on readings?

Before a centrifuge is started, the loaded tubes should be:

In a filter photoelectric colorimeter, which part selects the band of wavelengths that passes through the sample?

To make a buffer, 0.1 M NaOH (50 mL) is added to 0.2 M acetic acid (50 mL; pKa 4.76). The resulting pH is:

Which wavelength does the sodium filter of a flame photometer pass?

A blood tube breaks during a run in a centrifuge without sealed buckets. What should the technician do first?

All of the following are parts of a photoelectric colorimeter EXCEPT:

The standard cuvette used in routine colorimetry has a light path of:

A grossly lipaemic sample is tested by an end-point colorimetric glucose method zeroed with the reagent blank. Test absorbance 0.55; serum blank (serum with reagent lacking the enzyme) 0.15; 100 mg/dL standard 0.40. The glucose is:

An ion-selective electrode responds directly to which property of the ion in the sample?

The psychiatry ward asks for serum lithium on bipolar patients (therapeutic range 0.6-1.2 mmol/L). The flame photometer uses a lithium internal standard for sodium and potassium. What change is needed to measure lithium?

During a spectrophotometer performance check, the wavelength accuracy is verified with:

Which pair of chemicals in solution forms a buffer?

A patient's arterial pH falls from 7.40 to 7.10. How has the hydrogen ion concentration changed?