Over a week, calcium reagent blanks give rising absorbance and calibrations drift. The deionizer's meter now reads 0.5 MΩ·cm; last month it read 15 MΩ·cm. What is the best action?

Replace or regenerate the resin cartridge

Recalibrate the photometer's wavelength

Open a new lot of calcium reagent

Laboratory water: distilled, deionized and purity

18 Kerala PSC Lab Technician Gr II practice questions on Laboratory water: distilled, deionized and purity, part of Biochemistry. The first 18 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 · Easy
Freshly distilled water left open to the air soon becomes slightly acidic. Which gas does it absorb?
1. AAmmonia
2. BCarbon dioxide
3. COxygen
4. DNitrogen
Show answerHide answer
Answer: B · Carbon dioxide
Carbon dioxide from the air dissolves in pure water and forms carbonic acid, which lowers the pH to about 5.6 and raises the conductivity. Distilled water is therefore stored in tightly closed polythene or borosilicate containers and made fresh as needed. Absorbed ammonia would make the water alkaline instead. Oxygen and nitrogen dissolve without forming acids.
- A: Ammonia would make the water alkaline, not acidic.
- C: Oxygen dissolves without forming an acid.
- D: Nitrogen is inert and does not change the pH.
Question 2 · Medium
Which process gives pyrogen-free water for preparing sterile solutions?
1. ADistillation
2. BAutoclaving
3. CDeionization
4. DMembrane filtration (0.22 µm)
Show answerHide answer
Answer: A · Distillation
Pyrogens are mainly bacterial endotoxins, heat-stable lipopolysaccharides from the walls of Gram-negative bacteria. They do not evaporate, so distillation, often double distillation in an all-glass still, leaves them behind in the boiling flask; reverse osmosis and ultrafiltration also remove them. Autoclaving kills bacteria but does not destroy endotoxin, and ion-exchange beds and 0.22 µm filters do not reliably hold it back. Pyrogen-free water is stored sterile in sealed containers.
- B: Autoclaving sterilizes but leaves heat-stable endotoxin intact.
- C: Deionization removes ions, not endotoxin, and resin beds can grow bacteria.
- D: A 0.22 µm filter holds back bacteria but lets endotoxin through.
Question 3 · Medium
Resistivity is the reciprocal of conductivity. A water sample reads 0.1 µS/cm. What is its resistivity?
1. A0.1 MΩ·cm
2. B100 MΩ·cm
3. C1 MΩ·cm
4. D10 MΩ·cm
Show answerHide answer
Answer: D · 10 MΩ·cm
Resistivity = 1 ÷ conductivity = 1 ÷ (0.1 × 10^-6 S/cm) = 10 × 10^6 Ω·cm = 10 MΩ·cm. Because micro and mega cancel, resistivity in MΩ·cm is simply 1 divided by conductivity in µS/cm. A reading of 10 MΩ·cm meets the CLSI minimum for clinical laboratory reagent water. Ultrapure water reaches 18.2 MΩ·cm (0.055 µS/cm) at 25 °C.
- A: 0.1 MΩ·cm repeats the conductivity figure instead of inverting it.
- B: 100 MΩ·cm is impossible; even ultrapure water reaches only 18.2 MΩ·cm.
- C: 1 MΩ·cm corresponds to a conductivity of 1 µS/cm.
Question 4 · Hard
Over a week, calcium reagent blanks give rising absorbance and calibrations drift. The deionizer's meter now reads 0.5 MΩ·cm; last month it read 15 MΩ·cm. What is the best action?
1. AReplace or regenerate the resin cartridge
2. BRecalibrate the photometer's wavelength
3. CAutoclave the water before use
4. DOpen a new lot of calcium reagent
Show answerHide answer
Answer: A · Replace or regenerate the resin cartridge
A fall in resistivity from 15 to 0.5 MΩ·cm means ions such as calcium and magnesium are passing through, so the ion-exchange resin is exhausted. Calcium in the water raises the reagent blank, and the same water dilutes standards and reagents, so calibrations drift. The cartridge is replaced or the resin regenerated, and the water is confirmed at 10 MΩ·cm or more before reagents and standards are remade. Autoclaving kills bacteria but cannot remove ions.
- B: The photometer is not at fault; the water meter points to the cause.
- C: Autoclaving does not remove dissolved ions.
- D: A new reagent lot would still be blanked and diluted with the same impure water.
Question 5 · Easy
The purity of deionized water is monitored routinely by measuring its:
1. ASpecific gravity
2. BElectrical resistivity
3. CpH
4. DRefractive index
Show answerHide answer
Answer: B · Electrical resistivity
Dissolved ions carry electric current, so the purer the water, the higher its resistivity and the lower its conductivity. Deionizers carry an in-line meter, and a falling resistivity warns that the resin is becoming exhausted. CLSI sets at least 10 MΩ·cm at 25 °C for clinical laboratory reagent water. The pH of pure water is hard to measure because it has almost no buffering and takes up carbon dioxide.
- A: Specific gravity barely changes with trace impurities.
- C: The pH of pure water drifts with absorbed carbon dioxide and does not reflect its ion content.
- D: Refractive index is not sensitive to trace ions.
Question 6 · Easy
In a deionizer, which ion does the cation-exchange resin release in exchange for calcium, sodium and other cations?
1. AHydrogen ions
2. BHydroxyl ions
3. CSodium ions
4. DChloride ions
Answer and explanation in LearnFree account
Question 7 · Medium
Why is feed water often passed through reverse osmosis before it reaches the deionizer?
1. AIt removes most ions first, so the resin lasts longer
2. BIt warms the water for better ion exchange
3. CIt converts dissolved gases into ions
4. DIt adds ions that the resin then exchanges
Answer and explanation in LearnFree account
Question 8 · Medium
A mixed-bed deionizer removes all of the following from water EXCEPT:
1. ASulphate ions
2. BChloride ions
3. CCalcium ions
4. DBacteria
Answer and explanation in LearnFree account
Question 9 · Hard
Blood urea by the urease-Berthelot method gives high reagent blanks. The laboratory's distilled water turns yellow-brown with Nessler's reagent. What is the most likely problem?
1. AAmmonia in the distilled water
2. BHemolysis in the samples
3. CLoss of urease activity
4. DChloride in the distilled water
Answer and explanation in LearnFree account
Question 10 · Easy
By CLSI, clinical laboratory reagent water (CLRW) should have a resistivity of at least:
1. A0.1 MΩ·cm at 25 °C
2. B10 MΩ·cm at 25 °C
3. C1 MΩ·cm at 25 °C
4. D100 MΩ·cm at 25 °C
Answer and explanation in LearnFree account
Question 11 · Easy
In a deionizer, the anion-exchange resin removes chloride and sulphate by releasing:
1. AHydroxyl ions
2. BHydrogen ions
3. CSodium ions
4. DChloride ions
Answer and explanation in LearnFree account
Question 12 · Hard
A laboratory's reagent water reads 15 MΩ·cm, yet enzyme results drift and blanks vary from day to day. Culture of the storage tank water grows 10,000 CFU/mL of bacteria. What is the best action?
1. AAccept the water, as resistivity is adequate
2. BReplace the deionizer resin cartridges
3. CSanitize the system and add a 0.2 µm filter
4. DSwitch the analyzer to tap water
Answer and explanation in LearnFree account
Question 13 · Medium
A sample of distilled water acidified with nitric acid gives a white turbidity with silver nitrate. This shows that the water contains:
1. ASulphate
2. BCalcium
3. CChloride
4. DAmmonia
Answer and explanation in LearnFree account
Question 14 · Medium
In a water purification system, which stage mainly removes dissolved organic compounds and free chlorine from the feed water?
1. AActivated carbon filter
2. B0.2 µm membrane filter
3. CWater softener
4. DMixed-bed ion exchange
Answer and explanation in LearnFree account
Question 15 · Easy
The highest resistivity that any water can reach, set by the self-ionization of water at 25 °C, is about:
1. A18.2 MΩ·cm
2. B10 MΩ·cm
3. C1 MΩ·cm
4. D100 MΩ·cm
Answer and explanation in LearnFree account
Question 16 · Hard
Water drawn fresh from a deionizer outlet reads 16 MΩ·cm. The same water left overnight in an open beaker reads 1.2 MΩ·cm. What explains the fall?
1. AThe deionizer resin is exhausted
2. BSilica leached from the plastic beaker
3. CAir CO2 dissolved, forming ions
4. DBacteria in the water released salts
Answer and explanation in LearnFree account
Question 17 · Easy
Distilled water stored for a long time in ordinary soda-lime glass bottles becomes contaminated with:
1. AChloride from the glass
2. BSodium and silicates from the glass
3. CBacteria from the glass
4. DLead from the glass
Answer and explanation in LearnFree account
Question 18 · Medium
Water from a deionizer alone is not used for preparing injectable or cell-culture solutions because deionization does not remove:
1. ACalcium and magnesium ions
2. BSodium and potassium ions
3. CBacteria and pyrogens
4. DChloride and sulphate ions
Answer and explanation in LearnFree account

Laboratory water: distilled, deionized and purity

Freshly distilled water left open to the air soon becomes slightly acidic. Which gas does it absorb?

Which process gives pyrogen-free water for preparing sterile solutions?

Resistivity is the reciprocal of conductivity. A water sample reads 0.1 µS/cm. What is its resistivity?

Over a week, calcium reagent blanks give rising absorbance and calibrations drift. The deionizer's meter now reads 0.5 MΩ·cm; last month it read 15 MΩ·cm. What is the best action?

The purity of deionized water is monitored routinely by measuring its:

In a deionizer, which ion does the cation-exchange resin release in exchange for calcium, sodium and other cations?

Why is feed water often passed through reverse osmosis before it reaches the deionizer?

A mixed-bed deionizer removes all of the following from water EXCEPT:

Blood urea by the urease-Berthelot method gives high reagent blanks. The laboratory's distilled water turns yellow-brown with Nessler's reagent. What is the most likely problem?

By CLSI, clinical laboratory reagent water (CLRW) should have a resistivity of at least:

In a deionizer, the anion-exchange resin removes chloride and sulphate by releasing:

A laboratory's reagent water reads 15 MΩ·cm, yet enzyme results drift and blanks vary from day to day. Culture of the storage tank water grows 10,000 CFU/mL of bacteria. What is the best action?

A sample of distilled water acidified with nitric acid gives a white turbidity with silver nitrate. This shows that the water contains:

In a water purification system, which stage mainly removes dissolved organic compounds and free chlorine from the feed water?

The highest resistivity that any water can reach, set by the self-ionization of water at 25 °C, is about:

Water drawn fresh from a deionizer outlet reads 16 MΩ·cm. The same water left overnight in an open beaker reads 1.2 MΩ·cm. What explains the fall?

Distilled water stored for a long time in ordinary soda-lime glass bottles becomes contaminated with:

Water from a deionizer alone is not used for preparing injectable or cell-culture solutions because deionization does not remove: