CWEA LABORATORY ANALYST GRADE 3 CERTIFICATION
EXAM — 2026 COMPLETE PRACTICE EXAM (150
QUESTIONS WITH FULL OPTION RATIONALES)
SECTION 1 — WET CHEMISTRY METHODS (Questions 1–15)
Question 1. A BOD₅ test shows only 0.8 mg/L DO depletion after 5 days. Most appropriate corrective
action?
A. Report as zero
B. Raise incubation temperature
C. Decrease sample volume
D. Increase sample volume
Correct ANSWER✔✨-: D
A – Incorrect: sub-minimum depletion invalidates the dilution; it is not zero BOD.
B – Incorrect: method fixes 20°C; changing temperature voids the test.
C – Incorrect: less sample would lower depletion further.
D – Correct: more sample raises depletion into the valid 2.0–7.0 mg/L window.
Question 2. The acceptable DO depletion range for a valid unseeded BOD₅ dilution is:
A. 0.5–2.0 mg/L
B. 2.0–7.0 mg/L
C. 0.1–1.0 mg/L
D. 7.0–9.0 mg/L
Correct ANSWER✔✨-: B
A – Incorrect: below the 2.0 mg/L minimum measurable depletion.
B – Correct: 2.0–7.0 mg/L depletion with ≥1.0 mg/L residual is required.
C – Incorrect: far below method sensitivity.
D – Incorrect: would violate the ≥1.0 mg/L residual requirement.
,Question 3. BOD₅ incubation conditions are:
A. 35°C, 24 h, dark
B. 20°C, 5 days, dark
C. 25°C, 5 days, light
D. 20°C, 24 h, light
Correct ANSWER✔✨-: B
A – Incorrect: 35°C is for coliform/HPC incubation, not BOD.
B – Correct: 20±1°C for 5 days in darkness is standard.
C – Incorrect: wrong temperature; light promotes algae/DO production.
D – Incorrect: 24 h is not the 5-day endpoint.
Question 4. The purpose of seeding BOD dilution water is to:
A. Buffer the pH
B. Add acclimated microorganisms
C. Saturate with oxygen
D. Add nutrients only
Correct ANSWER✔✨-: B
A – Incorrect: buffering is done with phosphate buffer.
B – Correct: seed supplies organisms to oxidize substrate in sterile/low-bug samples.
C – Incorrect: aeration saturates DO, not seed.
D – Incorrect: nutrients are a separate additive.
Question 5. The strong oxidant used in the COD test is:
A. Potassium permanganate
B. Potassium dichromate
C. Hydrogen peroxide
D. Ozone
Correct ANSWER✔✨-: B
A – Incorrect: permanganate is a weaker oxidant (used in some organics tests).
B – Correct: dichromate in strong acid oxidizes most organics.
C – Incorrect: peroxide is not the COD oxidant.
,D – Incorrect: ozone not used in COD digestion.
Question 6. Chloride interference in COD is controlled by adding:
A. Silver sulfate
B. Mercuric sulfate
C. Sodium thiosulfate
D. Copper sulfate
Correct ANSWER✔✨-: B
A – Incorrect: silver sulfate is the catalyst, not chloride mask.
B – Correct: mercuric sulfate complexes chloride to prevent its oxidation.
C – Incorrect: thiosulfate neutralizes chlorine, not used in COD digest.
D – Incorrect: copper not used for chloride in COD.
Question 7. The catalyst that promotes oxidation of organic compounds in COD is:
A. Mercuric sulfate
B. Silver sulfate
C. Ferroin indicator
D. Antimony chloride
Correct ANSWER✔✨-: B
A – Incorrect: mercury masks chloride only.
B – Correct: silver sulfate catalyzes oxidation of straight-chain organics.
C – Incorrect: ferroin is a titration indicator.
D – Incorrect: not a COD catalyst.
Question 8. Total alkalinity is titrated to an endpoint of pH:
A. 8.3
B. 7.0
C. 4.5
D. 3.1
Correct ANSWER✔✨-: C
A – Incorrect: 8.3 is the phenolphthalein endpoint.
B – Incorrect: neutral pH is not an alkalinity endpoint.
, C – Correct: total alkalinity endpoint is pH 4.5.
D – Incorrect: over-titration biases high.
Question 9. Phenolphthalein alkalinity corresponds to an endpoint of pH:
A. 4.5
B. 8.3
C. 6.5
D. 10.0
Correct ANSWER✔✨-: B
A – Incorrect: 4.5 defines total alkalinity.
B – Correct: P-alkalinity endpoint is 8.3.
C – Incorrect: not a defined endpoint.
D – Incorrect: phenolphthalein color appears near 8.3, not 10.
Question 10. In the argentometric (Mohr) chloride titration, the indicator and endpoint color are:
A. Potassium chromate; red-brown
B. Ferroin; blue
C. DPD; pink
D. Eriochrome Black T; blue
Correct ANSWER✔✨-: A
A – Correct: chromate forms red-brown silver chromate at endpoint.
B – Incorrect: ferroin is for COD/ferrous titrations.
C – Incorrect: DPD is for chlorine residual.
D – Incorrect: EBT is for hardness.
Question 11. TSS is determined by filtering through a ______ and drying at ______:
A. 0.45 µm membrane; 180°C
B. 1.5 µm glass fiber; 103–105°C
C. Paper filter; 550°C
D. Glass fiber; 180°C
Correct ANSWER✔✨-: B
A – Incorrect: membrane/180°C applies to TDS-style drying, not TSS.
EXAM — 2026 COMPLETE PRACTICE EXAM (150
QUESTIONS WITH FULL OPTION RATIONALES)
SECTION 1 — WET CHEMISTRY METHODS (Questions 1–15)
Question 1. A BOD₅ test shows only 0.8 mg/L DO depletion after 5 days. Most appropriate corrective
action?
A. Report as zero
B. Raise incubation temperature
C. Decrease sample volume
D. Increase sample volume
Correct ANSWER✔✨-: D
A – Incorrect: sub-minimum depletion invalidates the dilution; it is not zero BOD.
B – Incorrect: method fixes 20°C; changing temperature voids the test.
C – Incorrect: less sample would lower depletion further.
D – Correct: more sample raises depletion into the valid 2.0–7.0 mg/L window.
Question 2. The acceptable DO depletion range for a valid unseeded BOD₅ dilution is:
A. 0.5–2.0 mg/L
B. 2.0–7.0 mg/L
C. 0.1–1.0 mg/L
D. 7.0–9.0 mg/L
Correct ANSWER✔✨-: B
A – Incorrect: below the 2.0 mg/L minimum measurable depletion.
B – Correct: 2.0–7.0 mg/L depletion with ≥1.0 mg/L residual is required.
C – Incorrect: far below method sensitivity.
D – Incorrect: would violate the ≥1.0 mg/L residual requirement.
,Question 3. BOD₅ incubation conditions are:
A. 35°C, 24 h, dark
B. 20°C, 5 days, dark
C. 25°C, 5 days, light
D. 20°C, 24 h, light
Correct ANSWER✔✨-: B
A – Incorrect: 35°C is for coliform/HPC incubation, not BOD.
B – Correct: 20±1°C for 5 days in darkness is standard.
C – Incorrect: wrong temperature; light promotes algae/DO production.
D – Incorrect: 24 h is not the 5-day endpoint.
Question 4. The purpose of seeding BOD dilution water is to:
A. Buffer the pH
B. Add acclimated microorganisms
C. Saturate with oxygen
D. Add nutrients only
Correct ANSWER✔✨-: B
A – Incorrect: buffering is done with phosphate buffer.
B – Correct: seed supplies organisms to oxidize substrate in sterile/low-bug samples.
C – Incorrect: aeration saturates DO, not seed.
D – Incorrect: nutrients are a separate additive.
Question 5. The strong oxidant used in the COD test is:
A. Potassium permanganate
B. Potassium dichromate
C. Hydrogen peroxide
D. Ozone
Correct ANSWER✔✨-: B
A – Incorrect: permanganate is a weaker oxidant (used in some organics tests).
B – Correct: dichromate in strong acid oxidizes most organics.
C – Incorrect: peroxide is not the COD oxidant.
,D – Incorrect: ozone not used in COD digestion.
Question 6. Chloride interference in COD is controlled by adding:
A. Silver sulfate
B. Mercuric sulfate
C. Sodium thiosulfate
D. Copper sulfate
Correct ANSWER✔✨-: B
A – Incorrect: silver sulfate is the catalyst, not chloride mask.
B – Correct: mercuric sulfate complexes chloride to prevent its oxidation.
C – Incorrect: thiosulfate neutralizes chlorine, not used in COD digest.
D – Incorrect: copper not used for chloride in COD.
Question 7. The catalyst that promotes oxidation of organic compounds in COD is:
A. Mercuric sulfate
B. Silver sulfate
C. Ferroin indicator
D. Antimony chloride
Correct ANSWER✔✨-: B
A – Incorrect: mercury masks chloride only.
B – Correct: silver sulfate catalyzes oxidation of straight-chain organics.
C – Incorrect: ferroin is a titration indicator.
D – Incorrect: not a COD catalyst.
Question 8. Total alkalinity is titrated to an endpoint of pH:
A. 8.3
B. 7.0
C. 4.5
D. 3.1
Correct ANSWER✔✨-: C
A – Incorrect: 8.3 is the phenolphthalein endpoint.
B – Incorrect: neutral pH is not an alkalinity endpoint.
, C – Correct: total alkalinity endpoint is pH 4.5.
D – Incorrect: over-titration biases high.
Question 9. Phenolphthalein alkalinity corresponds to an endpoint of pH:
A. 4.5
B. 8.3
C. 6.5
D. 10.0
Correct ANSWER✔✨-: B
A – Incorrect: 4.5 defines total alkalinity.
B – Correct: P-alkalinity endpoint is 8.3.
C – Incorrect: not a defined endpoint.
D – Incorrect: phenolphthalein color appears near 8.3, not 10.
Question 10. In the argentometric (Mohr) chloride titration, the indicator and endpoint color are:
A. Potassium chromate; red-brown
B. Ferroin; blue
C. DPD; pink
D. Eriochrome Black T; blue
Correct ANSWER✔✨-: A
A – Correct: chromate forms red-brown silver chromate at endpoint.
B – Incorrect: ferroin is for COD/ferrous titrations.
C – Incorrect: DPD is for chlorine residual.
D – Incorrect: EBT is for hardness.
Question 11. TSS is determined by filtering through a ______ and drying at ______:
A. 0.45 µm membrane; 180°C
B. 1.5 µm glass fiber; 103–105°C
C. Paper filter; 550°C
D. Glass fiber; 180°C
Correct ANSWER✔✨-: B
A – Incorrect: membrane/180°C applies to TDS-style drying, not TSS.
