CWEA Laboratory Analyst Grade 3 Certification Exam
150 Questions with correct answers and Detailed
Rationales 2026!
SECTION 1: SAMPLING AND FIELD MEASUREMENTS (Questions 1-25)
Question 1
A laboratory analyst is collecting a grab sample for dissolved oxygen analysis. What is the correct
preservation and maximum holding time?
A) Preserve with sulfuric acid to pH < 2, hold for 14 days
B) Analyze immediately, no preservation
C) Preserve with sodium thiosulfate, hold for 6 hours
D) Preserve with manganous sulfate, hold for 8 hours
ANSWER: B
Rationale: Dissolved oxygen samples must be analyzed immediately because oxygen can be consumed
or gained through biological activity or atmospheric exchange. No chemical preservation is effective for
DO, and prompt analysis is critical. If immediate analysis is impossible, samples should be fixed in the
field using the Winkler method reagents and stored in the dark. The other options are incorrect as
sulfuric acid and sodium thiosulfate would interfere with DO analysis.
Question 2
Which of the following is the correct preservation method for a sample collected for cyanide analysis?
A) Add sulfuric acid to pH < 2, cool to 4°C
B) Add sodium hydroxide to pH > 12, cool to 4°C
C) Add nitric acid to pH < 2, no cooling required
D) No preservation needed, analyze within 24 hours
,ANSWER: B
Rationale: Cyanide samples must be preserved with sodium hydroxide to raise the pH above 12, which
stabilizes cyanide by preventing its volatilization as hydrogen cyanide gas. Samples should also be cooled
to 4°C. Acidic conditions would cause cyanide to convert to volatile HCN gas and be lost. Nitric acid is
used for metal preservation, not cyanide. Without proper preservation, cyanide can be rapidly lost.
Question 3
When collecting a sample for total suspended solids (TSS) analysis, which container type is most
appropriate?
A) Plastic container
B) Glass container
C) Metal container
D) Any container as long as it is clean
ANSWER: B
Rationale: Glass containers are preferred for TSS analysis because they do not introduce particulate
contamination and are easier to clean thoroughly. Plastic containers can shed particles or adsorb
materials that could affect results. Metal containers may react with samples or contribute particulate
matter.
Question 4
A chain-of-custody (COC) form must include which of the following elements?
A) Sample collection date and time
B) Sample collector's signature
C) Analysis requested
D) All of the above
,ANSWER: D
Rationale: A complete chain-of-custody form must document the sample's journey from collection
through analysis, including collection date/time, collector signature, analyses requested, sample
identification, preservation method, and all transfers of custody. This ensures legal defensibility and
traceability of samples.
Question 5
What is the recommended holding time for a sample collected for Biochemical Oxygen Demand (BOD)
analysis?
A) 6 hours
B) 24 hours
C) 48 hours
D) 7 days
ANSWER: C
Rationale: BOD samples should be analyzed within 48 hours of collection (6 hours is ideal). Samples
must be kept at 4°C during storage to slow biological activity. Beyond 48 hours, significant bacterial
activity can alter the sample's organic composition, making results unreliable. The 24-hour option is not
the maximum, and 7 days is far too long.
Question 6
When collecting a composite sample for wastewater analysis, what is the typical sampling method?
A) Single grab at the midpoint of discharge
B) Equal volume aliquots collected at regular time intervals
C) Continuous flow proportional to flow rate
D) Both B and C are correct methods
, ANSWER: D
Rationale: Composite samples can be collected either as equal-volume aliquots at regular time intervals
(time-proportional) or as volume-weighted aliquots proportional to flow (flow-proportional). The choice
depends on the parameter being measured and the variability of the wastewater flow. Both methods
are acceptable for different applications.
Question 7
Which preservative is commonly used for nutrient analysis (nitrogen, phosphorus) in wastewater
samples?
A) Sodium chloride
B) Ethanol
C) Sulfuric acid
D) Acetone
ANSWER: C
Rationale: Sulfuric acid is used to preserve nutrient samples by lowering the pH to below 2, which
inhibits biological activity and prevents microbial consumption or transformation of nutrients. The acid
also stabilizes ammonia and prevents volatilization. Sodium chloride, ethanol, and acetone are not
appropriate preservatives for nutrient analysis.
Question 8
When collecting a sample for volatile organic compound (VOC) analysis, how should the sample be
collected?
A) Fill container completely with no headspace
B) Fill container halfway to allow for expansion
C) Collect in a plastic container
D) Add preservative after collecting
150 Questions with correct answers and Detailed
Rationales 2026!
SECTION 1: SAMPLING AND FIELD MEASUREMENTS (Questions 1-25)
Question 1
A laboratory analyst is collecting a grab sample for dissolved oxygen analysis. What is the correct
preservation and maximum holding time?
A) Preserve with sulfuric acid to pH < 2, hold for 14 days
B) Analyze immediately, no preservation
C) Preserve with sodium thiosulfate, hold for 6 hours
D) Preserve with manganous sulfate, hold for 8 hours
ANSWER: B
Rationale: Dissolved oxygen samples must be analyzed immediately because oxygen can be consumed
or gained through biological activity or atmospheric exchange. No chemical preservation is effective for
DO, and prompt analysis is critical. If immediate analysis is impossible, samples should be fixed in the
field using the Winkler method reagents and stored in the dark. The other options are incorrect as
sulfuric acid and sodium thiosulfate would interfere with DO analysis.
Question 2
Which of the following is the correct preservation method for a sample collected for cyanide analysis?
A) Add sulfuric acid to pH < 2, cool to 4°C
B) Add sodium hydroxide to pH > 12, cool to 4°C
C) Add nitric acid to pH < 2, no cooling required
D) No preservation needed, analyze within 24 hours
,ANSWER: B
Rationale: Cyanide samples must be preserved with sodium hydroxide to raise the pH above 12, which
stabilizes cyanide by preventing its volatilization as hydrogen cyanide gas. Samples should also be cooled
to 4°C. Acidic conditions would cause cyanide to convert to volatile HCN gas and be lost. Nitric acid is
used for metal preservation, not cyanide. Without proper preservation, cyanide can be rapidly lost.
Question 3
When collecting a sample for total suspended solids (TSS) analysis, which container type is most
appropriate?
A) Plastic container
B) Glass container
C) Metal container
D) Any container as long as it is clean
ANSWER: B
Rationale: Glass containers are preferred for TSS analysis because they do not introduce particulate
contamination and are easier to clean thoroughly. Plastic containers can shed particles or adsorb
materials that could affect results. Metal containers may react with samples or contribute particulate
matter.
Question 4
A chain-of-custody (COC) form must include which of the following elements?
A) Sample collection date and time
B) Sample collector's signature
C) Analysis requested
D) All of the above
,ANSWER: D
Rationale: A complete chain-of-custody form must document the sample's journey from collection
through analysis, including collection date/time, collector signature, analyses requested, sample
identification, preservation method, and all transfers of custody. This ensures legal defensibility and
traceability of samples.
Question 5
What is the recommended holding time for a sample collected for Biochemical Oxygen Demand (BOD)
analysis?
A) 6 hours
B) 24 hours
C) 48 hours
D) 7 days
ANSWER: C
Rationale: BOD samples should be analyzed within 48 hours of collection (6 hours is ideal). Samples
must be kept at 4°C during storage to slow biological activity. Beyond 48 hours, significant bacterial
activity can alter the sample's organic composition, making results unreliable. The 24-hour option is not
the maximum, and 7 days is far too long.
Question 6
When collecting a composite sample for wastewater analysis, what is the typical sampling method?
A) Single grab at the midpoint of discharge
B) Equal volume aliquots collected at regular time intervals
C) Continuous flow proportional to flow rate
D) Both B and C are correct methods
, ANSWER: D
Rationale: Composite samples can be collected either as equal-volume aliquots at regular time intervals
(time-proportional) or as volume-weighted aliquots proportional to flow (flow-proportional). The choice
depends on the parameter being measured and the variability of the wastewater flow. Both methods
are acceptable for different applications.
Question 7
Which preservative is commonly used for nutrient analysis (nitrogen, phosphorus) in wastewater
samples?
A) Sodium chloride
B) Ethanol
C) Sulfuric acid
D) Acetone
ANSWER: C
Rationale: Sulfuric acid is used to preserve nutrient samples by lowering the pH to below 2, which
inhibits biological activity and prevents microbial consumption or transformation of nutrients. The acid
also stabilizes ammonia and prevents volatilization. Sodium chloride, ethanol, and acetone are not
appropriate preservatives for nutrient analysis.
Question 8
When collecting a sample for volatile organic compound (VOC) analysis, how should the sample be
collected?
A) Fill container completely with no headspace
B) Fill container halfway to allow for expansion
C) Collect in a plastic container
D) Add preservative after collecting
