Question 1

What is the EPA MCL for nitrate in drinking water?

Accepted Answer

The US EPA Maximum Contaminant Level (MCL) for nitrate in drinking water is 10 mg/L as nitrate-nitrogen (NO3-N), equivalent to 44.3 mg/L as nitrate (NO3-). The MCL is set to protect infants under 6 months of age from methemoglobinemia (blue baby syndrome), a potentially fatal condition where nitrate reduces the oxygen-carrying capacity of hemoglobin. The Low Range DR300 Nitrate (0-0.50 mg/L) is appropriate for monitoring near the MCL in treated drinking water; the High Range (0-30.0 mg/L) covers source water and agricultural runoff scenarios.

Question 2

How does the cadmium reduction method for nitrate work?

Accepted Answer

The cadmium reduction method (Hach Method 8039, based on APHA 4500-NO3 F) reduces nitrate (NO3-) to nitrite (NO2-) by passing the sample through a cadmium reduction column or by reaction with cadmium sulfate granules in a reagent pillow. The resulting nitrite then reacts with a diazotizing agent (sulfanilic acid) and a coupling agent (gentisic acid or alpha-naphthylamine) to form a pink-red azo dye. The absorbance of this azo dye is measured at 507 nm. Because nitrite (if already present in the sample) also responds to this color reaction, true nitrate = total response minus any pre-existing nitrite in the sample.

Question 3

Why is cadmium used in nitrate testing and is it hazardous?

Accepted Answer

Cadmium metal is an efficient and reliable reducing agent that selectively converts nitrate to nitrite under controlled conditions. The Hach NitraVer 5 reagent and similar cadmium-based reagents are widely used because no other affordable field-stable reagent achieves comparable reliability for nitrate reduction at low concentrations. Cadmium is a regulated toxic heavy metal. Spent reagents, rinse water from cadmium columns, and sample waste from cadmium reduction tests must be collected and disposed of as cadmium-containing hazardous waste. Do not pour cadmium reagent waste down the drain. Some alternative nitrate methods (UV or enzymatic) avoid cadmium entirely but are typically used on bench-top instruments rather than pocket colorimeters.

Question 4

What is the difference between nitrate-nitrogen (NO3-N) and nitrate (NO3-)?

Accepted Answer

Nitrate-nitrogen (NO3-N) expresses nitrate concentration as the mass of nitrogen it contains. Nitrate (NO3-) expresses the concentration as the full molecular weight of the nitrate ion. The EPA MCL is 10 mg/L as NO3-N (nitrogen). To convert: NO3-N multiplied by 4.43 = NO3-. So the MCL of 10 mg/L NO3-N equals 44.3 mg/L NO3-. Always confirm which unit your permit or standard uses before comparing results. The Hach DR300 Nitrate reports results in mg/L as NO3-N.

To Convert	Multiply By	Example
NO₃-N → NO₃- (nitrate ion)	4.43	10 mg/L NO₃-N × 4.43 = 44.3 mg/L NO₃-
NO₃- → NO₃-N	0.226	44.3 mg/L NO₃- × 0.226 = 10 mg/L NO₃-N
DR300 Nitrate reports in mg/L as NO₃-N. US EPA MCL is stated as NO₃-N. EU Nitrate Directive is stated as NO₃-.

Application	Range	Key Context
Drinking water compliance	LR (0–0.50) or HR (0–30.0)	EPA MCL 10 mg/L NO₃-N; HR covers the MCL; LR for low-level groundwater monitoring
Groundwater monitoring	LR or HR	Agricultural areas; septic system plumes; varies from <0.5 to >30 mg/L
Agricultural runoff / irrigation water	HR	Nitrate leaching from fertilizers; stormwater compliance; irrigation quality for sensitive crops
Wastewater effluent	HR	NPDES total nitrogen compliance; biological denitrification process control
Aquaculture	LR or HR	High nitrate toxic to shrimp and sensitive fish; recirculating aquaculture systems accumulate nitrate
Food and beverage process water	LR	Nitrate limits in water used for certain food products; brewing water quality
Environmental monitoring	LR or HR	Lake and river eutrophication; nutrient load assessment

Interferent	Effect	Treatment
Nitrite (NO₂−)	Positive interference — measured together with nitrate	Measure nitrite separately; true nitrate = total reading minus nitrite value
pH <5 or >9	Cadmium reduction and azo coupling both pH-dependent	Adjust to pH 5–9 before testing; record adjustment in lab notes
Chloride (>200 mg/L)	High chloride can interfere with cadmium reduction efficiency	Dilute high-chloride samples (seawater, brine); multiply result by dilution factor
Residual chlorine	May oxidize the azo dye and produce falsely low results	Dechlorinate with sodium thiosulfate before testing; add 1 drop 3.5% Na₂S₂O₃ per 25 mL
Color >20 APHA units	Background color adds to absorbance measurement	Zero the instrument with sample water (no reagent) to subtract background color
Turbidity	Scatters light; positive interference	Filter through 0.45-micron membrane before testing colored or turbid samples

Item	Hach Item No.	Notes
NitraVer® 5 Nitrate Reagent Powder Pillow (pkg/100)	2107228	Primary reagent; cadmium-containing hazardous waste after use
NitraVer® 5 Nitrate Reagent Powder Pillow (pkg/25)	2107225	Smaller quantity for lower-volume applications
Sodium thiosulfate solution, 3.5% (for dechlorination)	2640349 (verify)	Add 1 drop per 25 mL sample to remove residual chlorine before testing
DR300 instrument cap	LPV445.97.08100	Must be installed before ZERO or READ
Batteries, AAA (4-pack)	Standard AAA alkaline	Replace all 4 simultaneously
All cadmium-containing reagent waste must be collected and disposed of as hazardous waste. Confirm current Hach item numbers at hach.com before ordering.

Hach DR300 Nitrate Pocket Colorimeter: Complete Review & Reference (2026)

EPA Drinking Water Standard — The 10 mg/L MCL

Unit Conversion — NO₃-N vs NO₃-

How the Cadmium Reduction Method Works

Standard Test Procedure (Both LR and HR)

Check sample pH — adjust if outside 5–9

Fill the sample cell to the specified volume

Add NitraVer® 5 Nitrate Reagent Powder Pillow — cap and shake 1 minute

Wait 1 minute — then read within 5 minutes

Applications

Interferences

Reagents & Parts Reference

Related DR300 & Hach Instrument References