Resolve low marine nitrate without switching to a high-range refill
Choose HI781-25 to keep low-level seawater nitrate monitoring supplied with 25 tests of the matched three-reagent formulation. It suits an existing HI781 Checker or HI83399 Marine Nitrate Low Range setup when you want low-range readings and can include the required filtration stage.
Manufacturer component illustration; use the supplied lot label, not photographed dates or volumes.
Match the matrix, method and reporting basis
0.00–5.00 ppm as NO3− direct; samples above 5 up to 50 ppm use the exact approved 1:10 dilution
Match this refill to the HI781 Checker or HI83399 Marine Nitrate Low Range method. It is not HI782-25 high-range chemistry or a freshwater nitrate substitute. The direct range is as nitrate (NO3−), not nitrogen; HI83399’s chemical-form display conversion does not change that reporting distinction.
What one refill supplies
One HI781-25 refill provides reagent capacity for 25 tests using liquid HI781A-0 and powders HI781B-0 and HI781C-0. It is not an instrument starter kit. Do not assume filters, holder, syringes or other method tools are included; check and replenish your existing setup separately.
Have the complete testing workflow ready
Filtration is mandatory, so have the HI740271 filter holder, dry HI740228 filter discs, HI740270 10 mL Luer-lock syringe, HI740226 5 mL syringe/tip, mixing vial, HI740272 blunt needle and model-appropriate cuvette/cap available. Keep scissors and a lint-free wipe ready. Approved dilution also needs an accurate 1 mL syringe and nitrate/nitrite-free artificial seawater.
Plan for the method, not just the reagents
Allow time for the complete reduction, filtration and timed color-development stages—not just a quick reagent addition. Follow all preparation, mixing, filtration and timing conditions in the HI781 Checker manual or the HI83399 Marine Nitrate LR method, as appropriate. Shared reagent chemistry does not make the two instruments’ operating instructions interchangeable.
Above 5 ppm? Check the approved dilution limits
For 5–50 ppm nitrate samples only, the approved 1:10 dilution can extend testing beyond the direct 0.00–5.00 ppm range. It requires nitrate/nitrite-free artificial seawater, the full model-specific preparation and timed method, and a ×10 result correction. Dilution reduces accuracy; it is not an unrestricted 0–50 ppm range. Follow the linked model manual for every dilution and measurement condition.
Handling and shipment checks
Nitrite and copper interfere. Review all three component SDS and the supplied labels for PPE, storage and waste handling. Confirm destination and carrier eligibility before ordering: component transport provisions do not establish a whole-refill shipping exemption, and Hanna specifies ground-only shipping for Dangerous Goods.
Frequently Asked Questions
When should I choose HI781-25 rather than HI782-25?
Choose HI781-25 for the documented marine low-range method, 0.00–5.00 ppm as nitrate. HI782-25 is high-range chemistry and is not a substitute in an HI781 method. Match the reagent code to the exact program, not just the word nitrate.
Is the range reported as nitrate or nitrate-nitrogen?
The direct 0.00–5.00 ppm range is as NO3−, not as N. HI83399 can display nitrate-nitrogen through its chemical-form function. Numerically equal NO3− and NO3−–N results mean different concentrations; do not relabel a nitrate reading as nitrogen.
Which instruments have a verified method?
The current HI83399 manual lists HI781-25 for Marine Nitrate Low Range. The HI781 Checker manual also specifies this refill. Other benchtop models are not added merely because they measure nitrate, and this page does not create or imply a separate Checker instrument listing.
What do the three reagent components do?
The procedure adds HI781A-0 and HI781B-0 before filtration, then zeroes the filtered preparation and adds HI781C-0 for final color development. The prescribed per-test doses are 4 mL A and one packet each of B and C. These method quantities do not establish the supplied bottle fill or full physical pack contents.
Are the filters, holder and syringes in the refill?
They are required for the procedure but are not established as included with HI781-25. The HI781 instrument starter-kit contents are not the reagent refill packing list. Check your reusable holder, 10 mL Luer-lock syringe, 5 mL syringe/tip, mixing vial, blunt needle and cuvette; replenish the dry filter discs separately as needed.
Why must the sample be filtered?
Filtration is a prescribed stage after adding reagents A and B and before zeroing. Use a dry filter disc and holder, apply constant light pressure and allow about 30 seconds. Excess pressure can tear the filter; a wet assembly or omitted filtration is not the documented method.
Can I measure samples above 5 ppm?
Only with the approved 1:10 preparation for 5–50 ppm samples: dilute 1 mL sample to 10 mL with nitrate/nitrite-free artificial seawater, mix, use 7 mL of that preparation and complete the normal method. Multiply by 10. The HI781 manual specifies degraded diluted accuracy of ±2.5 ppm ±5% of calculated reading; measure volumes carefully.
Which timing, interferences and safety checks matter?
On HI781, shake the A/B preparation for one minute, filter, zero, add C and shake for two minutes, then allow the eight-minute color-development time. Nitrite and copper interfere. Consult all three component SDS and the full method; the transport status of one component is not a whole-kit shipping guarantee.
