NP-Standard - Reference solution for nitrate and phosphate test kits

Product description

Reagents from test kits can lose their activity over time due to unfavorable storage conditions, high or low temperatures and exposure to oxygen. This can lead to incorrect results. The respective tests should therefore be regularly tested against a stable standard.

Tropic Marin® NP Standard consists of a solution with nitrate and phosphate, in known concentrations, prepared in the test laboratory. It allows you to quickly check the precision of test kits for nitrate and phosphate. The deviation can be determined and a correction factor can be calculated as necessary.




  • Easy testing of the precision of test kits
  • Calculating a correction factor can improve the accuracy of test results.
  • Only one standard for testing nitrate and phosphate tests
  • Works with all commercially available tests


Tropic Marin® NP-Standard contains 75 mg/l nitrate (NO3-) and 7.5 mg/l phosphate (PO43-)



Application and dosage


I. Checking the precision of the reagents

1.) This requires reverse osmosis or demineralized water. Fill the test vial with the required amount of demineralized water (according to the instructions for use of your test). Attach the blue tip on the enclosed 1ml syringe and draw the following amount from the Tropic Marin® NP Standard bottle, depending on the type of test and sample size:


Volume of the water sample (see instructions for test kit)Quantity of Tropic Marin® NP standard
5 ml0.13 ml
10 ml0.26 ml
15 ml0.39 ml

Add this amount of NP-Standard to the water sample. Perform the desired measurement with your test and determine the measured value M.

2.) The target value S after adding the standard is 2 mg/l nitrate or approx. 0.2 mg/l phosphate. If the measured value M corresponds to the target value S, the test is still fully functional. If the measured value M deviates by more than 50-75% from the target value S, we recommend not using your test any more. The reagents have probably lost their activity.

II. Calculation of the correction factor K

If the deviation is less than 50-75%, you can calculate a correction factor K using the following formula:

K = S / M

Example based on a nitrate measurement:

Quantity of Standard = 0.39 ml to 15 ml sample of reverse osmosis water

Target value S = 2 mg/l

Measured value M = 1.5 mg/l

-> The measured value M does not correspond to the expected target value S (2 mg/l).


Calculating the correction factor:

[For a better illustration, we do not use the units of the values (mg/l)]

Correction factor

K = S / M

K = 2 / 1.5

K = 1.33


Benefits of the correction factor:

In the future, multiply your measured values by the correction factor K to obtain the correct value W.

The correction factor K applied to the measured value M from our example:

Actual value

W = M x K

W = 1.5 mg/l x 1.33

W = 2 mg/l

2 mg/l corresponds to the target value. This proves that the correction factor is correct. Apply the correction factor to future measurements of your test kit.


Check of your test kits again regularly, e.g. every 2-4 weeks.



Other product recommendations

We recommend Tropic Marin® Multi-Standard for testing the precision of test kits for (then list the minerals) minerals.

Tropic Marin® offers a wide range of test kits with a very high degree of accuracy.