APPARATUS:
Eppendorfpipettes (10-100 µl, 100-1000 µl und 1-5 cm³)
Various volumetric flasks
Various cylinders und beakers
Various bottles to store the reagents
Photometer

REAGENTS:
Sulphanilamide:  Dissolve 1.0 g of sulphanilamide in 10 cm³ conc. HCl and 60 cm³ pure water. After cooling make up to 100 cm³. Store in brown glass bottle. Stable for several weeks.
NED: 100 mg N-(1-naphthyl)-ethylendiamine-dihydrochlorid (NED) are dissolved in 100 cm³
pure water and stored refrigerated in a glass bottle. Stable until a brown color develops.
NO₂-standard: MERCK 1 g NO₂ per liter (= 21.734 mmol/l)
Saltwater: 36 gNaCl per liter

STANDARDS:
Stock-Standard: NO₂-Standard (MERCK) = 21.734 mmol/l
Work standard: Dilute the stock-standard with salt water in different relations to obtain standards with different concentrations for a calibration (5-6 calibration points).
Calibration area: 1.087 µmol/l - 5.435 µmol/l
 

PROCEDURE:
To 5.0 cm³ of undiluted sample or standard 0.1 cm³ Sulphanilamide-solution and 0.1 cm³ NED-solution are added. Read the absorbance of the red azo dye after 30 minutes at 540 nm.

INTERFERENCES:
High concentrations of H₂S (> 1mM) in the sample disturbe the measurements. Therefore these samples should be acified with 10 µl HCL (65%/suprapur) per cm³ sample and then be stored for 1-2 days, without lid, in a cool room.
(Don`t forget the dilution factor of the acid when you calculate the final result!)