The Russian national standards require use one of two means for nitrate determination in vegetables: photometric and ionometric.
Photometric method is more difficult, because include stage of reducing  the nitrate to the nitrite. Ionometric method is more easy and popular.
At first, you must prepare water extract solution. It contains 1% potassium alum. The sample (10g) of vegetable must be chopped through homogenizer and flooded   in 50 ml of extract solution. You can replace homogenizer by meat chopper or grater. The mixture must be stirred by 5 minutes. Thereafter mixture is ready for analysis.
The extract solutions do not add to juices and drinks, because crystals of potassium alum dissolve directly into samples. Final concentration must be equal to 1%. Dried fruits flooded in double portions of extract solution. Then, the mixture warm up 5 minutes in water-bath.
For determination of nitrate in cabbage, solution must contain potassium permanganate (1g in 1000 ml of water) and sulfuric acid (0,6 ml in 1000 ml). This addition agents use for oxidation some of the cabbage's organic substances. It's important, because these substances can disable the ion selective electrode.
Before use, the electrodes must be calibrated in following standard solutions: 0,0001М, 0,001М, 0,01М и 0,1М. Standard solutions contained potassium alum (1%) for regulation of ionic force! (Why solutions with concentration 0,01М and 0,1М is need? I cannot understand! )
After calibration, you can measure concentration of nitrate in your sample. This is a standard ionometric procedure. Analysis result must be calculated in following units: milligram of nitrate on kilogram of vegetable. Units for juice result is mg/l