Porownanie trzech metod oznaczania azotanow w swiezych warzywach

Porównano chemiczną, instrumentalną i wskaźnikową metodą oznaczania zawartości azotanów w świeżych warzywach, zróżnicowanych w poziomie tych związków między 10 a 4000 mg N-NO3/kg. Stwierdzono bardzo wysoką korelację rzędu 0,99 między metodą zalecaną w normie PN-92/A-75112 (zmodyfikowana metoda kolorymetryczna wg Griessa) a metodą potencjometryczną. Metoda wskaźnikowa, chociaż powtarzalna, pozwala tylko na zakwalifikowanie warzyw do dość szerokich przedziałów zawartości azotanów.

Colorimetric analysis using direct reduction with cadmium as described in Polish Standard PN-92/A- 75112 was selected as the reference method. The Potentiometrie method using Mettler Toledo ion-selective nitrate electrode and the trace method using Merckquant (Merck) test papers were compared with it. The test material comparised 16 vegetables classified into 3 categories (leaf, fruit, root) harvested from an experimental plantation. They were grown and fertilized according to standard agricultural procedures. The level of nitrates (N-NO3), as determined by the chemical method, ranged from 12 to 3945 mg/kg of the product. Eight samples were used to test each vegetable species by each method. For the chemical method, by maximum deviation from the average was -6% and +6%, while the corresponding value for the Potentiometrie method was -6% and +4%. In spite that for 11 species a statistically significant difference at p=0.01 was shown to occur between the results of the chemical and Potentiometrie determinations, the difference between those methods was within 6% in 10 cases, within 10% in 6 cases, and higher than 10% only in 2 species, the Potentiometrie result being always higher. The correlation coefficient of almost 0,999 confirms a high agreement between the results obtained by the Potentiometrie and the chemical method. The test papers yielded consistent results; however, they made it possible merely to include the individual vegetables into one of the nitrate nitrogen-content ranges of 0-100, 50-150, 200-500, 500-1000, 1000-2000 and 2000-5000 mg/kg.