AOAC RI ERP EBOOK FOR FERTILIZERS

activity was verified periodically, and any source which did not produce solution capable of hydrolyzing 0.1 g urea/20 mL solution was discarded.

Determination

Ten grams of each liquid ureaformaldehyde fertilizer sample (containing ≤1.0g of urea) was transferred to a 15 cm Whatman No. 12 fluted filter paper (Fisher Scientific). This was leached with approximately 300 ml of de-ionized water (D-H 2 O) into a 500 ml volumetric flask. Next, 75-100 ml of saturated barium hydroxide, Ba(OH) 2 (Fisher Scientific) was added to precipitate out any phosphate present. Then, 20 ml of 10% sodium carbonate, Na 2 CO 3 (Fisher Scientific) solution was added to precipitate any excess Ba. This was diluted to volume, mixed, and filtered through a 15 cm Whatman No. 12 fluted filter paper. Then, 50 ml of the filtrate was transferred to a 250 ml Erlenmeyer flask, and 2-4 drops of methyl purple was added followed by addition of 2N HCl to form a reddish purple color (acidic). This was neutralized with 0.1 N NaOH to a green color. Finally, 20 ml of neutral urease solution was added and the flask closed with a rubber stopper. After one hour storage at room temperature, the flask was cooled in an ice-water bath and its content was titrated with 0.1N HClto a full purple color, and then a 5ml excess of 0.1 N HCl was added. Excess HCl was back titrated with 0.1 N NaOH to a neutral end point (green color). % = [ 0.1 − 0.1 ) 0.3003 ∗ ] * 0.3003 = this factor takes into account the molecular weight of urea, the conversion of the milliequivalent result of V x N, and the conversion to %. Percent urea calculated as follow:

Results and Discussion

The free urea contents of each fertilizer sample were measured in duplicates by the AOAC Official Method 2003.14 by Lab 1 and Lab 2. The results of analyses and t-test calculation of the twelve commercial liquid fertilizer sampleslisted above from Lab 1 and Lab 2 by the AOAC Official Method 2003.14 are listed in Table 2.