Improvements to the Methods for Testing Nitrates and Nitrites in Distilled Water

Nitrates and nitrites can be introduced into distilled water from water sources and are harmful chemicals to human health. Therefore, the Chinese Pharmacopoeia (1990 edition), Part II, added methods for testing nitrates and nitrites in water for injection, sterile water for injection, and distilled water. The reaction principle of this method is based on the use of zinc powder as a reducing agent, which dissolves in dilute acid and releases nascent hydrogen, reducing nitrates to nitrites, which then react with p-aminobenzenesulfonic acid-naphthylamine to form a pink azo compound. Its advantages are simplicity, speed, high specificity, high sensitivity, and a minimum detection limit of 0.007 μg. However, our experiments showed that adding 0.01 g of zinc powder as a reducing agent resulted in a high false-positive rate, affecting the interpretation of the test results. Using 0.01 g of arsenic-free zinc as the reducing agent significantly reduced the false-positive rate, eliminating interference from false positives and yielding more satisfactory results. The report is as follows:

1. Reagents and Solutions

1.1 Zinc Powder: Analytical grade, produced by Taishan Chemical Plant, Guangdong.

1.2 Arsenic-free zinc, analytical grade, produced by Shanghai Xinhua Chemical Plant.

1.3 Acetic acid solution (6 mol/L) and p-aminobenzenesulfonic acid-naphthylamine test solution were prepared according to the appendix of Part II of the Chinese Pharmacopoeia (1990 edition).

1.4 Experimental water was deionized water with a specific conductivity above 10⁷, and passed nitrate and nitrite tests. It was prepared in our laboratory.

1.5 Sodium nitrate control solution: Dissolve 0.01 g of analytical grade sodium nitrate in water to 1000 ml. Dilute 7 ml of the solution to 100 ml with water. Dilute 1 ml of the solution to 100 ml with water. 0.07 μg/ml.

1.6 Sodium nitrite control solution: Dissolve 0.01 g of analytical grade sodium nitrite in water to 1000 ml. Dilute 7 ml of the solution to 100 ml with water. Dilute 1 ml of the solution to 100 ml with water. 0.007 μg/ml.

2. Experimental Methods

Following the methods in Part II of the Chinese Pharmacopoeia (1990 edition), six batches of samples from five pharmaceutical companies and hospitals were simultaneously tested using two reducing agents (0.01 g zinc powder and 0.01 g arsenic-free zinc) and a blank control (without reducing agent). The results are shown in Table 1.

Table 1. Samples and Recovery Tests for Two Reducing Agents and Blank Control

Note: Blank control is without reducing agent.

3. Recovery Test

Take 1 ml each of sodium nitrate control solution and sodium nitrite control solution, add 14 ml of water, and determine the recovery rate according to the method in Part II of the Chinese Pharmacopoeia (1990 edition). The results are shown in Table 1.

As shown in Table 1, the results of the zinc powder reducing agent do not match the blank control; the color is darker, the false positive rate is high, and it interferes with the judgment of the test results. The results of the arsenic-free zinc reducing agent match the blank control; there is no false positive interference, and the recovery rate is good.

4. Effect of Standing Time on the Color Development Results of the Two Reducing Agents

Take 15 ml of water, and determine the color development results at different standing times according to the method in Part II of the Chinese Pharmacopoeia (1990 edition). The results are shown in Table 2.

Table 2 Effect of Standing Time on the Results of the Two Reducing Agents

As shown in Table 2, the reducing agent zinc powder turns pink after 5 minutes, reddish-purple after 10 minutes, and purplish-red after 15 minutes, with a relatively deep color development. The color development results of the reducing agent zinc without arsenic are consistent with the blank control, with no false positives, and it is stable within 240 minutes, meeting the 15-minute requirement of the Chinese Pharmacopoeia (1990 edition), Part II.