What indicates a positive result in a nitrate reduction test after adding powdered zinc?

A positive result in a nitrate reduction test after adding powdered zinc is indicated by no color change. In this test, bacteria reduce nitrate to nitrite, and if nitrite is present, a red color develops upon adding reagents. However, if the zinc powder is added and there is no color change, it confirms that the nitrate has been further reduced beyond nitrite, likely to nitrogen gas or other end products.

When zinc dust is added, it serves to check for unreduced nitrate. If the medium remains colorless after the addition of zinc, it confirms that all nitrate was reduced by the test organism. Therefore, the absence of any color change signifies that a positive result has been achieved, as it shows complete reduction of nitrate.

In contrast, a red coloration would indicate that nitrite is present, suggesting only partial reduction of nitrate, while yellow coloration and bubble formation are not relevant to the specific interpretation of the nitrate reduction test in this context.