This is a test for chlorides in NaOH sample.

1.0g of NaOH was dissolved in 5ml of distilled water, acidified with 4ml of dilute nitric acid and diluted to 15ml with distilled water. This mixture was poured into a test tube containing 1ml of AgNO3 solution.
However, I obtained a dirty brown ppt. Do you have any idea what the problem might be?
Thanks.

Yes. You did this in light, the silver chloride is photosenstive, it darkens when exposed to light. Brown is not unusual (dark bluish, with slight yellow).

I obtained a lot of dirty brown ppt the moment the mixture was poured into AgNO3 solution. Does this mean my NaOH sample contains a lot of chlorides? Could other silver halides be present?

However for my control solution of 50ppm sodium chloride solution,a turbid solution (cloudy white ppt) was obtained after standing for a while.

Please advise how should I conduct the test?

With my observation it turns to be dirty brown

.

Based on the information provided, it seems like the presence of a dirty brown precipitate (ppt) indicates the formation of silver chloride (AgCl). The appearance of AgCl indicates the presence of chloride ions (Cl-) in the original NaOH sample.

However, if the expected result was a white precipitate, there may be several reasons for the observed brown coloration. Here are a few possible reasons:

1. Contamination: The test tube or any of the reagents used might have been contaminated with other substances or impurities that can affect the reaction and lead to an unexpected coloration.

2. Presence of impurities in the NaOH sample: The NaOH sample used might have contained impurities, such as metal ions or organic compounds, which can react with the reagents and cause a color change.

3. Incorrect preparation or measurement: If the amounts of reagents used were not accurately measured or mixed, it could have affected the reaction and led to the observed brown coloration.

To determine the exact cause of the unexpected result, it is essential to carefully review the steps and conditions of the test and ensure they were conducted accurately. Additionally, verifying the purity of the reagents and the NaOH sample is crucial. It could also be helpful to repeat the test with fresh reagents and a new sample of NaOH to see if the same results persist.