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I'm currently working on attempting to identify a particular substance of which I know the cation is Na, however I am unsure as to what the corresponding anion is based off of these characteristics.

The salt is soluble, and when drops of AgNO3 are added to the dissolved substance the liquid turns a dark brown and a nearly black precipitate forms on the top. The liquid remains relatively transparent however. In addition, I observed that when chlorinated water was added to the solution, the solution slowly turns white with a light tint of yellow, like a very light creamy color, it releases some pungent odor however I am not familiar enough with scents to determine whether or not it is an ammonia or sulphur odor. Of the sodium compounds, the following are options:

  • Sodium bisulfate
  • Sodium borate
  • Sodium citrate
  • Sodium chloride
  • Sodium fluoride
  • Sodium nitrate
  • Sodium nitrite
  • Sodium oxalate
  • Sodium sulfate
  • Sodium sulfite
  • Sodium thiosulfate

As far as I'm aware the reaction with chlorinated water is just a reaction with Cl

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The black precipitate can either be

  1. metallic silver, formed by reduction of the $\ce{Ag+}$ ions (most likely)
  2. an insoluble silver salt

Possible candidates for the sodium salt:

╔══════════════════╦═══╦═══╗
║      SALT        ║ 1 ║ 2 ║ 
╠══════════════════╬═══╬═══╣ 
║Sodium bisulfate  ║   ║   ║ 
║Sodium borate     ║   ║ ? ║ 
║Sodium citrate    ║   ║   ║ 
║Sodium chloride   ║   ║   ║ 
║Sodium fluoride   ║   ║   ║
║Sodium nitrate    ║   ║   ║
║Sodium nitrite    ║   ║   ║
║Sodium oxalate    ║   ║   ║
║Sodium sulfate    ║   ║   ║
║Sodium sulfite    ║ ✔ ║   ║
║Sodium thiosulfate║ ✔ ║   ║
╚══════════════════╩═══╩═══╝

The reaction with chlorinated water tends to show it is the first case (maybe with sodium thiosulfate).

Regarding the pungent odor, $\ce{H2S}$ has a strong rotten-egg odor.

A specific test for sodium thiosulfate is adding an acid (such as $\ce{HCl}$ to the unknown salt solution). A yellowish-white sulfur colloid should slowly appear.

A specific test for sodium sulfite is the decoloration of fuschine (should be available in a microbiology lab): add a buffer at pH = 7 to 1 mL of your unknown salt solution, then 1 mL of fuschine. After a few seconds, it should turn transparent.

Can you conduct those test to get more insight on this? If the first one is positive, no need to do the second one.

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