B Qualitative Analysis for Halogens, Sulfur, and Nitrogen

Halogens. Acidify about 2 mL of the FAQS by dropwise addition of 6 M nitric acid. Working at the hood, boil the solution gently for 2-3 min to expel any hydrogen sulfide or cyanide that may be present. Cool the solution and then add several drops of 0.3 M aqueous silver nitrate solution. A heavy precipitate of silver halide indicates the presence of chlorine, bromine, or iodine in the original organic compound. A faint turbidity of the solution should not be interpreted as a positive test. 

Tentative identification of the particular halogen is made on the basis of color. Definitive identification must be made by standard inorganic qualitative procedures (Ref. 1 at the end of Section 25.5) or by means of TLC (Sec. 6.2), using the following procedure. 

Sulfur. Acidify a 1-2 ml sample of the FAQS with acetic acid, and add a few drops of 0.15 Mlead acetate solution. A black precipitate of PbS signifies the presence of sulfur in the original organic compound. 

Transfer precipitated silver halides to a container labeled for them so that the silver can be recovered. Put the precipitated lead sulfide in a container for heavy metals. Any FAQS that remains may be flushed down the drain, as can the solution from the test for nitrogen. 

The solubility of an organic compound in water, dilute acid, or dilute base can provide useful, but not definitive, information about the presence or absence of certain functional groups. In reality, however, the assignment of an unknown to a formal solubility class may be arbitrary because a large number of compounds exhibit borderline behavior. We recommend that the solubility tests be done in the order presented here. 

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