Question

I have Pre-lab assignment that requires me to make a flow chart to determine two unknown compounds each unknown is one of the following five white solids: benzoic acid ; sodium benzoate; glycine ; cinchonidine (free base) ; cinchonidine2HCl. Before the lab period, we have to work out and draw a flowchart of tests in which neutral water, acidic water (HCl) and basic water (NaOH) are added to each unknown to determine its solubility at the three different pH's. Thank you!

The info on the manual:

use the solubility results from additions of neutral water, HCl (aq) and NaOH (aq) to your unknowns to determine the identities of your unknowns.  

The unknown samples will be ~ 200 - 250 mg in amount, use ~ 25 - 50 mg in a clean small test tube for the solubility tests and add ~ 1 - 2 mL of neutral, acid or base solution.  Keep in mind that enough acid or base has to be added to completely protonate or deprotonate the sample molecules, be sure to add at least 10 mole equivalents of acid or base per mole of sample to ensure complete reaction.  Litmus paper is available should you need it to check the pH of a solution.

NOTE: if the unknown sample dissolves in acid, add base to verify that it precipitates (and vice-versa if it dissolves in base); and note that only glycine does not precipitate at either extreme of pH.

Answer 1

1. Take small amount of solid and add neutral water and shake well to dissolve.

• Compounds which get dissolved in neutral water are: sodium benzoate, glycine and cinchonidine hydrochloride (Compunds which will NOT dissolve in neutral water are benzoic acid and cinchonidine).
• If compunds dissolve in nuetral water. Divide the solution in 2 parts.
• Add ninhydrin (dissolved in EtOH) to one part and heat the mixture on water bath. If color changes to bue then the compund is glycine.
• Add Cerric ammonium nitrate solution to the second part. If the color changes to red, then the compund is cinchonine hydrochloride and if the color does not change then the compound is benzoic acid. This happens because of free alcoholic OH group present in cincholine which is absent in sodium benzoate.

2. If the compund does not dissolve in water then the compund maybe be benzoic acid or cincholine.
3. Add basic water to the compund. If it dissolves completely then the compound is benzoic acid. To confirm this, add the compund to the saturated solution of sodium carbonate, brisk effervescence will be observed.
4. Add acidic water to the compound, if it dissolves then the compound is concholine.