CHEMISTRY : Quantitative Analysis

Preparing Acid-free salts from ACID vs METAL Reactions

INTRODUCTION

This is a procedure to obtain the salt formed in the reaction between an acid and a metal in a the pure form as much as possible.

ACID (aq) + METAL (s) ---> SALT (aq) + HYDROGEN (g)

The main contaminating factor is xs unreacted acid, which will be present when evaporating and crystallizing the salt out after filtering unreacted metal. Ideally, the reaction is left for a long time until all acid is reacted leaving behind salt water and unreacted metal which can easily be filtered out. However the reaction may be complete after a very long time, example 20 days, and almost always there is left traces of unreacted acid after such long time.

xx Acid + yy Metal --> Salt + Hydrogen + x Acid + y Metal (still unreacted)

PRINCIPLE:

The principle involves of a simple method to neutralize the xs acid with an alkali such as sodium hydroxide or Sodium carbonate or ammoinium hydroxide solution. Sodium carbonate is the prefered choice for reasons expailned later.

2 Al + 6 HCl ===> 2 AlCl3 (aq) + 3 H2 (g)

Immagine the picture, we have the metal salt, example Aluminium Chloride, and un reacted Hydrochloric Acid, and we must neutralize the acid out. Adding an alkali solution will 'prefer' to neutralize the acid, rather then precipitating the metal into the corresponding base. So, the acid is removed, leaving the metal salt. Of course the neutralization reaction between the added alkali and unreacted acid will produce another sodium salt (sodium chloride in this case).

2HCl + Na2CO3 ===> CO2(g) + 2 NaCL(aq) + H20(l)

However, appart from being in trace ammounts, it is a more cleaner and safer contaminant from the acid beacuse it will not affect the pH, and on evaporation (to crystallize the salt out), the acid gets concentrated and interfere if for example the metal salt is used to precipitate its metal by an alkali.

Sodium Carbonate is the preferred choice od alkali to be used becasue it does not form complex compounds as Hydroxide and Ammonia solution can do with few metal hydroxides. Also all carbonates of metals (except that in group I in the periodic table ) are insoluble, while certain hydroxides are slightly soluble.

PROCEDURE:

(1) Make the Acid + Metal reaction solution to boil and dropwise add Sodium Carbonate solution until no effervesence (production of CO2 by acid) is produced and so a precipitate starts to form.

(2) The precipitate (metal carbonate) formation which does not dissolve on boiling, is the end point of the reaction.

(3) Pass the mixture solution from a filter paper to remove any unreacted metal and the little Carbonate formed.

(4) evaporate by heat or in air the solution filtered to obtain the Metal salt.

CONCLUSIONS

It is a quite easy procedure, but actually traces of sodium salt remains together with the wanted metal salt.

It is very difficult to purify the salt from any other chemicals (hence xs acid) in a common home lab. Ideally organic solvents which does not dissolve the metal salt are used to wash away the unreacted acid, and then the salt is evaporated to vaporize the organic solvent. Such a solvent is for example absolute ethanol which reacts with several mineral acids forming volatile compounds and does not dissolve almost all metal salts.

In the procedure, boiling while adding sodium carbonate solution is very important to speed up the reaction, and for better results.

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