How to Prepare Pure Hydrochloric Acid for the Home Lab
Dense fumes when vapors of Hydrochloric acid and ammonia interact
Description of Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride gas dissolved in water. It is a corrosive and toxic strong acid that emits choking fumes when concentrated. The impure technical grade is called muriatic acid which is available at home improvement stores as well as pool care centers. It is usually contaminated with iron, chlorine, and organics giving it a yellow tint.
To perform titrations or make pure chloride salts, a purer form of hydrochloric acid will be needed. There are a few methods that are useful for obtaining chemically pure hydrochloric acid. These methods are as follows:
- Displacement of hydrogen chloride gas from impure muriatic acid and leading said gas into pure distilled water
- Using a less volatile acid such as sulfuric acid to displace hydrogen chloride from a suitable chloride salt and leading resulting gas into pure distilled water
- Aqueous chemical reduction of free elemental chlorine with suitable reducing agent(ex. sulfur dioxide) and distilling resulting mixture
- Heating hydrated heavy metal chlorides and collecting resulting vapors.
Traditional proceedure of hydrochloric acid synthesis
The oldest method of preparation of hydrochloric acid is to create hydrogen chloride gas by adding concentrated sulfuric acid to a chloride salt such as sodium chloride. This hydrogen chloride gas is led into chilled water until an acid of the desired concentration is achieved. The sulfuric acid is a source of protons and the hydrogen chloride gas is volatile and is removed as it is formed. This is a useful method but requires a lot of apparatus. Also, in the later stages of the reaction, external heating is needed to drive the reaction because the sodium bisulfate formed slows evolution of hydrogen chloride.
Another variation on this theme is to take muriatic acid purchased from a home center or pool supply place and add sulfuric acid to this at a controlled rate. The heat generated keeps the reaction going and the gas can be led into purified water. When hydrogen chloride evolution ceases, the sulfuric acid is diluted and it can be concentrated by heating it in pyrex beaker or flask until specific gravity of 1.84 is reached. This way no sulfuric acid is wasted. Of coarse the sulfuric acid will retain impurities found in the technical muriatic acid. So it is advised to not use the sulfuric acid for synthesis after performing this procedure.
Hydrochloric Acid from Salt
Calcium Chloride Method
If using sulfuric acid is not option, then you should consider the calcium chloride method. In this method concentrated muriatic acid can be dripped on anhydrous calcium chloride. This will evolve some hydrgen chloride. The best proceedure is to take an erlenmeyer flask and place the solid calcium chloride pellets in the flask. Then secure a pressure equalizing addition funnel filled with concentrated hydrochloric and add dropwise collecting the gas evolved. This is from the article in Journal Chemical Education circa 1995 concerning this preparation.
Again the dehydrating agent,calcium chloride, can be regenerated by heating the solid mass to 200 Celsius.
Pure hydrochloric acid from impure muriatic acid
On youtube, I found the most interest video regarding the preparation of pure hydrochloric acid from impure muriatic acid. It is so simple and requires no expensive glassware. It is based on taking two small plastic or glass containers and putting an equal volume of muriatic acid and distilled water in each one. Then carefully placing both open containers in a larger container and sealing it undistrubed for about a week or so. After that time, you will have two containers with hydrochloric acid of the same concentration! What happens is the fumes from the muriatic acid are trapped in a closed system and the pure distill water absorbs them. As this occurs, more vapors are evolved from the muriatic acid to continue the process until an equilibrium is reached. This produces dilute hydrochloric acid only but that is all that is needed for preparing most pure chlorides. More hydrogen chloride can be dissolved to concentrate the diluted acid.
Purify hydrochloric acid
Reduction of elemental Chlorine
The chemical reduction of elemental chlorine gas can generate hydrogen chloride or hydrochloric acid solution. If the reducing agent is somewhat acidic and nonvolatile then the hydrochloric acid generated can be distilled off. For example, bubbling chlorine gas through sodium metabisulfite solution would yield the corresponding bisulfate and hydrochloric acid in solution. Ferrous sulfate with excess dilute sulfuric acid would produce ferric sulfate and hydrochloric acid.
This by no means practical for large scale work, but may be a way to reclaim some HCl from chlorides. Some metal chlorides, especially the hydrated heavy metal ones, give off HCl fumes when heated leaving a mass of metal oxide or oxychloride behind. Several metal salts are capable such as magnesium chloride, iron(II)chloride, iron(III)chloride, aluminum chloride etc. It advisable to find a chloride that decomposes at a relatively low temperature to minimize difficulties.
An example is if you use muriatic acid to remove mill scale, after time, the acid will become green/yellow with iron(II)chloride and iron(III)chloride. In time it would lose its potency. To alleviate this, a glass distilation set up can be set up and unused hydrochloric acid can be collected. Then when the iron chloride mix is more concentrated, the heat can be increased to liberate free HCl to be collected. Depending on what iron chloride was present, black iron oxide or red iron oxide would be left behind.