Muriatic Acid test for carbonate minerals

Hydrochloric acid is known as spirits of salt or acidum salis and its formula is HCl. Other name for HCl is muriatic acid. In muriatic acid Muriatic means pertaining to brine or salt. HCl is a strong acid, highly corrosive with many industrial applications. Hydrochloric acid is found naturally in gastric acid.

Hydrochloric acid used in the manufacture of other industrial chemicals like phosphoric acid, chlorine dioxide, ammonium chloride, fertilisers, dyes. HCl is used as a refining ore in the production of tin and tantalum, as a lab reagent, and as a metal treating agent. Muriatic acid is used to remove scale and dust from boilers and heat exchange equipment, to clean membranes in desalination plants, to increase oil well output, to prepare synthetic rubber products by treating isoprene, and to clean and prepare other metals for coatings.

HCl is used in the neutralization of waste streams, the recovery of zinc from galvanized iron scrap, the production of chloride chemicals, the production of vinyl chloride from acetylene and alkyl chlorides from olefins, the manufacture of sodium glutamate and gelatine, the conversion of cornstarch to syrup, sugar refining, electroplating, soap refining, leather tanning, and the photographic, textile, brewing, and rubber industries. It is used to maintain pH balance in swimming pools, spas, etc.

Hydrochloric acid is also used as a bactericide, a fungicide, and a virucide to disinfect restroom, kitchens and food preparation areas, and in commercial buildings, industrial buildings, in hospitals, in nursing homes, around household dwellings. Hydrochloric acid is used in food processing as a starch modifier.

Worldwide hydrochloric acid production is 20 million tones annually. Hydrochloric acid’s major manufacturers include Dow chemicals, Georgia Gulf Corporation, Tosoh Corporation, Akzo Nobel, and Tessenderlo.

Physical properties

Hydrochloric acid is a solution of hydrogen chloride in water. Hydrogen chloride exists as either a colorless liquid with an irritating, pungent odour or a colorless to slightly yellow gas which can be shipped as a liquefied compressed gas.

Chemical properties

HCl is one of the most corrosive of the non-oxidizing acids in contact with copper alloys, and is handled in dilute solutions. If hydrochloric acid reacted with metals it produces hydrogen gas, which in turn creates the chance of an explosion. It produces poisonous gas, including chlorine, in a fire.

Aqueous hydrochloric acid attack and corrode nearly all metals, except mercury, silver, gold, platinum, tantalum, and certain alloys. It may be colored yellow by traces of iron, chlorine, and organic matter.

Muriatic Acid test for carbonate minerals and carbonated rocks

Carbonate minerals are the major constituents of sedimentary rocks; however, they are also found in igneous and metamorphic rocks, either as primary or as secondary minerals. In carbonate rocks formed either by chemical or mechanical deposition; these minerals constitute the main rock-forming components. Sometimes they are also found in great abundance in pelitic, psammitic and psephitic rocks.

According to most geologists “acid test” means placing a drop of dilute (5 per cent to 10 per cent) hydrochloric acid on a rock or mineral and watching for bubbles of carbon dioxide gas to be released. The bubbles signal the presence of carbonate minerals such as calcite, dolomite or one of the minerals. These minerals are aragonite (CaCO3) witherite (BaCO3), strontianite (SrCO3), cerussite (PbCO3) and alstonite [(Ba,Ca)CO3].

The identification and discrimination of carbonate minerals in hand specimens or thin sections, are made easier by the use of simple chemical staining methods.

It is recommended that the thin section or the cut and polished surface of the hand specimen be etched with diluted HCl acid before staining. Diluted acetic acid or formic acid may also be used in place of HCl acid. Different carbonate minerals in a rock specimen, immersed in an appropriate HCl solution for an appropriate period of time, will show-different reactions. Minerals that show a weak reaction should be treated with warm HCl acid for about 30 seconds to 1 minute.

The bubbling release of carbon dioxide gas can be so weak that you need a hand lens to observe single bubbles slowly growing in the drop of hydrochloric acid or so vigorous that a flash of effervescence is produced. Carbonate minerals are unstable in contact with hydrochloric acid. When acid begins to effervesce (fizz/bubbles) on a specimen a reaction similar to the one shown below is taking place.

Many other carbonate minerals react with hydrochloric acid. Each of these minerals consists of one or more metal ions combined with a carbonate ion (CO3—). The chemistry of these reactions is similar to the calcite reaction above. The mineral reacts with hydrochloric acid to produce carbon dioxide gas, water, a dissolved metal ion and dissolved chlorine.

If you place a drop of hydrochloric acid on powdered dolomite a visible reaction will occur. This is because the surface area has been increased, making more dolomite available to the acid. (You can easily make dolomite powder by scratching a specimen of dolomite across a streak plate.

Some rocks contain carbonate minerals and the acid test can be used to help identify them. Limestone is composed almost entirely of calcite and will produce a vigorous fizz with a drop of hydrochloric acid. Dolostone is a rock composed of almost entirely of dolomite. It will produce a very weak fizz when a drop of cold hydrochloric acid is placed upon it, a more obvious fizz when powdered dolostone is tested and a stronger fizz when hot hydrochloric acid is used.

A few rocks can produce an extreme reaction with hydrochloric acid. These are usually rocks composed of calcite or aragonite with abundant pore space or extremely high surface areas. When a drop of dilute hydrochloric acid is placed on these specimens an eruption of acid foam can rise up off of the rock and spread to an unexpected diameter.


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