Magnesium hydroxide forms available

Mag nesia. ndAlumina. Suspension. A mixture of salts, available as Maalox, Mylanta, Gelusil, and Aludrox, contains magnesium hydroxide [1309-42-8] Mg(OH)2, and variable amounts of aluminum oxide in the form of aluminum hydroxide and hydrated aluminum oxide, ie, 2.9—4.2% magnesium hydroxide and 2.0—2.4% aluminum oxide, Al O, for a mixture of 4.9—6.6% combined magnesium hydroxide and aluminum oxide. This mixture may contain a flavoring and antimicrobial agents in a total amount not to exceed 0.5% (see Aluminum compounds, aluminum oxide). 

White, very fine, odorless powder. Available in a very bulky form termed Light or in a dense form termed Heavy. Takes up C02 and H20 from the air, the light form more readily than the heavy, mp 2800°. Highly reflec ttve in visible and near uv region. Combines with water to form magnesium hydroxide. Very slightly sol in pure water, soly increased by 002 sol in dil acids insol in alcohol. Imparts a slight alkaline reaction to water. pH of satd aq soln 10.3. Keep well closed. 

The fine crystallographic form in which MGH is most readily available leads to inconveniently high viscosities. New ways of preparing magnesium hydroxide have been developed to counter this, although they increase the price. MGH is nevertheless extensively used in cable sheathing and in polypropylene. 

Magnesium hydroxide is potentially available in a number of product forms, which are all being developed and optimised for flame-retardant uses. Three distinct types of magnesium hydroxide can be recognised natural, synthetic seawater or brine, and synthetic large crystal. 

Where sea-water is available, electrolysis provides an efficient method of sewage treatment. The sewage is mixed with sea-water and passed through an electrolytic cell. Hypochlorite is formed and rapidly sterilises the sewage, which then sediments out the latter process is assisted by the gelatinous precipitate of magnesium hydroxide which forms in the alkaline solution. 

Antacids are bases used to neutralize the acid that causes heartburn. The most common antacid ingredients are magnesium and aluminum hydroxides, and bicarbonate or carbonate salts (Table 9.4). Baking soda (sodium bicarbonate) was used to relieve indigestion before many of the other commercial products became available. The bicarbonate ion, a basic anion of a weak acid, reacts with the hydronium ion from hydrochloric acid to form carbonic acid, which decomposes to give carbon dioxide and water. Note this is the same mechanism by which the blood buffering system neutralizes acid. 

When zinc and magnesium containing iron are dissolved in dilute hydrochloric acid, the solution contains not only zinc and magnesium chloride but also iron chloride. However, under these conditions, the iron does not respond to a,a -dipyridyl directly unless no more than traces of iron are involved. The sensitive test for valent iron fails because Zn+ and Mg+ ions form colorless complex ions with a,a -dip3rridyl and thus consume the reagent. However, this interference can be averted, if concentrated ammonium hydroxide is added after the a,a -dipyridyl has been introduced. The zinc and the magnesium dipyridyl compounds are converted into the respective ammine ions and the reagent is released and made available to the ferrous ions. 

The electrons thus released pass around the external electric circuit to the air cathode where they are available for the reaction between water and oxygen to form more hydroxyl ions. The cathode reaction is exactly the same as for the alkaline fuel cell shown in Figure 1.4. The metal oxide or hydroxide should remain dissolved in the electrolyte. Cells that use a salt-water electrolyte work reasonably well when they use aluminium or magnesium as the fuel . 

Pure magnesium should have a driving potential of 850 mV to protect steel but in practice the metal corroded very rapidly with a very low efficiency. The metal suffers low polarization in the presence of chloride or sulfate ions and produce highly soluble chloride and sulfate salts. These ions are usually artificially introduced into the electrolyte as a backfill when a deficiency is expected, the hydroxide which is preferentially formed because of its low solubility becomes enriched with the backfill anions and itself functions as a backfill. Uniform general corrosion can then be obtained and well-designed inserts help to keep most or all of the anode metal available for sacrificial consumption. In freshwater or electrolytes which contain none of these ions, the hydroxide and carbonate may form, but these do not seriously polarize the anode (Morgan, 1993). 

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