phosphorous acid

noun Chemistry.
a colorless, crystalline, water-soluble acid of phosphorus, H 3 PO 3 , from which phosphites are derived.

Origin:
1785–95

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World English Dictionary
phosphorous acid
 
n
1.  Also called: orthophosphorous acid, Systematic name: phosphoric acid a white or yellowish hygroscopic crystalline dibasic acid. Formula: H3PO3
2.  any oxyacid of phosphorus containing less oxygen than the corresponding phosphoric acid

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American Heritage
Science Dictionary
phosphorous acid   (fŏs'fər-əs, fŏs-fôr'əs)  Pronunciation Key 
A white or yellowish crystalline solid used as a reducing agent. It is hygroscopic (easily absorbs moisture from the atmosphere). Chemical formula: H3PO3.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

phosphorous acid

one of several oxygen acids of phosphorus, used as reducing agent in chemical analysis. It is a colourless or yellowish crystalline substance (melting point about 73 C, or 163 F) with a garliclike taste. An unstable compound that readily absorbs moisture, it is converted to phosphoric acid (H3PO4) in the presence of oxygen or when heated above 180 C (360 F). Phosphorous acid (H3PO3) forms salts called phosphites, also used as reducing agents. It is prepared by dissolving tetraphosphorus hexoxide (P4O6) or phosphorus trichloride (PCl3) in water.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
In other areas phosphorous acid is still recommended.
The third group includes phosphorous acid-containing products.
These are the first reported data concerning the physical mode of action of phosphorous acid against downy mildew.
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