GUEST AUTHOR David Dingledy
State University College Fredonia, N e w York
I I
Textbook Errors, 56
Detolorization of Glass
A
common error in general chemistry textbooks is a stat,ement that manganese dioxide (pyrolusite, glassmakers' soap) decolorizes commercial glass by virtue of its ability to oxidize the blue-green iron(11) to yellow-brown iron (110.' This is reasonable from the chemist's point of view since manganese dioxide is a strong oxidizing agent at low temperatures. For example, it is used in general chemistry laboratory experiments to oxidize hydrochloric acid and thus generate chlorine. Furthermore, it decomposes at 53j°C, releasing oxygen, and --
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Suggestions of material suitable for this column and guest columns suitable for publication directly should be sent with as m w y derail, n pos,~hlr, . a n d ~ a r t i c ~ l ~ rwliyt h refereneri tu mderr~tcxtl,ods, tu Karol J. IIyielr, IJrp~rrmcl.c~~f('herwstry, I l n i w ~ufi ~%uthm, ~ Califmuin. 1.0s angel^.% . (.'difurt.ia90M7. sincethe purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. The error must occur in at least two independent standard hooks to be presented.
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Journal of Chemical Education
would therefore appear to be a suitable source of oxygen a t the higher temperature of the glass melt. However, the following considerations show that manganese dioxide is not added to glass because it is an oxidizing agent: First, although the blue-green color of glass which has escaped decolorization is not esthetically pleasing for the packaging of many products (for example, milk), the yellow-brown of oxidized iron, while of lesser intensity, is also usually not acceptable to the packager and is rather difficult to attain in practice. Second, a really satisfactory oxidizing agent for the glass melt releases oxygen a t melt temperatures; for modern commercial glasses held at furnace temperatures higher than 1000°C, the oxidizing action of manganese dioxide is relatively ineffective. The presence of iron in commercial glasses in amounts of 0.05 to 0.10% (as Fez03) is unavoidable because of the small amounts of iron present as an impurity in the raw materials used in bottle and window glass manufacture (1). The iron in glass is present in both
the iron(II1) and iron(I1) forms, and if the melt is made under the usual conditions with oxidizing agents such as sodium nitrate (niter) and sodium sulfate (salt cake) the hished glass is blue-green or yellow-green depending on the point at which the iron(I1)-iron(II1) equilibrium is established. It is relatively difficult to force the direction of the equilibrium in the direction of the iron(II1) species; this can be done by using oxidizing agents that are effective at elevated temperatures, such as arsenic or antimony pentoxides or cerium oxide. Cost and other considerations usually rule out these more effective agents; in addition the finished glass has a yellow color which makes it somewhat uusatisfactory for bottles and jars. The true function of manganese in glass is to act as a physical decolorizer, masking the blue-green color from the iron by addition of a complementary red color. This addition results in approximately uniform absorption of light in the visible spectrum and the glass appears colorless (gray) to the eye. The result is esthetically more pleasing for many purposes, as those who have seen milk in a greenish milk bottle that accidentally escaped decolorization will probably agree. Manganese dioxide is suitable as a decolorizer in several respects, and has been used as such since ancient times (8). It decomposes at a relatively low temperature (535°C) forming a small number of Mn(II1) ions in the melt; these have a strong light absorption in the 4700-5200 A region in the visible spectrum. This is the desired complementary color region. Decolorization by addition of a complementary color is indicated by Figure 1, which shows the absorption spectra of glasses containing iron, manganese, and cobalt. The total absorption for a glass containing iron plus decolorizers is approximately uniform. The color of manganese in glass depends upon the equilibria between the various oxidation states present: Oxidation state
Color
7 6 4 3 2 0
Deep purplwed Green Black Red-brown Pale yellow Pale oink-silver
The intensely colored permanganate ion is not formed under the usual glass melting conditions. Of the remaining ions, the red-brown Mn(II1) ion is most effective in coloring the glass; however, it is estimated that only about 0.001 of the manganese is present in this form, being in equilibrium with the Mn(I1) form having a weak yellow coloration in glass (5). (It may be noted that Mn(I1) ions in water are pale pink because of hydration of the ions). Maintaining the proper concentration of Mn(II1) ion in the glass melt is diicult, since the oxidation state of the glass depends upon variables of furnace atmosphere, batch material changes, additions of fining (degassing) agents which are also oxidizing agents, and so forth. I n earlier days when commercial glasses
.00022 MANGANESE
Figure 1. Absorbance (log l/lol in the visible spectrum of sodium borate glosre$ 0.75 sm thick. Numerical valuer are weight concentrotionr of the elements.
were melted by the batch in pots, oxidizing agents were added along with an excess of manganese dioxide, and the color level brought down after melting by "blocking" (that is, reduction of the melt), perhaps by stirring with a green sapling pole (4). This effectively reduced the excess Mn(II1) ion to nearly colorless Mn(I1) ion. I n present con~mercialcontinuous processes, the melt can no longer be held for adjustment to the proper oxidation state and therefore manganese is no longer used as a decolorizer. Selenium, which colors the glass pale pink in its elemental form and is much less sensitive to the state of oxidation of the melt, is now used universally as the commercial glass decolorizer, along with cobalt (5). Present usage requires quantities of the order of magnitude of an ounce of selenium to the ton of glass and one-sixth this amount of cobalt (6). Acknowledgment
The author wishes to acknowledge the support of the Research Foundation of the State University of New York and of the State University of New York Atmospheric Sciences Research Center, and to thank the public library of the city of Buffalo for the opportunity to examine their incunabulun~copy of Pliny. Literature Cited (1) TOOLEY, F. V., Editor, "Hsndbook of Glass Manufacture," Ogden Publishing Company, New York, 1953, p14. (2) PLINYTHE ELDER, ''Natural History," A. and J. Bntmnicus, Brescia. 1496. Book 36. eh. 26. (3) WEYL, W; A,, ''r~olour& Glasses," The Society of Glass Technology, Shefield, England, 1951, p. 126. , "Glass," revised by L. M. Angus-Butterworth, (4) M m s o ~ P., Pitman, London, 1932, p. 34. (5) Glass Indzlsl~y,39, 487 (1958). (6)HODKIN,F. W., AND COUSEN,A,, "A Textbook of Glass Technology," D. van Nostrand Co., New York, 1925, p. 134.
Volume 42, Number
3, March 1965 / 161
