GlasswareCleaning Farooq

Химия, год. ХIV, кн. 6 (2005)
Chemistry, Vol. 14, Iss. 6 (2005)
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• Letters to the Editor •
A GLASSWARE CLEANING MIXTURE
Chemists often encounter glassware in the laboratory with stubborn dirt
which does not come off easily with ordinary detergents. Usually we use chromic acid (mixture of concentrated sulfuric acid and sodium dichromate) and
piranha solution (concentrated sulfuric acid and hydrogen peroxide mixture).
Such mixtures are extremely effective and glassware after washing does show
an unbroken film – a well known criterion of clean surfaces.
The inherent hazards, e.g., the carcinogenicity of dichromate and the explosive risk of piranha solution if traces of organics are present outweigh their
usage for common purposes. There is alternative safer washing mixture which
the authors have tried and found useful.
An approximately 2-4 M solution of sodium hydroxide is prepared in a
saturated solution of potassium permanganate, depending upon the level of
dirt. This mixture is stable and retains the original color of permanganate. The
dirty glassware is allowed to soak in it for 10-30 minutes depending upon the
extent of dirt. Prolonged soaking is however not recommended, since alkalis
attack glass slowly. The glassware then may acquire a brown layer of manganese dioxide which shows the presence of reducible matter adhering to the
glass surface. After the soaking period, the glassware is rinsed with water and
then with approximately 3% hydrogen peroxide and dilute (~0.001 M) nitric or
hydrochloric acid solution, any acid would work. This treatment instantly removes the brown tinge on the glass. The washed glassware then shows unbroken film of water indicating the surface is clean.
The chemical dirt which is most likely composed of chemicals in a matrix
of lipophilic substances stubbornly adhering, are solubilized by alkali and subsequently oxidized by permanganate with the formation of manganese dioxide
which is insoluble in basic medium. This imparts a brown tinge on the surface.
Final treatment with acidified hydrogen peroxide converts the oxide to water
soluble Mn(II). Manganese is relatively non-toxic as compared to chromium
(VI) which is known human carcinogen [1]. Unlike piranha solution, alkaline
KMnO4 does not pose an explosive hazard while cleaning.
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A variation of the above method was found in ref. [2] for cleaning burettes.
The procedure Fowles recommended was to fill a burette with a strong neutral
solution of potassium permanganate for some days prior to use, run out the
solution and add warm HCl to dissolve any precipitated manganese dioxide.
This treatment also gives excellent results. Potassium permanganate is reduced
in neutral medium of MnO2. Treatment with concentrated HCl removes precipitated manganese dioxide by forming water soluble Mn(II); in the process
Cl2 is also formed. What KMnO4 can not clean, in situ generated chlorine can.
Safety: Gloves and requisite eye protection should be worn all the time
while handling any cleaning mixture. Potassium permanganate solutions can
temporary stain the skin or clothing brown which can be readily washed off
with medicinal hydrogen peroxide.
References
1. http://www.atsdr.cdc.gov/toxprofiles/phs151.html
2. Fowkes, G. Volumetric Analysis. Ball & Sons, L., 1957.
Çàìúðñåíà ñ õèìèêàëè õèìè÷åñêà ñòúêëàðèÿ ñå èçìèâà åôèêàñíî ñ
2-4 Ì ðàçòâîð íà íàòðèåâà îñíîâà â íàñèòåí ðàçòâîð íà êàëèåâ ïåðìàíãàíàò.
Ñúäîâåòå êèñíàò â òàçè ñìåñ 10-30 ìèí, ñëåä êîåòî ñå èçïëàêâàò ñ âîäà è ñ
3% ðàçòâîð íà âîäîðîäåí ïåðîêñèä èëè ìíîãî ðàçðåäåíà àçîòíà èëè ñîëíà
êèñåëèíà. Äà ñå ðàáîòè ñ ðúêàâèöè è ïðåäïàçíè î÷èëà! Îöâåòÿâàíåòî
íà êîæàòà è äðåõèòå îò êàëèåâèÿ ïåðìàíãàíàò ñå ïðåìàõâà ñ ìåäèöèíñêè âîäîðîäåí ïåðîíñèä.
M. Nasiruddin Khan, Farooq Wahab,
Department of Chemistry,
University of Karachi, Karachi 75270 (PAKISTAN)
E-mail: [email protected]
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