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NATURAL INDICATORS​
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B A C K G R O U N D
We know the color changes that we see in the nature all around us. We also notice the color changes in iced tea when lemon juice is added. In the study of acidbase chemistry we use litmus paper to indicate if a solution is acidic, basic, or neutral based on the color changes. Is there a common chemical process occurring in all the above phenomena? Can we know the ‘mystery’ of the acid-base indicators, like litmus paper, by carefully examining the change in color of some natural dyes?
The substances in plant products such as tea, red cabbage or grapes react with acids(or bases), resulting in changes at the molecular level which causes their colors to be different at different pH levels. All pH indicators, such as litmus paper, change colors depending upon whether they donate or accept protons, (acids are proton donors and bases are proton acceptors). Therefore, pH indicators are themselves acids or bases!!!!! Chemists pick selective natural dyes to indicate specific pH levels based on their color changes. For example, a commonly used acid-base indicator, named Bromthymol Blue, is yellow in acid and blue in base and intermediate in between at pH 7. Yellow form of Bromthymol Blue, (acid-form, HBb) when donates a proton to OH- ions at higher pH changes to anion, (base-form, Bb-) which is blue. The point at which the color changes from intermediate to blue gives information on the acid strength of the acid-form, HBb. Chemists test various indicators for their acid strength and choose an indicator for specific pH levels.
http://chemistry.about.com/library/weekly/aa112201a.htm​
 


Handbook of Acid-Base Indicators
(Hardcover)
by R. W. Sabnis
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Product Details
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Hardcover: 416 pages
Publisher: CRC
(October 4, 2007)
Language: English
ISBN-10: 0849382181
ISBN-13: 978-0849382185
Product Dimensions: 9.9 x 7.2 x 1.1 inches
Shipping Weight: 2 pounds
Editorial Reviews
Product Description
While acid-base indicators continue to find new applications in an ever-widening range of scientific disciplines, there is no current book that focuses entirely on the subject, nor one that brings together the relevant advances that have evolved over the last three decades. The Handbook of Acid-Base Indicators compiles the most up-to-date, comprehensive information on over 200 water-based and solvent-based indicators into a single source.
Organized alphabetically, entries include: common name, other names, CA index name, CAS registry number, Merck index number, chemical structure, chemical/dye class, molecular formula, molecular weight, pH range, color change at pH, pKa, physical form, solubility, UV-visible (Lambda-max), melting point, and boiling point. This resource also offers unique coverage including protocols for synthesizing indicator compounds; data relating to adverse effects, toxicity, and safety; and major applications for each indicator. The Handbook of Acid-Base Indicators contains practical information for widespread applications that include semiconductors, displays, nanotechnology, OLEDs, fuel cells, sensors, security, surface coatings, adhesives, insecticides, agricultural chemicals, textiles, packaging, cosmetics, personal care products, pharmaceuticals, and the detection and treatment of disease.
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