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    A PH Indicator

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    Xavier Chan Kf
    Natural pH indicators are substances that change color based on whether the environment is acidic or basic. Many plant pigments act as natural pH indicators, including anthocyanins found in red cabbage, which change from red in acid to purple and green in base. Chemists select natural dyes like litmus, alizarin, and curcumin to indicate pH levels based on their color changes between acid and base. All pH indicators act as acids or bases themselves, donating or accepting protons to change color depending on the solution's pH.

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    A PH Indicator

    Uploaded by

    Xavier Chan Kf
    370 views2 pages
    Natural pH indicators are substances that change color based on whether the environment is acidic or basic. Many plant pigments act as natural pH indicators, including anthocyanins found in red cabbage, which change from red in acid to purple and green in base. Chemists select natural dyes like litmus, alizarin, and curcumin to indicate pH levels based on their color changes between acid and base. All pH indicators act as acids or bases themselves, donating or accepting protons to change color depending on the solution's pH.

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    SImple PH indicator introduction

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    A pH Indicator

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    Natural pH indicators are substances that change color based on whether the environment is acidic or basic. Many plant pigments act as natural pH indicators, including anthocyanins found in red cabbage, which change from red in acid to purple and green in base. Chemists select natural dyes like litmus, alizarin, and curcumin to indicate pH levels based on their color changes between acid and base. All pH indicators act as acids or bases themselves, donating or accepting protons to change color depending on the solution's pH.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    370 views2 pages

    A PH Indicator

    Uploaded by

    Xavier Chan Kf
    Natural pH indicators are substances that change color based on whether the environment is acidic or basic. Many plant pigments act as natural pH indicators, including anthocyanins found in red cabbage, which change from red in acid to purple and green in base. Chemists select natural dyes like litmus, alizarin, and curcumin to indicate pH levels based on their color changes between acid and base. All pH indicators act as acids or bases themselves, donating or accepting protons to change color depending on the solution's pH.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    of 2

    A pH indicator is something that can tell you whether a substance is an acid or a base.

    A visual acid-base indicator is just a weak acid with differently colored acid and conjugate base forms. 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 acid base chemistry we use litmus paper to indicate if a solution is acidic, basic or neutral based on the color changes. Flowers and leaf pigments often fit this description. Many flowers, fruits and vegetables contain chemical substances that change color in solutions of different pH. All pH indicators change color depending on whether they donate or accept proton (acids are proton donors and bases are proton acceptors). Therefore pH indicators are themselves acids or bases.

    Introduction:
    Chemists choose selective natural indicators (such as litmus) to indicate specific pH levels based on their color changes. Many plant pigments act as acid-base indicators, and we can now appreciate why some of these pigment molecules behave the way they do. The blue and red pigments of flowers were isolated and extensively studied by R. M. Willstatter, just before the outbreak of the First World War. Many indicators can be extracted from plants; others like phenolphthalein and methyl orange are synthetic. The substances in the plant products such as tea, red cabbage or grapes react with acids or bases resulting in changes at the molecular level which causes their color to be different at different pH levels. Red cabbage juice a natural pH indicator which is known to contain anthocynins which are pigments that react in a different way to acids and bases. Cyanidin-based compounds make apples, autumn leaves, roses, strawberries, and cranberry juice red. They make blueberries, cornflowers, and violets blue. They also make some grapes, blackberries, and red cabbage purple.

    Cyanidin

    Examples:
    Here are few examples of natural pH indicators: Alizarin is an orange dye present in the root of the madder plant; it was used to dye wool in ancient Egypt, Persia, and India. In an 0.5% alcohol solution, alizarin is yellow at pH 5.5 and red at pH 6.8. Several synthetic modifications of alizarin are also used as acid/base indicators.

    Cochineal is an acid-base indicator made from the bodies of dried female cochineal insects, found in Mexico and Central America. You'll have to grind about 70,000 insects to make one pound of dry indicator. The powder is about 10% carminic acid, which is yellow in acidic solution and deep violet in alkaline solution. Cochineal solutions are not used much as acid/base indicators these days. Curcumin, or turmeric yellow, is a natural dye found in curry powder. It turns from yellow at pH 7.4 to red at pH 8.6. Esculin is a fluorescent dye that can be extracted from the leaves and bark of the horse chestnut tree. You'll need to shine a black (ultraviolet) light on the indicator to get the full effect. Esculin changes from colorless at pH 1.5 to fluorescent blue at pH 2. Anthocyanin is probably the most readily available acid/base indicator; it is the plant pigment that makes red cabbage purple, cornflowers blue, and poppies red. It changes color from red in acid solution to purplish to green in mildly alkaline solution to yellow in very alkaline solution. The color changes for red cabbage juice are shown to the right. Litmus is a blue dye extracted from various species of lichens. Although these lichens grow in many parts of the world, almost all litmus is extracted and packaged in Holland. Litmus is red at pH 4.5 and blue around pH 8.3. While most litmus is used to make litmus papers, some is used as a coloring for beverages. Logwood is a dye obtained from the heartwood of a tree that grows in Central America and the West Indies. The extract contains hematoxylin and hematein, which turn bright red in alkaline solution.

    Conclusion:
    All pH indicators changes colors depending upon whether they donate or accept protons. Therefore, pH indicators are themselves acids or bases!!! Chemists pick selective natural dyes to indicate specific pH levels based on their chemical changes.

    Reference:
    1. 2. 3. 4. Ingrid Jansons, Natural pH Indicators: What are they and how do they work?, Chapter 8: Acids and Bases, Nelson Science 10, 2001. Phanstiel O., J. Chem. Ed. 62, 322, 1985. Summerlin L.R., Chemistry of Common Substances, Silver Burdett, New York, 1986. http://antoine.frostburg.edu/chem/senese/101/features/water2wine.shtml.

    http://www.pharmainfo.net/reviews/natural-ph-indicators-review

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