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Indicators and Buffers

Indicators and Buffers. Indicators. Indicators are a substance that change color in the presence of (whatever they check for) They do this because of Le Ch â telier’s principle. All you need an equilibrium reaction with different colored products and reactants.

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Indicators and Buffers

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  1. Indicators and Buffers

  2. Indicators • Indicators are a substance that change color in the presence of (whatever they check for) • They do this because of Le Châtelier’s principle. All you need an equilibrium reaction with different colored products and reactants. • The pen used to check for counterfeit money is a starch indicator

  3. How an acid base indicator works • A generic indicator will follow this reaction, HId is the reactant indicator, and Id- is its product • HId + H2O  H3O+  + Id- • The color differences are important • in an acidic solution (high H3O+) you see reactant • HId + H2O H3O+  + Id- • in a basic solution (low H3O+) you see product • HId + H2OH3O+  + Id-

  4. Acid Base indicators • Acid base indicators change color at certain pH levels • They don’t have to change at 7 (most don’t) • Universal indicator solution(phenolphthalein, bromthymol blue and methyl red dissolved in ethanol and water) changes color at each integral pH value

  5. Other pH indicators • Litmus and phenolphthalein are indicators • Red cabbage has a pigment that changes colors at different pH values

  6. Buffers • Buffers are solutions that don’t change in pH when acids or bases are added. • They use weak acids/bases and Le Châtelier’s principle. • WA = weak acid • HWA + H2O  H3O+ + WA-

  7. How? • pH is determined by the concentration of H3O+ • Concentration is measured by mol /L • Moles of H3O+ / L (primarily of) H2O

  8. What it does • adding H3O+ forces the equation to the left • so the [H3O+] remains constant • removing H3O+ (adding OH-) forces the equation to the right • so the [H3O+] remains constant • There is a breaking point where the pH will change.

  9. What does this have to do with my life? • Your blood is a buffered solution • The pH must remain between 7.35-7.45 • Outside of that range can kill you • below this range is called acidosis • above is called alkalosis

  10. homework • Read section 10.5 • Pg 354-358 • Questions 26-30; 97-100

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