chapter 5

1. Chapter 5 Applications of Titrimetric Analysis

3. Acid-Base Titrations and Titration Curves Acid-Base Titrations an Titration Curves Monoprotic vs. Polyprotic Monoprotic Acid with only one H+ (or proton) to donate per formula HCL-strong acid CH2COOH-Acetic Acid-weak acid Polyprotic Two or more H+ to donate per formula H2SO4-sufuric acid H3PO4-phosphoric acid

4. Acid-Base Titrations and Titration Curves Acid-Base Titrations an Titration Curves Monobasic vs. Polybasic Monobasic Accepts just one H+ per formula NaOH-strong base NH4OH-ammonium hydroxide-a weak base NaHCO3-sodium bicarbonate-a weak base Polybasic Will accept two or more H+ per formula Na2CO3-sodium carbonate Na3PO4-sodium phosphate

5. Titration of Hydrochloric Acid With NaOH as titrant-HCl as substance titrated H+ + OH- ? H2O pH increases as titration proceeds # of H+ decreases Lower the [H+], the higher the pH

6. Titration of Hydrochloric Acid Graph (fig. 5.1, pg. 100) Titration curve Sharp increase in the pH occurs at the equivalence point Point of neutralization The point at which the slope of the titration curve is a maximum = inflection point Verifiable results Each point can be verified by calculating [H+] and pH after each addition Plot the results Will show steady increase in pH over wide range of ml/NaOH added

7. Titration of Hydrochloric Acid Graph (fig. 5.1, pg. 100) Titration curve-different concentrations Lower the concentration of acid, fewer H+ ions present Higher the initial pH The higher the pH level of the initial steady increase Lower the concentration of base, lower the level of the pH after the inflection point (5.1b)

8. Titration of Weak Monoprotic Acids Titration of a weak acid with a strong base Weak acid (5.1c) Higher starting pH Equivalence point reached at higher pH level Similar to lower concentration of HCl Weak acid = fewer H+ ions

9. Titration of Monobasic Strong and Weak Bases Acid = titrant; base = substance titrated Strong base titrated with strong acid pH starts high an ends at a low pH Opposite observed when titrating an acid with a base (fig. 5.3a) Weak bases titrated with strong acid (ammonium hydroxide titrated with HCl) Starts at lower pH than strong base Similar to weak acids start out at higher pH (fig. 5.3b & 5.4)

10. Equivalence Point Detection How is the equivalence point found and what is the relationship with titration curves Inflection point at center of curves is at the equivalence point Point at which all of the substance titrated has been exactly consumed by the titrant Strong acid titrated w/strong base & vice versa pH 7.00 = middle of the inflection point Weak acid with strong base Occurs at pH higher than 7 Weaker the acid, the higher the pH value that corresponds to the equivalence point Equivalence point for weak bases Progressively lower pH vs. strength of the base (concentration)

11. Equivalence Point Detection Problems for the analyst Choosing indicators Must change color close enough to an equivalence point Accuracy not diminished Fortunately there are a large number of indicators available Exception are the extremely weak acids and bases (fig. 5.5)

12. Titration of Polyprotic Acids: Sulfuric Acid and Phosphoric Acid Polyprotic acids Have inflection point for each H+ H2SO4 and H3PO4 (fig. 5.6 & 5.7) H2SO4 Essentially one inflection point Both hydrogens strongly acidic Dissociation in water nearly complete Neutralized at the same time Hence only one inflection point H3PO4 Hydrogens very different from each other Expect neutralization one at a time 3rd dissociation very small Just two inflection points

13. Titration of Potassium Biphthalate Potassium Biphthalate or potassium hydrogen phthalate or KHP Used in standardizing a base solution It�s the salt representing partially neutralized phthalic acid Monoprotic weak acid KHC8H4O4 + OH- ? K2C8H4O4 + H2O High FW Stable at oven drying temps Titration curve similar to acetic acid Fig. 5.9 Compared to acetic acid in fig. 5.1c

14. Titration of Potassium Biphthalate

15. Alkalinity Important application of acid-base titrations Treatment of municipal water and wastewater Alkalinity of a water sample Its acid-neutralizing capacity Titrate the water sample with standard acid until a particular pH is achieved Alkalinity value depends on the pH used for the end point

16. Alkalinity Total alkalinity Titrating the sample usually to a pH of 4.5 Neutralizes ALL hydroxides, carbonates, bicarbonates, and other bases This pH considered to be the equivalence point Expression of alkalinity Millimoles of H+ required to titrate 1L of water Multiply molarity of the acid (millimoles per milliliter) by buret reading in milliliters Example 5.1, pg 105

17. Buffering Effects and Buffer Solutions Titration curves help us to understand buffering and buffer solutions Buffer solutions Solution that resists changes in pH Even with addition of strong acids or bases Dilution with water Typical composition Weak acid or base and its conjugate base or acid Conjugate base Product of an acid neutralization Gains back a H lost during neutralization Conjugate acid Product of a base neutralization that can lose the H that it gained during the neutralization

18. Buffering Effects and Buffer Solutions Buffer solutions Typical composition Conjugate acid-base pair Acid and its conjugate base - -Conjugate acid-ammonium ion formed by neutralization of ammonia with an acid Base and its conjugate acid - -conjugate base-acetate ion formed during the neutralization of acetic acid with hydroxide -Acetate is conjugate base -Gains a H and becomes acetic acid again -acetic acid and acetate ion = acid-base pair

19. Buffering Effects and Buffer Solutions Buffer solutions For weak acid or weak base neutralization titration Conjugate acid-base pair exists in the flask leading to the inflection point Acetic acid with sodium hydroxide Acetic acid and acetate ion exists in reaction flask prior to inflection point In titration curve, pH of the solution does not change appreciably even with addition of NaOH

20. Buffering Effects and Buffer Solutions Buffer capacity If conjugate acid-base pair is high Buffer solution become very effective Portion of curve leading to the inflection point is extended Extends the buffer capacity Ability to resist neutralization These mixtures require even more strong acid/base to reach neutral Region leading up to the inflection point often called the buffer region

21. Buffering Effects and Buffer Solutions Buffer solutions Prepared by appropriate combination of conjugate acid-base pairs Commercially prepared buffer solutions available Mostly used for pH meter calibration Not for adjusting/maintaining chemical reaction systems at a given pH Often need to prepare solution for this purpose Question of what proportions to mix for the desired pH

22. Complex Ion Formation Reactions Introduction Neutralization reactions only one type of titrimetric analysis Formation of a precipitate Transfer of electrons These involve complex ion formation Involves transition metals Often used for qualitative and quantitative colorimetric analysis Complex ion that forms analyzed according to the depth of a color

23. Complex Ion Formation Reactions Terminology Complex ion Polyatomic charged aggregate Positively charged metal ion with either neutral molecule or negative ion These called the ligand Monatomic, negative ion: F-, Cl-, etc. Polyatomic molecule or ion: H2O, CN-, NH3 Classification of ligands Number of bonding sites available for coordinate covalent bonds with metal ions One in which the shared electrons are contributed to the bond by only one of the two atoms involved With complex ion-forming reactions, both electrons donated by the ligand

24. Complex Ion Formation Reactions Terminology Complex ion Classification of ligands, cont�. One pair of available electrons Monodentate Two pair Bidentate Chelating agents Involve ligands with two or more bonding sites Used for removing or masking metal ions in a solution Dissolved ions interfere with analysis