TITRATION NOTES I. Titration and Curves - Titration: lab technique in which one solution is used to analyze another (analyte/titrant) - point: point in a titration where just enough standard solution has been added to react completely with all the unknown solution (commonly acids and bases) o moles of acid = moles of base (neither is in excess) - point: defined by the change in color of the indicator; the end point should be as close to the equivalence point as possible - All acid-base reactions are neutralization reactions and go to - The titration equation works for any acid-base combo (strong, weak) when provided with and asking for concentrations and volumes (not ) #H + M a V a formula = M #OH - bv b formula II. Molarity and Units Reminder - ml M = mmol - mmol ml = Molarity III. Strong Acid Strong Base Titrations 1) NI Equation: 2) Produces a: 3) at equivalence point: Controlled by: Volume of NaOH added 4) Particulate views A (Initial) B (Before eq. pt.) C (At eq. pt.) D (After eq. pt.) Practice #1: Calculate the after 29.50 ml of 0.2500 M HCl have been added to 50.00 ml of 0.1500 M NaOH.
IV. Weak Acid Strong Base Titration 1) NI Equation: 2) Produces a: 3) at equivalence point: Controlled by: Volume of NaOH added 4) Particulate views A (Initial) B (Before eq. pt.) C (At eq. pt.) D (After eq. pt.) Note: The weaker the acid, the greater the at the equivalence point (due to the production of a stronger conjugate base). Practice #2: A 25.0 ml sample of 0.400 M HC2H3O2 is titrated with 25.0 ml of 0.200 M NaOH. Find the at this point in the titration. (acetic acid Ka = 1.8 x 10-5 ) Practice #3: A 25.0 ml sample of 0.400 M HC2H3O2 is titrated with 50.0 ml of 0.200 M NaOH. Find the at this point in the titration. (acetic acid Ka = 1.8 x 10-5 )
V. Weak Base Strong Acid Titration 1) NI Equation: 2) Produces a: 3) at equivalence point: Controlled by: Volume of HCl added 4) Particulate views A (Initial) B (Before eq. pt.) C (At eq. pt.) D (After eq. pt.) Practice #4: A 30.0 ml sample of 0.100 M NH3 is titrated with 5.0 ml of 0.200 M HBr. Find the of the solution at this point in the titration. (ammonia Kb = 1.8 x 10-5 ) Practice #5: A 30.0 ml sample of 0.100 M NH3 is titrated with 0.200 M HBr. Find the of the solution at the equivalence point. (ammonia Kb = 1.8 x 10-5 ) (5.22)
Practice #6: A 30.0 ml sample of 0.100 M NH3 is titrated with 20.0 ml of 0.200 M HBr. Find the of the solution at this point in the titration. (ammonia Kb = 1.8 x 10-5 ) VI. Weak Acid Weak Base Titration at equivalence point: Depends on: Volume of HCl added VII. Titrations of Polyprotic Acids - Diprotic Acid - Triprotic Acid Volume of base added Volume of base added
VIII. Acid-Base Indicators - These mark the end point of a titration by changing color. - Although the equivalence point, defined by the stoichiometry, is not necessarily the same as the endpoint, careful selection of an indicator will ensure the error is negligible. - The most common indicators are complex (HIn). (In = indicator) - They are one color when the proton is attached and a different color when the proton is removed. - Example: phenolphthalein HIn H + + In - K a = [H+ ][In - ] [HIn] - Adding an acid will increase the [H + ] and cause the indicator equilibrium to shift - Adding a base will remove H + via a reaction with OH and cause the indicator equilibrium to shirt - The color change range for an indicator is ±pka - Example: Phenolphthalein s Ka is 1.0 x 10-9, so the pale pink color is seen over the range of. We would choose this indicator for an expected equivalence point of 8-9 when titrating an acid with a base (approaching the equivalence point from the side of the curve) - Example: Methyl orange s Ka is 1.0 x 10-4, so the pale pink color is seen over the range of 3-5. We would choose this indicator for an expected equivalence point of when titrating a base with an acid (approaching the equivalence point from the side of the curve) - More general selection: o Strong acid strong base: choose an indicator near 7 o Weak acid strong base: choose an indicator whose color change should be in the basic region o Strong acid weak base: choose an indicator whose color change should be in the acidic region Indicator Acid Transition Base Litmus Red Purple, violet Blue Phenolphthalein WA/SB pka = 9 Methyl Orange WB/SA pka = 4 Bromothymol Blue SA/SB pka = 7 Colorless Red < 3 Yellow < 6 Pale pink 8-10 Orange 3-5 Green 6-8 Bright pink Yellow > 5 Blue > 8 Practice #5: An indicator HIn (Ka = 1.0 x 10-6 ) where HIn is red and In - is blue, is placed in a solution of strong acid. The solution is then titrated with NaOH. At what will a color change occur? Practice #6: Two drops of an indicator, HIn (Ka = 1.0 x 10-8 ) where HIn is yellow and In - is blue, are placed in 100. ml of 0.30 M HCl. a) What color is the original solution? b) This solution is titrated with 0.20 M NaOH. At what will the color change be visible? c) What color will the solution be after 300. ml of NaOH has been added?
TITRATION CALCULATION FLOWCHART STOICHIOMETRY Write neutralization reaction BCA table using mmol Must determine limiting reactant Chose one of the three options below to calculate Only HA/A - or B/BH + remains Only A - or BH + remains Only H + /BH + or OH - /A - remains BUFFER: Henderson-Hasselbach STRONG C.A. or C.B.: Write equilibrium equation with RICE table MIXTURE: Use [H + ] or [OH - ] only Recalculate concentrations using total volume Use K a or K b expression for A - or BH + to find [H + ] or [OH - ]