1 acid content of beverages a titration exercise susb - 010 2

Acid Content of Beverages A titration exercise SUSB - 010 2

characterize theacidity How can we characterize the acidity of a natural mixture of unknown acids? strongweak What do chemists mean when they talk about acids being strong or weak? ? QUESTIONS ? quantitativemeasure What is a quantitative measure of the strength of an acid? titrations How do we conduct titrations and what can we learn from them? 3

Concepts: Strong/Weak Acids Acid Dissociation / K a Concentration Titration Titration curve Equivalence point End point Indicator Mole Relationships Polyprotic acids Total available acid pH & pK a Logarithms Techniques: Apparatus: BuretpH Meter Graduated Cylinder Titration 4 pH Measurement

MOLES, LITERS & CONCENTRATION UNITS mL mgsmall numbers of grams. 1 kg1 mole1 Liter In lab, we generally measure volumes in mL & weights in mg or small numbers of grams. We dont normally use even close to 1 kg, 1 mole or 1 Liter of anything. 0 To avoid writing quantities with zer0s after decimal point, procedures and data sheets often specify: volumes in m mL (= 1 / 1000 liter) and molar quantitites in m mmol (= 1 / 1000 mol) & weights in m mg (= 1 / 1000 g) 0.03421 L 00 0.002845 mol m 34.21 mL m 2.845 mmol 0 0.0757 g m 75.7 mg 5

E.g., 6 M HCl has6 mmol of HCl in 1 mL, also 1.0 m mol of NaOH weighs 40 m g Similarly, for atomic and molecular weights: atomic weight of carbon is 12 m g / m mol molar mass of vanillin is 152 mg/mmol same numerical value For molar concentrations, M (molarity) has same numerical value in mol / L and in mmol / mL mm HCl 6.0 M 6 mol mol 1000 mmol Y = Y = Y L L mL 1000

Background Strong and Weak Acids (and bases) Acids and bases can be characterized by the extent to which they dissociate in solution STRONG Fully Dissociated E.g., in water, HCl dissolves to give a STRONG acid HCl (aq) H + (aq) + Cl - (aq) WEAK Partially Dissociated 7 HCl (g) + H 2 O (l) proceeds virtually to completion (no undissociated HCl )

Acetic Acid ( CH 3 COOH), dissolves in water to give a WEAK acid CH 3 COOH (aq) CH 3 COO - (aq) + H + (aq) Almost all CH 3 COOH exists in ASSOCIATED form Associated proceeds only slightly in forward direction. Dissociated 8 group defines an organic acid The

Analogously, NaOH dissolves in water to give a STRONG base: NaOH (aq) Na + (aq) + OH - (aq) 9 NaOH (s) + H 2 O (l) proceeds virtually to completion (no undissociated NaOH ), dissolves in water to give a WEAK base, NH 3 (aq) NH 4 + (aq) + OH - (aq) NH 3 (g) + H 2 O (l) whereas, ammonia (NH 3 ) proceeds only slightly in forward direction.

A QUANTITATIVE measure of strength or weakness of an acid ( or base ) is its DISSOCIATION CONSTANT, K a For reaction HA H + + A - acid dissociation constant, K a, is defined as [ H + ] [ A - ] K a = ------------- [ HA ] LARGE K a ( >> 1) STRONG ACID SMALL K a ( 9 pH < 9 38">

PHENOLPTHALEIN Use PHENOLPTHALEIN, which is itself, a weak DI-PROTIC ACID. ColorlessPink PHENOLPHTHALEIN Color change for PHENOLPHTHALEIN occurs when pH increases from 9. pH > 9 pH < 9 38

Before end point At end point Past end point 39 If you bring in your own beverage, remember that you will need to be able to tell when the color changes from its original color to pink.

1.) Determine TOTAL AMOUNT OF ACID in measured sample of beverage.HOW? by titrating the sample with NaOH of known concentration. NaOH reacts with ALL OF THE ACID*, dissociated, or not. Will not detect any acids with pK a s > 9 (pH at which phenolphthalein signals end point) We make two measurements: This tells us TOTAL DISSOCIATED + ASSOCIATED ACID in the beverage. 40

2. Measure pH of beverage to determine H + concentration of beverage using a pH METER This tells us CONCENTRATION OF DISSOCIATED H + in beverage, [ H + ] 41 Electronic device designed to measure hydrogen ion concentration in aqueous solutions

PROCEDURE Beverages you bring in vary widely in acid content. Cannot predict how much of it to use. In such instances, we always do a: To get reasonable precision from buret readings, always try to use net volumes between 20 mL 30 mL in a titration. PRELIMINARY TITRATION Objective: To determine how much of the beverage we need to use to consume the desired amount (20 30 mL) of NaOH. 42

If 30.0 mL requires 15.45 mL of base X39 X = 20 X 30.0 / 15.45 = 39 3958 So, if we use between 39 and 58 mL of our beverage, we will assure that the amount of NaOH required will be in the correct range (20-30 mL) X 30.0 X -------- = ----- 15.45 20 Y 30.0 Y -------- = ----- 15.45 30 X X mL requires 20 mL of base Y Y mL will require 30 mL Y58 Y = 30 X 30.0 / 15.45 = 58 43

Example: 20.0 mL of a beverage requires 10.0 mL of NaOH to reach the phenolphthalein end point. How much beverage will use 30.0 mL of the NaOH solution? 44 A.20 mL B.30 mL C.60 mL

Example: How much beverage 20.0 mL of a beverage requires 10.0 mL of NaOH to reach the phenolphthalein end point. How much beverage will use 30.0 mL of the NaOH solution? C C60 mL 45 X mL beverage X mL beverage 20.0 mL beverage = 30.0 mL NaOH 10.0 mL NaOH 20.0 mL beverage X mL beverage X mL beverage = 30.0 mL NaOH 10.0 mL NaOH

one Having determined how much juice you need to conduct one titration, you can compute how much beverage you will need to complete the exercise. You are asked to do no more than 4 titrations (in addition to the preliminary titration) and report the best 3 out of those 4. So, if x mL is required to do one titration, you need, at most, (4x + 20) mL of beverage. If you do not have enough beverage to complete exercise, use juice provided in laboratory. (But you must do another preliminary titration.) Based on the preliminary titration, Calculate this quantity Based on the preliminary titration, Calculate this quantity. If you brought sufficient beverage, continue using your beverage. 46

For simplicity, assume beverage contains single weak monoprotic acid HA H + + A - with acid dissociation constant K a For this, we need pH of untitrated (and undiluted) beverage. I.e., [ H + ] = 10 -2.7 = 2.0 X 10 -3 All that remains it to calculate (effective) K a for acid(s) in beverage. [ H + ] [ A - ] K a = --------- [ HA ] ( 2.0 X 10 -3 ) 2 = ----------------- 0.124 = 3.2 X 10 -5 Suppose result of titration is Total Acid = 0.124 M & measured pH = 2.7 47 From pH Total Acid [ H + ] = [ A - ] Should subtract 2.0 X 10 -3 But to 2 sig figs, can ignore

REMEMBER: Second QUIZ will be given at the beginning of the acid in beverages lab period. (15 minutes) It will cover: SUSB-030,SUSB-004,SUSB-039 48

Stamp out Chemophobia U CHEMISTRY TELL EM This message is approved by the USB Department of Chemistry

NEXT EXERCISE Strength of Vinegar by Acid Base Titration Read SUSB-011 and Do Prelab A TEST EXERCISE 105 Points 51

1 acid content of beverages a titration exercise susb - 010 2

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