Chem 144 Experiment 5 Part 1 and 2: The Quantitative Determination of an Acid in Carbonated Beverages

Safety

-Make sure to wear goggles ALWAYS -Do not consume any sodas -Pour all solutions down the drain and flush them down with some tap water

Monoprotic Acid

-Only one proton available to react with a base -Always react with bases with a 1/1 stoichiometry -HCl is an example: HCl + NaOH --> H2O + NaCl

Equivalence Point

-The point where the # of moles of base added (the titrant) is stoichiometrically equal to the # of moles of acid originally present in the analyte

Objectives

-To understand the difference between polyprotic and monoprotic afcids -To learn the basics of the pH scale -To learn how to perform an acid-base titration -To compare and contrast two methods (traditional indicator dye vs. modern pH electrode) for detection of the estimated equivalence point of an acid-base titration -To calculate the molarity (mol/L) of citric acid in two sodas

End Point

-Use an indicator (phenolphthalein) to distinguish color change and estimate the equivalence point -Color change signals that the amount of titrant added is equal to the analyte in a sample -*For citric acid there will be one observable end point b/c all 3 protons of the acid have similar strengths* -Reached when enough base has been added to neutralize all 3 of the acidic protons -Will be overestimate the equivalence pt slightly

Procedure Part 2: Modern Titration using a pH electrode - Setting up the Vernier LabQuest Device and Calibrating the pH Electrode

*Make sure to work in a group of 4* 1) Turn on LabQuest; carefully remove storage solution bottle from pH electrode and ensure that the bulb at the tip of pH electrode is intact to then plug in electrode to LabQuest 2) Calibrate the pH electrode w/ known pH solution by clicking the Sensors drop-down menu on LabQuest then clicking "calibrate now" (make sure to check "one-point calibration") --> place probe into solution of known pH --> enter known calibration value (7.0) and keep value once reading has been stabilized 3) Use utility clamp to suspend pH electrode on ring stand

Procedure Part 1: Traditional Titration using an Indicator Dye - Cleaning the Buret

*Make sure to work in a pair* 1) Set up sing stand with buret clamp and place buret in the clamp 2) Place large beaker ("waste") under the tip of the buret and a clean plastic funnel at the opening of the buret --> use CLEAN beaker to fill the buret with distilled water and drain it completely and repeat 3x to rinse inside of buret carefully --> discard rinse water into "waste" beaker --> make sure stopcock does not leak 3) Use a clean beaker (150mL) to collect about 100mL of NaOH from the supply bench 4) Use clean and dry plastic funnel to add 5 to 10 mL of ~0.05 M NaOH solution and carefully rotate the buret to wet the walls completely then allow the liquid to drain out of the tip into your "waste" beaker thereby rinsing the inside of the buret completely with the titrant solution 5) Repeated this procedure TWO TIMES MORE to ensure the walls are well rinsed

Titration that will be used in part 2

*Modern Titration* -Monitors pH directly with an electrode -Will work in highly colored solutions b/c does not require observation of a visual end pt -Utilizes magnetic stir plate to ensure thorough mixing of the reactants -A pH electrode monitors the pH of the solutions as the titrant (NaOH) is added from the buret into known volume of acid solution -Will use Vernier LabQuest device to record data -Estimate of the equivalence pt can be detected as the pt of most rapid pH change as a fxn of titrant volume in a plot of pH (y-axis) vs. titrant volume (x-axis) -Known volume of acid solutions and volume of standard NaOH solution make it possible to calculate the concentration of citric acid in sample

Titration that will be used in part 1

*Traditional Titration* -Must be possible to see the color change from colorless to light pink at the end pt of titration -Begin w/ measuring volume from scale to initiate the titration -NaOH solution will slowly be added to flask from buret --> as it approaches end pt and basic solution is added drop-by-drop, temporary faint pink color may become evident in the Erlenmeyer flask (color disappears as flask is swirled by hand) BUT permanent forms when pH of solution has reached pt at which dye begins to change color -*End point will concide closely w/ equivalence pt* -Volume reading of NaOH solution measured @ end pt will provide volume of standard NaOH solution necessary to titrate the acid in the flask (once corrected by subtraction of initial volume reading of the buret) --> need this to calculate the concentration of citric acid in the sample

Introduction

-Citric acid = C6H8O7 is common in carbonated beverages --> present in much higher concentrations than other acids (except carbonic acid) -Eq of how soda goes flat (aka remove carbonic acid): H2CO3 --> H2O + CO2 -Once carbonic acid is removed, the amount of citric acid can be quantitatively determined by an acid-base titration b/c the large differences in the concentrations

pH

-Equation: pH = -log[H+] -Measuring the pH of the solution in which the titration occurs will determine the end point of the titration -Can be measured directly w/ pH electrode that's sensitive to H+ or indirectly by the use of an indicator dye that changes color at the desired pH -End pt will occur at pH = 8.5

Polyprotic Acid

-Has multiple acidic protons that can react w/ base -Citric = triprotic acid aka 3 moles of base are needed to completely neutralize one mole of citric acid --> all 3 acidic protons = similar strength so they ALL react w/ base to similar extent -Ex: H3C6H5O7 + 3OH --> 3H2O + C6H5O7

Titration

-Use titrant aka one solution of known concentration --> in this lab, NaOH is the titrant -This titrant determines concentration of another solution aka analyte --> in this lab, C6H8O7 is the analyte -We are worried about acid-base titration in this lab

Procedure Part 1: Traditional Titration using an Indicator Dye - Initial Rapid Titration

1) After recording initial buret volume reading, place buret tip below rim of Erlenmeyer flask and start adding NaOH solution to flask in approx 2mL increments while swirling --> indicator dye will initially give some pink hue to solution upon swirling 2) Stop titration when end point = reached aka pink color is persistent 3) Record final volume reading of the buret and use this to determine titrant volume at end point (this final volume reading - initial volume reading)

Sources of Error Associated with Use of Buret

1) Air bubbles may be trapped in the stopcock or buret tip --> will result in an error in volume dispensed 2) Not reading the buret volume (the meniscus) at eye level will result in parallax error 3) Over or underestimating the end point color will also result in an error in the volume dispensed

Procedure Part 1: Traditional Titration using an Indicator Dye - Molarity (mol/L) of Citric Acid

1) Calculate volume of NaOH used to reach end point (Volume of NaOH = Vf - Vi) for each trial 2) From V NaOH and molarity of NaOH recorded, calculate the moles of NaOH for each trial 3) Use stoichiometry of rxn of citric acid w/ NaOH to calculate moles of citric acid in each trial 4) From volume of soda used (V soda), calculate the molarity of citric acid in each trial 5) Calculate the mean and standard deviation of the molarity of citric acid for your four accurate trials

Procedure Part 2: Modern Titration using a pH electrode - Molarity (mol/L) of Citric Acid

1) First step finds volume of NaOH used to reach estimated equivalence pt for each of the 3 accurate trials (this is NOT the total volume of NaOH used in each titration) -In Data menu, press New Calculated Column; under Expression, click on Fxns button and choose Calculus then Derivative -Type "Y","X" into expression box -For 1st derivative to appear on plate, right click graph then go to Axes options to check box for calculated column and click done (MAKE SURE AXES ARE LABELED) -From graph, note the peak of 1st derivative curve corresponds to volume of NaOH at estimated equivalence pt and record this volume -Repeat this procedure for 2 other trials and record volume of NaOH and pH @ estimated equivalence pt for each trial 2) From V NaOH and molarity of NaOH, calculate moles of NaOH for each trial 3) Using stoichiometry of rxn of citric acid w/ NaOH, calculate moles of citric acid in each trial 4) From volume of soda used, calculate the molarity of citric acid in each trial 5) Calculate the mean and standard deviation of the molarity of citric acid for each of the 3 trials

Procedure Part 2: Modern Titration using a pH electrode - Initial Preparations

1) Repeat cleaning process for buret and prep for 1st soda samplelike last week --> DIFFERENCE FROM LAST WEEK: do not add indicator dye and pour sample into 250mL beaker, not an Erlenmeyer flask 2) Don't forget to record actual volume of soda for EACH trial 3) Place beaker w/ soda sample on magnetic stir plate (close to center of plate) and add clean magnetic stir bar 4) Ensure pH electrode is about 3cm from bottom of beaker and clear of the stir bar 5) Fill buret and adjust NaOH solution level to 0.00mL in buret then record NaOH concentration to 3 sig figs --> position buret using clamp directly above beaker and make sure you are able to access stopcock of buret/read volumes on side of the buret

Procedure Part 1: Traditional Titration using an Indicator Dye - Accurate Titrations

1) Subtract 4mL from titrant volume recorded in previous step 2) Prep another trial by refilling buret and prepping soda solution --> remember to keep soda w/in 5% of recommended volume and record the actual soda volume 3) Record initial buret reading and rapidly add the estimated volume of NaOH calculated in step 1 while swirling the flask (about 5mL at a time) 4) Continue titration by adding NaOH (drop-wise) until pink = persistant after swirling then record final buret reading 5) Repeat this procedure for accurate titrations 3x for a total of 4 accurate trials 6) Calculate the ratio of titrant volume (NaOH) to soda volume for each of your four accurate titration trials (Volume of NaOH / Volume of Soda) and perform Q-test @ 95% confidence to ensure all values are not sus

Procedure Part 2: Modern Titration using a pH electrode - Accurate Titrations

1) Turn on stir plate and adjust it to moderate speed before starting titration 2) Start titration by using stylus to press Play button then press keep --> enter initial buret volume to 2 decimal places (should be 0.00mL) then record initial volume/pH 3) Add NaOH solution to stirred beaker until pH reaches 6.0 and stop once pH is stabilized on program then enter volume of NaOH solution from buret--> now add NaOH drop-wise manner until change of 0.1-0.2 pH units = observed then enter volume of NaOH solution from buret --> repeat until 10.5 reached then press stop 4) Remove pH probe from utility clamp and place it into 400mL beaker filled w/ 100mL of distilled water --> dispose of titrated soda sample as directed and rinse beaker w/ distilled water for subsequent trials 5) Repeat this titration procedure 2x more for total of 3 trials 6) Loosen pH electrode from LabQuest device to rinse w/ distilled water then return to storage bottle

Procedure Part 1: Traditional Titration using an Indicator Dye - Preparing the Soda for Titration

1) Use 100-mL graduated cylinder to measure the required volume of decarbonated soda (found in table 2-1) --> record ACTUAL volume of soda according to scale on graduated cylinder then add distilled water to the soda for total volume of 100mL --> pour solution into 250mL Erlenmeyer flask 2) Rinse remaining soda solution in graduated cylinder into the Erlenmeyer flask w/ 2 washes of 5mL each of distilled water 3) Add 5 drops of phenolphtalein indicator dye to flask w/ soda solution and gently swirl to mix

Procedure Part 1: Traditional Titration using an Indicator Dye - Filling the Buret

1) W/ closed stopcock, use clean/dry funnel to fill buret w/ NaOH above the 0.00mL mark --> record the actua concentration of NaOH solution TO 3 SIG FIGS 2) Remove funnel and ensure waste beaker is still under buret 3) Free tip of buret of air bubbles by quickly opening stopcock and allowing small amount of solution to flow thru the tip 4) Adjust level of solution to (or a lil below) 0.00 mL mark --> touch tip of buret to the beaker to remove any hanging drops 5) See if buret leaks w/ stopcock closed and record initial volume of solution in buret (should be btwn 0.00-2.00mL) by looking @ meniscus @ eye level 6) Remove waste beaker and replace w/ Erlenmeyer flask w/ acidic soda solution in it to be titrated --> lower buret to place buret tip inside the Erlenmeyer flask before opening stopcock 7) After each titration, refill the buret w/ NaOH using clean/dry funnel until the solution level = btwn 0.00-2.00mL marks (remember to remove funnel after buret = filled and before beginning titration!