Graphical Analysis of Titration - Analytical Chemistry - Lab 6 | CHEM 321 | Lab Reports Analytical Chemistry

Chem 321L

Experiment 6

EXPERIMENT 6

IDENTIFICATION OF A DIPROTIC ACID VIA TITRATION AND GRAPHICAL

ANALYSIS OF TITRATION CURVES USING MICROSOFT EXCEL

A. Prelab

• Read Skoog et al. sections 14C (pp. 37 8-381) and 15E (pp. 407-416) for background

information.

• You have one week to complete the experimental portion of the lab.

B. Experimental Overview

This week you will perform a titration of an unknown solid diprotic acid (mw~100-200 g/mol) w ith your

standardized 0.1 M NaOH solution . The reaction sequence proceeds as follows:

H2A + OH-  HA- + H2O

HA- + OH-  A2- + H2O

You will use a pH meter to measure solution pH and to accurately map out the titration curve of your

unknown acid with a strong base. You will then determine the position o f the e quivalence points of the

reaction graphically using the titration curve, first derivative titration curve, and a second derivative

titration curve. You will determine pKa value(s) and calculate the molar mass of your unknown and use

this information to identify your unknown based on comparison with literature values. You will work in

groups of three and share data.

C. Procedure

• Calibrate the pH meter according to the pH meter manual. You will calibrate your pH meter

with at least 3 pH buffers. The meter will automatically perform a linear regression and

calibrate the meter for you.

• Prepare your notebook. You will record pH vs. mL base added as you titrate.

• Weigh ~0.12 g of your unknown to the nearest 0.0001 g and dissolve in 25 mL of H2O in a

150 mL beaker for titration. Heat to assist dissolu tion if necessary. Allow the solution to cool

prior titration.

• Set up your titration apparatus with the beaker on a magnetic stirrer and add a stirring bar.

Secure you pH probe with a clamp so that is immersed in solution without touching the

bottom of the beaker.

• Condition and fill a 50 mL buret with your standardized 0.1 M NaOH solution. Record the

concentration of your group’s s tandard in your notebook. Adjust your initial volume reading to

precisely 0.00 mL.

• Titrate your solu tion with NaOH in <0.2-mL (2-4 drop) increments. Manually p lot your titration

curve on graph paper to help monitor your progress. Collect more data (dropwise) as you

approach and titrate through each equivalence point. Continue measuremen ts until you have

titrated at least 30 .0 mL of base..

Data Analysis:

• Use a spreadshee t to create the following graphs.

• Plot your data as pH vs. volume of base added (mL). This is a titration curve.

Partial preview of the text

Download Graphical Analysis of Titration - Analytical Chemistry - Lab 6 | CHEM 321 and more Lab Reports Analytical Chemistry in PDF only on Docsity!

Experiment 6

EXPERIMENT 6

IDENTIFICATION OF A DIPROTIC ACID VIA TITRATION AND GRAPHICAL

ANALYSIS OF TITRATION CURVES USING MICROSOFT EXCEL

A. Prelab

Read Skoog et al. sections 14C (pp. 378-381) and 15E (pp. 407 - 416) for background information.
You have one week to complete the experimental portion of the lab.

B. Experimental Overview

This week you will perform a titration of an unknown solid diprotic acid (mw~100-200 g/mol) with your standardized 0.1 M NaOH solution. The reaction sequence proceeds as follows: H 2 A + OH-^  HA-^ + H 2 O HA-^ + OH-^  A^2 -^ + H 2 O You will use a pH meter to measure solution pH and to accurately map out the titration curve of your unknown acid with a strong base. You will then determine the position of the equivalence points of the reaction graphically using the titration curve, first derivative titration curve, and a second derivative titration curve. You will determine pKa value(s) and calculate the molar mass of your unknown and use this information to identify your unknown based on comparison with literature values. You will work in groups of three and share data.

C. Procedure

Calibrate the pH meter according to the pH meter manual. You will calibrate your pH meter with at least 3 pH buffers. The meter will automatically perform a linear regression and calibrate the meter for you.
Prepare your notebook. You will record pH vs. mL base added as you titrate.
Weigh ~0.12 g of your unknown to the nearest 0.0001 g and dissolve in 25 mL of H 2 O in a 150 mL beaker for titration. Heat to assist dissolution if necessary. Allow the solution to cool prior titration.
Set up your titration apparatus with the beaker on a magnetic stirrer and add a stirring bar. Secure you pH probe with a clamp so that is immersed in solution without touching the bottom of the beaker.
Condition and fill a 50 mL buret with your standardized 0.1 M NaOH solution. Record the concentration of your group’s standard in your notebook. Adjust your initial volume reading to precisely 0.00 mL.
Titrate your solution with NaOH in <0.2-mL (2-4 drop) increments. Manually plot your titration curve on graph paper to help monitor your progress. Collect more data (dropwise) as you approach and titrate through each equivalence point. Continue measurements until you have titrated at least 30.0 mL of base.. Data Analysis:
Use a spreadsheet to create the following graphs.
Plot your data as pH vs. volume of base added (mL). This is a titration curve.

Experiment 6

Plot the first derivative of pH vs. Volume of base added (mL). This is a first derivative titration curve.
Plot the second derivative of pH vs. volume of base added (mL). This is a second derivative titration curve.
Identify the equivalence points from the titration plot that provides the least ambiguous result.
Compile a table of equivalence points and half-equivalence points as determined from each of your three plots. Use the half-equivalence points from the least ambiguous plot (typically the first or second derivative plot) to determine the pKas of your unknown from your titration curve data. Identify and label the pKas on your titration plot. Calculations
Show an example calculation for one row of your spreadsheet. Show what Excel equation was used.
Calculate your Kas for each pKa as determined experimentally from your titration plots. You can find the first pKa by finding the pH of the solution half-way to the first equivalence point. Additional pKas are found half-way between higher equivalence points. Calculate Kas for your unknown from these pKas. Note that your pKa values have an uncertainty of approximately ±0.5 units or more. Additionally, not all equivalence points result in a well-defined inflection point. This occurs if your unknown’s Kas are not separated by a factor approximately >10^3.
Calculate your unknown’s pKas based on the literature value Kas.
Calculate the number of moles of NaOH titrated at each visible equivalence point.
Calculate the molar mass of your unknown for each visible equivalence point. The first equivalence point is typically easier to detect. However, if the second equivalence point is more clearly defined on the titration curve you can identify the theoretical position of your first equivalence point based on the position of the second. If your molar mass calculation does not yield a value between 100-200 g/mol consider whether you have correctly identified the positions of your two equivalence points. If both equivalence points are visible calculate a mean molar mass from both points. Acid Formula Molar Mass Kas Malonic Acid HOOCCH 2 COOH 104.1 1.42x10-^3 , 2.01x10-^6 Maleic Acid HOOCCH:CHCOOH 116.1 1.3x10-^2 , 5.9x10-^7 Oxalic Acid HOOCOOH • 2H 2 O 126.1 5.06x10-^2 , 5.42x10-^5 Sodium Dihydrogen Phosphate NaH 2 PO 4 •H 2 O 138.0 6.32x10-^8 , 4.5x10-^13 Succinic Acid HOOCCH 2 CH 2 COOH 118.1 6.21x10-^5 , 2.31x10-^6 Tartaric Acid HOOC(CHOH) 2 COOH 168.1 9.20x10-^4 , 4.31x10-^5

D. Lab Report

Title Page (include lab title, your name, lab partners names, section, and unknown #)
Purpose (be brief, 1 paragraph) state the goal of this lab and the principles on which the experiment is based
Data Table: VNaOH, pH, Vmid, ΔpH, ΔV, ΔpH/ΔV, Vmid, Δ(ΔpH/ΔV), ΔV, and Δ^2 pH/ΔV^2 and your NaOH concentration.
Three computer plots: the titration curve, a first derivative titration curve, and a second derivative titration curve. Identify and label the equivalence points, the half-equivalence points the titration curve.. Identify and label the equivalence points on your first and second derivative titration curves.

Graphical Analysis of Titration - Analytical Chemistry - Lab 6 | CHEM 321, Lab Reports of Analytical Chemistry

Related documents

Partial preview of the text

Download Graphical Analysis of Titration - Analytical Chemistry - Lab 6 | CHEM 321 and more Lab Reports Analytical Chemistry in PDF only on Docsity!

EXPERIMENT 6

IDENTIFICATION OF A DIPROTIC ACID VIA TITRATION AND GRAPHICAL

ANALYSIS OF TITRATION CURVES USING MICROSOFT EXCEL

A. Prelab

B. Experimental Overview

C. Procedure

D. Lab Report