CHEMISTRY 59-320CHEMISTRY 59-320ANALYTICAL CHEMISTRYANALYTICAL CHEMISTRY

Fall - 2010Fall - 2010

Lecture 11

Chapter 7: Let the titration begin

Example of multiple choice• The degrees of freedom used to calculate the confidence interval are

(a) the number of data points which are being evaluated.(b) the number of data points - 1.(c) the value associated with the number of data points used to determine the value of t.

• How many significant figures are in the number 6.230 x 1023? (a) 4(b) 3(c) 23

• 1. What are the three essential attributes of a laboratory notebook? (a)1. Record what you did.2. Write in complete sentences.3. The notebook should be understandable by a stranger.(b)1. Record what you did.2. Record what you observed.3. Always write balanced equations.(c)1. Record what you did.2. Record what you observed.3. The notebook should be understandable by a stranger.

• The proper use of a buret involves (a) reading the bottom of the meniscus in a manner that avoids a parallax error and making sure that you understand that the buret reads from 50.00 mL at the top to 0.00 mL at the bottom.(b) reading the bottom of the meniscus in a manner that avoids a parallax error and making sure that you understand that the buret reads from 0.00 mL at the top to 50.00 mL at the bottom.(c) reading the top of the meniscus closest to the volume mark on the buret with your eye at the same height as the top of the liquid

Example of multiple choice

• In a 0.001 M aqueous solution of NaCl, the solute is (a) aqueous.(b) NaCl.(c) H2O.

• Police have a hit-and-run case and need to identify the brand of red auto paint. The percentage of iron oxide, which gives paint its red color, was determined to be 43.35 ± 0.33% by one method of analysis using five measurements. The color reference provide by the automotive manufacture is 44.21%. What can you conclude about whether the sample matches the reference at a 95% confidence level? (a) The sample matches the reference.(b) The sample does not match the reference.(c) There are an insufficient number of trials to determine if the sample is a match.

To answer this one: A t test (Case 1) is used to compare one set of measurements with a known value to decide if they are the same: for 4 degrees of freedom t table = 2.776 at 95% confidence level and ? = 43.35 ± (2.776)(0.33)/√5 = 43.35 ± 0.43; therefore, the 95% confidence interval is 42.92 to 43.78%, which does not include 44.21%. The values are not the same. (section 4-5).

Sample of calculations

• Find the absolute uncertainty of the following calculation, and express the answer to the proper number of significant figures. 12.34(± 0.04) + 178.1(±0.2) - 18.493(±.002) = ???

Sample of short answers

• 7-6 : What is the difference between a direct and back titration?

• 0-1: What is the difference between qualitative and quantitative analysis?

All the questions in Midterm and Final Exam will be

prepared based on

Terms to Understand listed at the end of each chapter

Volumetric analysis• A procedure in which we measure the volume of reagent

needed to react with analyte.

7-1 Titrations

• The principal requirements for a titration reaction are that (1) a large equilibrium constant, and (2) proceed rapidly.

• The most common titrations are based on (a) acid-base, (b) oxidation-reduction, (c) complex formation, and (d) precipitation reactions.

• Equivalent point: when the quantity of added titrant is the exact amount for stoichiometric reaction with the analyte.

• End point: marked by a sudden change in a physical property of the solution.

• An indicator: a compound with a physical property that changes abruptly around the equivalent point.

Direct titration: This curve shows how pH varies as 0.100 M NaOH is added to 50.0 mL of 0.100 M HCl.

Back titration: Lab 1

• The equivalence point is the ideal (theoretical) result we seek in a titration. What we actually measure is the end point, which is marked by a sudden change in a physical property of the solution

7-2 Titration calculations• The key step is to relate moles of titrant to moles of analyte.

Solution:

Kjeldahl nitrogen analysis• Developed in 1883, the Kjeldahl nitrogen analysis remains one of

the most accurate and widely used methods for determining nitrogen in substances such as protein, milk, cereal, and flour.

• The solid is first digested (decomposed and dissolved) in boiling sulfuric acid, which converts nitrogen into ammonium ion

Example: Kjeldahl analysis

• Problem 7-12. The Kjeldahl procedure was used to analyze 256 μL of a solution containing 37.9 mg protein/mL. The liberated NH3 was collected in 5.00 mL of 0.0336 M HCl, and the remaining acid required 6.34 mL of 0.010 M NaOH for complete titration. What is the weight percent of nitrogen in the protein.

• Solution: in class

7-3 Spectrophotometric titrations

• Absorption of light used to monitor the progress of a titration.

• When iron binds to the protein, a red color with an absorbance maximum at a wavelength of 465 nm develops.

• The absorbance is proportional to the concentration of iron bound to the protein.

7-4 The precipitation titration curve

• Titration curve: a graph showing how the concentration of the reactants varies as titrant is added.

• p function:

The titration curve of I- and Ag- has three distinct region as shown in the plot:(1) Before the equivalence point.(2) At the equivalence point.(3) After the equivalence point.

• The equivalence point is the steepest point of the curve. It is the point of maximum slope (a negative slope in this case) and is therefore an inflection point (at which the second derivative is 0):

7-5 Titration of a mixture

• If a mixture of two ions is titrated, the less soluble precipitate forms first. If the solubilities are sufficiently different, the first precipitation is nearly complete before the second commences.

• The silver electrode responds to changes in Ag+ concentration, and the glass electrode provides a constant reference potential in this experiment.

0 0.0591 [ ]log

[ ]Ag

RE E

n O

O ne R

• Titration curve for 40.00 mL of 0.050 2 M KI plus 0.050 0 M KCl titrated with 0.084 5 M AgNO3.

• Because Ksp(AgI) << Ksp(AgCl), AgI precipitates first.

• Coprecipitation: occurs when a substance whose solubility is not exceeded precipitates along with one whose solubility is exceeded.

7-7 End-point detection for precipitation titrations

• 1. Volhard titration: formation of a soluble, colored complex at the end point. The Volhard titration is a titration of Ag+ in HNO3 solution.

• 2. Fajans titration: adsorption of a colored indicator on the precipitate at the end point. After the equivalence point, there is excess Ag+ in solution. Adsorption of Ag+ onto the AgCl surface places positive charge on the precipitate

In all argentometric titrations, but especially with adsorption indicators, strong light (such as daylight through a window) should be avoided. Light decomposes silver salts, and adsorbed indicators are especially light sensitive.

Solution:

• Direct titration: One in which the analyte is treated with titrant and the volume of titrant required for complete reaction is measured.

• primary standard: A reagent that is pure enough and stable enough to be used directly after weighing. The entire mass is considered to be pure reagent.

• trace analysis: Chemical analysis of very low levels of analyte, typically ppm and lower.

• blank titration: One in which a solution containing all reagents except analyte is titrated. The volume of titrant needed in the blank titration should be subtracted from the volume needed to

titrate unknown. • Self-study: section 7-6 Calculating titration curves with a

spreadsheet section 7-8 Efficiency in experimental design