Transcript CHEMISTRY 59-320 ANALYTICAL CHEMISTRY Fall

CHEMISTRY 59-320
ANALYTICAL CHEMISTRY
Fall - 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.
•
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
Back titration: Lab 1
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.
0.0591  [ R] 
EE 
log 

n
[
O
]


0
Ag
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