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EXPERIMENT 4
Purpose: In the first part of this experiment you will decompose
a nickel ammine compound using heat. In the second part
you will analyze the same nickel ammine compound for the
percent of ammonia in the complex. In the third part you
will titrate with EDTA to determine the percent of nickel
present. This will also allow you to determine the percent
of sulfate and water if it is present. The overall goal of
the experiment is to calculate the empirical formula and
chemical formula of a transition metal coordination
compound.
Procedure: ("WORK IN PAIRS")
A. Thermal Decomposition of the Nickel Ammine Compound
Add a small amount of the blue nickel ammine compound to a clean,
dry evaporating dish. Place a piece of moist pink litmus paper on
the underside of a watch glass. Set the watch glass on top of the
evaporating dish and then gently heat the dish. Record any color
change that occurs.
If the litmus turns blue, ammonia gas is present; it reacted
with water to produce hydroxide ions (a basic solution). Now
remove the watch glass and continue to heat vigorously until a
yellow-green-brown color appears. The water, ammonia, and NO gas
are probably given off, leaving NiO solid residue.
B. Analysis for Ammonia by Titration with Standard HCl
(To save time, one person should be doing this HCl
titration while the other person does the EDTA titration)
Accurately weigh, on the analytical balance, two samples
(0.1500 g each) of the nickel compound and transfer each to a
125 mL Erlenmeyer flask. Dissolve each sample in about 30 mL
of distilled water. If it does not all dissolve immediately,
it will dissolve during the titration.
Add 10 drops of bromocresol green indicator (pH range: 3.8-5.4)
to each solution and titrate the samples with standard 0.1 M HCl to
a light yellow end point. (If the blue color returns, add more acid.
The endpoint color should hold for at least 2 minutes.)
BE SURE TO RECORD THE HCl CONCENTRATION FROM THE REAGENT BOTTLE.
Remember to read your buret to two places after the decimal. We will
assume the HCl is reacting with only the ammonia.
C. Analysis of Nickel by Titration with EDTA
The nickel complex will be analyzed for its nickel content using a
compleximetric titration using EDTA (see below).
O-Na+ O-Na+
| |
O=C-CH2 CH2-C=O
\ /
N-CH2-CH2-N
/ \
O=C-CH2 CH2-C=O
| |
OH OH
Disodium Dihydrogen Ethylenediaminetetraacetic Acid
Nickel is often determined by titration with the ligand
ethylenediaminetetraacetic acid (EDTA) in the form disodium
dihydrogen EDTA. In this titration, a standard solution of
EDTA is added until the endpoint is detected by a visual metal
indicator. This is a compound which produces a color when it
is coordinated into a metal ion and which gives a different color
when it is uncoordinated. Murexide, the ammonium salt of purpuric
acid, will be used as the indicator in this experiment. It undergoes
a color change from light yellow to violet at the endpoint. When not
coordinated, the murexide idicator is violet.)
Since EDTA can be obtained as a primary analytical standard
(which means it is available as a pure solid), a standard solution
can be prepared volumetrically. This solution will be furnished for
you. Be sure to record the exact concentration. In solution EDTA is
in the minus four oxidation state(-4).
Accurately weigh, on the analytical balance, two samples
(0.3000 g each) of the nickel compound and transfer each to a
300 mL Erlenmeyer flask. Add 100 ml of distilled water, a few
milligrams of the solid murexide indicator (about the size of a pea)
and 3 ml of a previously prepared 4 M ammonia/3 M ammonium chloride
buffer (pH 10). Titrate the nickel in your sample with the standard
EDTA solution to the endpoint (blue or violet depending on the amount
of indicator added). Read the buret to two decimal places (0.01 ml).
D. Lab Report
On your lab report, you will calculate the percent nickel
contained in each sample. Assuming the salt contains only divalent
nickel, ammonia, sulfate, and possibly water, you will calculate the
simplest (empirical) formula for the complex consistent with your
data.
Chemistry 102 LAB REPORT 4 Name__________________
Date Submitted______________ (25 points) Section__________________
ANALYSIS OF NICKEL AMMINE COORDINATION COMPOUND
1. Thermal Decomposition of Nickel Ammine Coordination Compound
a. (1 point) Write a reaction showing why the pink litmus
paper turns blue):
____________________________________________________________
b. (1 point) The complex was prepared using a "hydrated" nickel
sulfate coordination compound. Write a possible "formula"
for the yellow-green (brown) residue produced when the
complex was heated. HINT: READ YOUR LAB CAREFULLY TO ANSWER
THIS QUESTION.
________________
2. Analysis for Ammonia by Titration with Standard HCl
a. (3 points) Write the net ionic equation for the reaction
which took place when the complex was titrated with HCl:
__________________________________________________________
b. (1 point) "CIRCLE" the pH of the solution.
less than 7 7 more than 7
c. (2 points) Complete the following table. (Watch sig figs!)
|---------------------|------------------------|
|Weight of Complex | mL of HCl used |
------------|---------------------|------------------------|
Trial I | | |
------------|---------------------|------------------------|
Trial II | | |
------------|---------------------|------------------------|
d. (2 points) Calculate % ammonia in the complex.
Show your work.
Trial I:
Trial II:
% ammonia: Trial I:______ Trial II:_______ Average:_______
3. Analysis of Nickel by Titration with EDTA
a. (1 point) Write a balanced "net ionic" equation for the
reaction of EDTA with nickel ion.
_________________________________________________________________________
b. (4 points) Complete the following table. (Watch sig figs!)
|---------------------|------------------------|
|Weight of Complex | mL of EDTA used |
------------|---------------------|------------------------|
Trial I | | |
------------|---------------------|------------------------|
Trial II | | |
------------|---------------------|------------------------|
c. (2 points) Calculate the percentage nickel by weight in the complex.
Record the average percentage nickel ion:_______________________
4. Calculation of Percent Sulfate and Water from EDTA Titration Data:
a. (2 points) Assuming that sulfate ion is the only anion
present, calculate the percent by weight of sulfate in
your sample. Hint: The starting compound for the complex
is nickel sulfate with "six" water molecules of hydration!!
Record the average percent of sulfate ion:________________
b. (1 point) Now add up the ammonia, nickel, and sulfate
percentages.
Record the total: ___________
If they do not total 100%, what do you think might
make up the difference?
______________________________
5. Calculation of the Empirical Formula for Nickel Ammine Compound: ___________
(5 points) Calculate the simplest (empirical) formula in
terms of divalent nickel, ammonia, sulfate ion and possibly
water using percentages and moles. HINT: The formula should
be based on (contain) one atom of nickel and one molecule of
sulfate ion.
a. Ni(+2):
b. ammonia:
c. sulfate(-2):
d. water:
e. EMPIRICAL FORMULA
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