Analysis of Copper In a Brass Sample INTRODUCTION A. GENERAL In this experiment the percentage of copper present in a brass sample will be determined spectrophotometrically using a Spectronic 20. Brass is an alloy consisting of tin, lead, copper, and zinc. The brass sample,has already been ground to a fine powder. A standard curve (graph) will be made for Cu'^^ in which a plot of absorbance (instrument reading from the Spectronic 20) versus molarity of Cu'*"^ for various solutions will be made. These standard Cu"''^ solutions will be made by dissolving the appropriate amount of CuS04*5H20 in distilled water. The brass samples will be made by dissolving the brass in concentrated HNO3 to produce Pb"*'^, Cu"^^, and Zii^^ in solution along with the finely divided white hydrated tin (IV) oxide solid. After the properly prepared solutions have been filtered to remove the tindV) oxide, the absorbance of these solutions will be measured using the Spectronic 20. The molarity of Cu"^^ in these solutions will be obtained from the standard curve. Then the percent copper in the brass sample will be calculated. Before starting this experiment you should read Appendix VIII concerning the concentration unit: Molarity. B. METHOD OF ANALYSIS In each of the five solutions (three for determining the standard curve and two for the determination of copper in brass) the molarity of Cu"^^ present will be deter mined spectrophotometrically using the Bausch & Lomb Spectronic 20. The only species in the solution that absorbs at 620 mp (this wavelength corresponds to the visible portion of the electromagnetic waves) is Cu"^^. In this aqueous solution Cu"^^ is really present as the aquo complex [Cu(H20)6]+2. The other species, Pb"^^, H"^, NOs", S04~2, and H2O do not absorb at the 620 mp wavelength of light (and tin is precipitated as hydrated tin (IV) oxide). lo FIGURE 1 Solution Containing In the Spectronic 20, light of wavelength of 620 mp. and certain initial inten sity, Iq, see (Fig. 1) is allowed to pass through the sample. The wider the sample tube (width = b) and the greater the molarity (M) of Cu"^^, the more absorption will occur causing the intensity of the 620 mp wavelength (I) to be less as it leaves the solution. The following equation describes the process quantitatively: A = logi = abM where A = absorbance (quantity actually measured by the Spectronic 20), M = molarity of the absorbing species (Cu"*"^ in this experiment), b = tube diameter, and a constant which is characteristic of each absorbing species. Since the same tube (a special tube called a cuvette is used for spectrophotometry) is used throughout the experiment, b remains constant. Hence, we can define a new constant, K = ab. Therefore, the above equation becomes; A = KM = K[Cu+2i A plot of A (the absorbance value measured by use of the Spectronic 20) versus the molarity of Cu"^^ should be a straight line going through the origin (A = 0.000 and M= 0.000). Three standard Cu"'"^ ...

1. Analysis of Copper In a Brass Sample
INTRODUCTION
A. GENERAL
In this experiment the percentage of copper present in a brass
sample will be
determined spectrophotometrically using a Spectronic 20.
Brass is an alloy consisting of tin, lead, copper, and zinc. The
brass sample,has
already been ground to a fine powder.
A standard curve (graph) will be made for Cu'^^ in which a plot
of absorbance
(instrument reading from the Spectronic 20) versus molarity of
Cu'*"^ for various
solutions will be made. These standard Cu"''^ solutions will be
made by dissolving
the appropriate amount of CuS04*5H20 in distilled water. The
brass samples will
be made by dissolving the brass in concentrated HNO3 to
produce Pb"*'^, Cu"^^, and
Zii^^ in solution along with the finely divided white hydrated
tin (IV) oxide solid.
After the properly prepared solutions have been filtered to
remove the tindV) oxide,
the absorbance of these solutions will be measured using the
Spectronic 20. The
molarity of Cu"^^ in these solutions will be obtained from the
standard curve. Then

2. the percent copper in the brass sample will be calculated.
Before starting this experiment you should read Appendix VIII
concerning the
concentration unit: Molarity.
B. METHOD OF ANALYSIS
In each of the five solutions (three for determining the standard
curve and two for
the determination of copper in brass) the molarity of Cu"^^
present will be deter
mined spectrophotometrically using the Bausch & Lomb
Spectronic 20. The only
species in the solution that absorbs at 620 mp (this wavelength
corresponds to the
visible portion of the electromagnetic waves) is Cu"^^. In this
aqueous solution Cu"^^
is really present as the aquo complex [Cu(H20)6]+2. The other
species, Pb"^^,
H"^, NOs", S04~2, and H2O do not absorb at the 620 mp
wavelength of light (and tin
is precipitated as hydrated tin (IV) oxide).
lo
FIGURE 1
Solution

3. Containing
In the Spectronic 20, light of wavelength of 620 mp. and certain
initial inten
sity, Iq, see (Fig. 1) is allowed to pass through the sample. The
wider the sample tube
(width = b) and the greater the molarity (M) of Cu"^^, the more
absorption will occur
causing the intensity of the 620 mp wavelength (I) to be less as
it leaves the solution.
The following equation describes the process quantitatively:
A = logi = abM
where A = absorbance (quantity actually measured by the
Spectronic 20), M =
molarity of the absorbing species (Cu"*"^ in this experiment), b
= tube diameter, and
a constant which is characteristic of each absorbing species.
Since the same tube
(a special tube called a cuvette is used for spectrophotometry) is
used throughout
the experiment, b remains constant. Hence, we can define a new
constant, K = ab.
Therefore, the above equation becomes;

4. A = KM = K[Cu+2i
A plot of A (the absorbance value measured by use of the
Spectronic 20) versus the
molarity of Cu"^^ should be a straight line going through the
origin (A = 0.000 and
M= 0.000).
Three standard Cu"'"^ solutions will be made by dissolving a
known amount of
CuS04*5H20 in distilled water. Hence, the molarity of these
solutions will be
known. Then the absorbance value for each of these three
solutions will be mea
sured using the Spectronic 20. A plot of the three absorbance
values versus the
corresponding Cu"^^ molarities will be done. The best straight
line is drawn through
these three points and the origin.
With the standard curve, the molarity of Cu"^^ in any Cu'^^
solution can be
determined by measuring the absorbance value of the solution

5. (always using the
same cuvette and Spectronic 20) and referring to the standard
curve to find the
molarity of Cu"""^ that corresponds to that absorbance value.
Thus, the molarity of
Cu"^^ in the brass sample solutions can be obtained from the
standard curve after
their absorbance values have been measured on the Spectronic
20.
PROCEDURE
A. PREPARATION OF BRASS SAMPLE SOLUTIONS
Weigh two small clean dry Erlenmeyer flasks on the Mettler top
loader balance to
three decimal places. Then add enough of the unknown brass
sample so that the
sample size will be between 0.7 to 0.8 g. Weigh the Erlenmeyer
flask with the brass
sample to three decimal places on the Mettler top loader
balance. To each Erlenm
eyer flask {.this must be done in the hood) add 12 mL of 8 M

6. nitric acid (measured in
a 10 mL graduated cylinder) very slowly. The reaction of 8 M
nitric acid with the
finely divided brass sample occurs vigorously. The 8 M acid
must be added slowly to
prevent splattering of the brass sample out of the container. The
reaction is
performed in the hood to prevent the toxic brown NO2 gas
(Cu*" + 4HNO3
Cu(N03)2(aq) 2NO2 + 2H2O) from entering the lab. After the
initial vigorous
reaction, place a watch glass on top of each Erlenmeyer flask
and mix occasionally
over a thirty minute period in the hood. (During this 30 minute
period the
CuS04*5H20 solutions can be prepared and measured on the
Spectronic 20.)
After the brass has reacted with the 8 M nitric acid, wash down
the sides of the
Erlenmeyer flask with some distilled water from a squeeze
bottle. Quantitatively
(with no spillage) transfer this mixture to a clean 50 mL
volumetric flask by pouring
the mixture in the Erlenmeyer flask down a glass rod (the spout

7. of the Erlenmeyer
flask will be touching the glass rod) into the 50 mL volumetric
flask. After the
solution is poured into the volumetric flask, add more distilled
water to the Erlenm
eyer flask, washing the sides of the walls. Pour this
quantitatively into the volumet
ric flask as mentioned above. Repeat this two more times. Also
wash the glass rod
with distilled water fi*om the wash bottle and allow the wash
water to go into the 50
mL volumetric flask. Then add enough distilled water until the
50.00 mL mark is
reached by the bottom of the meniscus of the solution. Stopper
and mix very well by
continually shaking the flask in the upright and inverted
positions.
The cloudy pale blue solutions must be filtered to remove the
tin (IV) oxide or
an incorrect high reading will be obtained for the absorbance
reading due to the
scatter of the light waves. Filter each of the solutions through
dry Whatman No. 42
filter paper. Collect the filtrate in clean, diy beakers. If the

8. filtrate is not a clear pale
(blue) color {no cloudiness ?nust bepreseiit), then filter through
another dry Whatman
No. 42 filter paper and collect in another clean, dry beaker. One
to three filtrations
may be needed, depending on how much tin was present in the
brass sample.
Now measure the absorbance of these two solutions as described
in the section
"Use of the Spectronic 20."
Return all equipment and reagent bottles back to their proper
storage area.
Make sure that the equipment is cleaned.
*1116 Other metals in brass react similarly.
B. PREPARATION OF KNOWN CuSO^-SH^O SOLUTIONS
Weigh three 50 mL volumetric flasks to three decimal places on
the Mettler top

9. loader balance. To each of these volumetric flasks add
respectively about 1.000,
2.000 and 3.000 grams of CuS04*5H20. Again weigh the
volumetric flasks with the
CuS04*5H20 on the Mettler top loader balance to three decimal
places. Add 6.0 mL
of concentrated nitric acid (measured with graduated cylinder)
and approximately
25 mL of distilled water to each flask. Mix until CuS04®5H20
dissolves. Then add
distilled water until the bottom of the meniscus of the solution
hits the 50.00 mL
mark. Stopper and mix well (as described in previous section
for making the brass
solutions). The three solutions have already been prepared for
you.
Measure the absorbance of each solution as described in the
section on the "Use
of the Spectronic 20."
C. USE OF THE SPECTRONIC 20
How to use the Spectronic 20 and measure the absorbance of

10. each solution is
described in Appendix III. Remember that the wavelength
should be set at 620m|i.
CALCULATIONS
A. STANDARD CURVE
Suppose the weight taken to prepare 50.00 mL of each of the
standard solutions
were 0.307, 0.622, and 0.868 gof CuS04»5H20 respectively.
The molarity of Cu"*"^in
the first solution would be;
_ grams/molecular weight
liters
M = 0.307/249.7 _ 0.0246
0.05000
where 249.7 is the molecular weight of CuS04*5H20. Check to
see that you get
0.0498 and 0.0695 for the molarity of the other two solutions.

11. Suppose the measured absorbance values of each of these
solutions were
respectively, 0.095, 0.215, and 0.285. Then make a graph of the
absorbance (y-axis)
versus the molarity of Cu"^^ (x-axis), using graph paper that
has many line divisions.
An example graph is shown in Figure II. Note that you should
not show the dotted
lines as shown in the graph in Figure II. These dotted lines were
shown to indicate
how to get the points for this straight line.
In order to make a graph of absorbance (A) versus molarity (M),
using graph
paper, the following steps should be done:
1. Determine the range of the data for each axis. Assign rounded
values to the
major divisions (every 5 or 10 small divisions) on your graph
paper so that
the range of data is included and as much as possible of the
available graph
is used. Label each axis and only the major divisions.

12. 2. Plot each set of A and M data as a point on the graph paper.
Make the points
only large enough to be distinct.
3. Draw one straight line which follows the points as closely as
possible. If the
points do not lie on the line, position the line so that the
average distance
between the line and the points above the line are approximately
the same
as the average distance between the line and the points below
the line.
Adjust the slope so that these distances above and below are
approximately
the same at both ends of the line.
4. Use the line to determine the unknown molarity. Draw a fine
horizontal
line at the unknown absorbance so that it intersects the standard
line. At
the point of intersection, draw a fine vertical line corresponding
to the
unknown molarity.

13. 5. Use ink throughout. Title the graph and attach it to your lab
report.
B. PERCENT COPPER IN THE BRASS SAMPLE
Suppose 0.480 g of brass was dissolved in 8 M nitric acid and
distilled water so as to
produce 50.0 mL of solution as described in this experiment.
After filtering, the
absorbance value was found to be 0.260 as measured by the
Spectronic 20. From the
standard curve the molarity of Cu'^^ would be 0.0628.
Therefore, the percent copper
in this example brass sample would be:
0.3
0.2
0)
u
c

14. (Q
.A
w
o
CO
A
<
0.1
0.0
:e:
.t
0.0000 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600
Molarity of
0.0700 0.0800

15. FIGURE 2 Data from Absorbance vs. Molarity of
s
molarity Cu solution ( mclG /4iter) x vol (4-) of Cu soln x at wt
of Cu— x 100
( rnn1r> 1
%Cu =
weight (g) brass
9' Cu ~ 0.0628m6le/-4ite x 0.0500 4ito x 63.5g Cu/^neie ^
0.480 g brass
% Cu = 41.5%
NAME DATE SECTION
INSTRUCTOR GRADE
EXPERIMENTS:

16. REPORT FOR THE ANALYSIS OF COPPER IN A BRASS
SAMPLE
DATA/RESULTS*
I. PREPARATION OF STANDARD CURVE