Determination of the copper content in a brass sample

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).

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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"'"^ 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 (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 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 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 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 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 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.

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.

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.

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

(Q

.A w

o CO A

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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

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:

REPORT FOR THE ANALYSIS OF COPPER IN A BRASS SAMPLE

DATA/RESULTS*

I. PREPARATION OF STANDARD CURVE

Solution 1 Solution 2 Solution 3

1. Weight (g)CuS0.*5H20 dissolved in 50.0 mL of ^ solution

2. Molarity of Cu"^^ in the 50.0 mL of solution

3. Absorbance value obtained using the Spectronic 20

II. ANALYSIS OF COPPER IN THE BRASS SAMPLE SAMPLE NUMBER

Trial 1 Trial 2

1. Weight (g) of dry flask plus brass

2. Weight (g) of dry flask

3. Weight (g) of brass 4. Absorbance value obtained

using the Spectronic 20 5. Molarity of Cu"*"^ as read

from the standard curve

6. Percent copper in the brass sample

7. Average percent copper in the brass sample

CALCULATIONS

'A graph of your standard curve must be submitted wth the Data and Results form.

EXERCISES

1. When the brass sample solutions were prepared by dissolving the brass sample in 8 M nitric acid, suppose the cloudy solution was not filtered. Would the percent copper reported be higher, lower, or equal to the actual percent copper in the brass sample?

2. When the brass sample was dissolved in 8 M nitric acid in a flask and then transferred to the 50.0 mL volumetric flask, suppose some of the mixture was spilled onto the table top. Would the percent copper found experimentally be higher, lower, or equal to the actual percent copper in the brass sample?

3. If 0.521 g of brass were dissolved in 8 M nitric acid, diluted to 50.0 mL in a volumetric flask with distilled water, filtered, and found to have a Cu'^^ molar- ity of 0.0931 mole/liter, then what is the percent copper in the brass sample?

4. Suppose the Cu"^^ determination described in question 3 was done by using a 50.0 mL volumetric flask. If the student thought a 25.0 mL volumetric flask was used instead of a 50.0 mL volumetric flask, what would be the true percent of copper in the brass sample? What would be the percentage actually reported by the student?

5. Should the straight line always go through the origin of a standard curve in which absorbance values are being plotted as a function of molarity? Why?

6. How many significant figures can the absorbance reading be measured, using the Spectronic 20?

7. During the preparation of the brass solution suppose a trace of some other metal ion that absorbs at 620 mp enters the solution. Would this cause the percent copper found to be higher, lower, or equal to the true percentage of copper in the brass sample?

8. Suppose that a student performed the experiment and calculations perfectly except for the following. Unlcnown to the student, Cu(N03)2®3H20 instead of CuS04®5H20 was used to make up the known Cu'^^ solution. Using his data the student found a percentage of copper in the brass samples of 50.0%. Would this percentage be correct? If so, why? If not, what is the correct percentage? Show how you got your answer.

9. If a solution of 0.750 g of CuS04*5H20 in 80.0 mL of solution has an absor bance of 0.100, what is the absorbance of a solution of 1.200 g of CUSO4 ® 5H2O in 50.0 mL of solution? Show your set-up and the result of your calculation.