Percentage of copper in brass experiment

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.