Breaking Brass:
An investigation to determine the percent composition of copper in a brass alloy
STUDENT EXAMPLE LAB REPORT
Chemistry
INFINITY Period
Experiment Performed:
Brass Dissolved 1/15/15
Solutions Made 1/16/15
Absorbance Calculated 1/21/15
Abstract
The strength and durability of copper is a concern to many people who use copper for different purposes. Copper is used for many reasons, but one that is of special interest in the actual percentage of the copper in brass is laying pipes underwater and using brass on boats. This investigation determined the percent composition of copper in a brass screw by using nitric acid to dissolve the brass sample to produce a brass solution. Then, spectral analysis was used to determine the absorbance of the brass solution, and copper nitrate solutions for comparison. The molarity of the brass solution was then found, and from there the percent composition was determined. The percent of copper in this brass sample was determined to be 62.22%. The percentage is on the borderline of being too low to use for laying underwater pipes and on boats. The larger percentage of copper in the brass, the stronger and more malleable the alloy will be.
Introduction:
Brass is an alloy comprised of varying percentages of copper and zinc. The varying proportions can create a different set of properties for the resulting brass alloy. Brass is a substitutional alloy, meaning that its atoms can interchange with one another within the same crystal structure (Alloys).
The uses of brass include, but are not limited to, locks, gears, bearings, doorknobs, plumbing, electrics, and musical instruments. The main uses of brass are in areas where the metal will be subject to minimal friction. Brass creates a combination of high workability and durability. It is important that in the use of brass that sparks do not get struck (Non-Sparking). Brass is extremely malleable, making it easier to cast compared to other metals such as bronze or zinc.
One concern with the use of brass is its capability of corrosion. With higher levels of zinc present (above 15%), brass could be subject to dezincification, which is the corrosion and dissolution of zinc (Dealloying). Dezincification occurs when brass is exposed to oxygen and moisture (Dealloying). To combat this issue in seawater uses, tin can be added to brass. This creates dezincification resistant brass, DZR, also called Brass C352 (Copper Alloys). Also, an aluminum oxide (Al2O3) layer around the brass makes the brass stronger as well as more corrosion resistant (Aluminum).
Additionally, copper is germicidal, or an agent that destroys germs (Germicide). Contingent upon the type and concentration of the germ, the brass alloy can kill germs within hours of contact. Brass alloys began to be investigated in 2007 for their antimicrobial properties. Frequently touched items were fitted with copper alloys to study copper’s germicidal assets. In 2011 it was discovered that these “coppered rooms” demonstrated a 97% reduction in surface pathogens (Copper Touch Surfaces). It was also found that patients had a lower risk of contracting hospital acquired infections when in coppered ICU rooms (Copper Surfaces). The higher the percentage of copper in the brass, the more germicidal properties the alloy displayed.
There are different types of brass used in different situations due to their different properties. Alpha brasses have more than 65% copper and less than 35% zinc. Alpha-beta brasses have 55%-65% copper and 35%-45% zinc. Beta brasses have 50%-55% copper and 45%-50% zinc. White brass has less than 50% copper and greater than 50% zinc. The more copper there is in the brass alloy the stronger and more malleable it will be (Brass Knowledge).
Method:
Brass is an alloy of copper and zinc.
In the lab, we measured out a brass sample of 3.14 grams, or two brass screws. This sample was dissolved completely using 12.0 Molar nitric acid stock solution. This created a 50 milliliter sample of a brass solution with an extremely high concentration. The chemical equation of this dissolution is shown below:
4HNO3(l)+Cu(s)+Zn(s)→CuNO32(l)+ZnNO32(l)+H2(g)
To use this sample in the lab, it was first diluted down using 200 milliliters of distilled water to then make a 250. milliliter sample of diluted brass solution.
The stock 0.40 Molar copper nitrate (CuNO32) solution was serially diluted to make different concentrations of copper nitrate to use for comparison. Solutions of molarities 0.25 M, 0.20 M, 0.16 M, 0.13 M, and 0.10 M were created from the 0.40 M stock solution.
The absorbance was taken for all of these samples, as well as the absorbance of the brass sample using spectral analysis with a colorimeter. Colorimeters evaluate the color intensity of a solution to determine the absorbance. From there the concentration can be determined. White light is shone through a color filter and then through the sample to hit a detector. Light will either be absorbed by the solution, or will go through the sample. The amount of the light the sample absorbs determines the absorbance level the colorimeter calculates. The absorbance is dependent upon the molarity of the solution. Using samples with known molarities, we can find the absorbance of the known samples using the colorimeter. When dealing with an unknown, we can use these absorbances and molarities of known solutions to find an unknown molarity of a solution, using the unknown’s absorbance.
A=ab[C]
Absorbance = (absorptivity)(path length)(concentration)
In this investigation, serially diluted copper nitrate solutions will be used as the known solutions with known molarities to find the unknown molarity of the brass solution. The independent variable will be the concentration (molarity), [C]. The absorption, A, will be the dependent variable. Path length, or b in the equation above, and absorptivity, or a, will both be constants.
The blue color of both the brass solution and the copper nitrate solution means that blue light is reflected off of the solution. Blue light has a wavelength of about 475 nm. For this investigation using the colorimeter, red light of wavelength 635 nm was used to create the greatest contrast from the blue light it reflects and therefore create the largest absorbance. When we started to use the colorimeter, the colorimeter was successfully calibrated using water as the blank sample, because it is a clear solution with a 0.0 Molarity.
Data and Analysis:
Figure 1
Concentration (M) / Absorbance.4 M / .835
.25 M / .414
.2 M / .372
.16 M / .253
.133333 M / .223
.1 M / .136
Brass Solution
(Unknown Molarity) / .250
Figure 1 shows the table of concentrations obtained from the serially diluted stock copper nitrate solution and their respective absorbance measurements acquired from the colorimeter.
Figure 2
Figure 2 shows the molarity vs. absorbance data as a linear graph, using a line of best fit. The linear equation and R2 value were calculated and are included in the figure. As the R2 value approaches 1, the data points are a closer fit to the calculated linear function. The unknown molarity of the brass solution can be calculated from its absorbance and the linear equation shown above.
Y = absorbance = 0.250 y = 2.2837x – 0.1011
X = molarity = unknown 0.250 = 2.2837x - 0.1011
x = 0.1537
Molarity of the brass solution = 0.1537 M
This molarity represents the molarity of the copper nitrate in the brass solution. Multiplying this molarity by the liters of solvent used will give the number of moles of copper nitrate.
Molarity of the copper nitrate = moles of copper nitrateLiters of solvent
0.1537 = moles.200 L
The .200 L is from the 200 mL of water was added to the brass solution to dilute it to its current molarity of .1537 M.
moles of copper nitrate=0.03075
According to the chemical equation below, the number of moles of copper nitrate produced is the same as the number of moles of copper reacted.
4HNO3(l)+Cu(s)+Zn(s)→CuNO32(l)+ZnNO32(l)+H2(g)
moles of copper= 0.03075
Multiplying the moles of copper by the atomic mass of copper will equal the grams of copper in the brass.
0.03075 mol ∙63.546 g=1.954 g Cu in brass
Dividing the grams of copper in the brass by the total grams of the brass and multiplying that by 100 gives you the percent composition of copper in brass, which was calculated to be 62.22%
1.954 g3.14 g ∙ 100=62.22%
Discussion and Conclusion:
The actual percent of copper in the brass screw is reported to be 65%
To determine the percent error of this investigation, we set the theoretical as 65% and the actual as 62.22%.
% Error=Theoretical-ActualTheoretical∙100
% Error=65-62.2265∙100=4.274%
The source of most of the error was from the tools for measurement we were supplied with. After dissolving the brass screws, there was a built-in error with the tools for volume measurement. The beaker that the brass solution was dissolved in only went up by +/- 25 milliliters and the graduated cylinders we used to measure the brass solution as well as the copper nitrate and water for diluting the copper nitrate solutions had only a +/- .05 milliliters. This limited the accuracy of which we could measure the actual amount of solute for the brass solution, and also the amounts to dilute the copper nitrate solutions. This resulted in our milliliters of solvent (200 milliliters) that we used for our calculations above, being skewed from the limited accuracy of our materials.
Another source where a small error could be found was from the limited availability of the stock concentration of copper nitrate. We were only allowed 15 milliliters of the solution, restricting us to only being able to make a limited number of accurately measured diluted copper nitrate solutions for comparison.
The percent of copper in this sample of brass is on the borderline of being characterized as either alpha or alpha-beta brasses. The more copper and less zinc there is in brass, the less susceptible to dezincification. Dezincification is a real concern for such purposes as laying copper pipes underwater. The salt and the moisture from the water lead to faster dezincification. For this purpose, it would be more beneficial to either use an alloy of brass with less zinc or another alloy that eliminates the use of zinc altogether. For other purposes of brass, this percent composition of copper is perfectly acceptable. The amount of copper will create a strong and malleable alloy that can be shaped and used for purposes such as doorknobs, screws, musical instruments, and many more uses.
References:
Alloys. (n.d.). Retrieved January 27, 2015, from http://mihaela.academicdirect.ro/free/Alloys.pdf
Aluminium corrosion resistance. (n.d.). Retrieved January 27, 2015, from http://www.aluminiumdesign.net/design-support/aluminium-corrosion-resistance/
Brass Knowledge Base:What Is Brass ? Different Type Of Brass. (n.d.). Retrieved January 29, 2015, from http://exports-unlimited.com/knowledge_base.html
Copper Alloys - Brass - Bronze | NBM Metals. (n.d.). Retrieved January 27, 2015, from http://www.nbmmetals.com/
Copper surfaces in the ICU reduced the relative risk of acquiring an infection while hospitalized. (2011, January 1). Retrieved January 27, 2015, from http://www.biomedcentral.com/1753-6561/5/S6/O53
Copper Touch Surfaces. (2011, July 1). Retrieved January 27, 2015, from http://www.coppertouchsurfaces.org/press/releases/20110701.html
Dealloying (selective leaching). (n.d.). Retrieved January 27, 2015, from http://corrosion-doctors.org/Forms-selective/dezinc-valve.htm
Germicide. (n.d.). Retrieved January 27, 2015, from http://www.merriam-webster.com/dictionary/germicide
Non-Sparking Tools. (n.d.). Retrieved January 27, 2015, from http://www.ccohs.ca/oshanswers/safety_haz/hand_tools/nonsparking.html