Spectrophotometric Det of Copper in Brass PDF

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Spectrophotometric Determination of Copper in Brass Prepared by Zoey Bryant, Yuhong Wu, and Michelle Greene

Purpose Determine the mass percent of copper in a sample of brass using spectrophotometry.

Summary In order to determine the amount and percentage of copper in a known amount of brass, we will be using the spectrophotometric method to determine absorbance of two solutions. The first solution! contains a known amount of copper, while the second is a brass solution where the percentage of copper is measured. By measuring the absorbance of the copper solution, we will determine the molar absorptivity for copper. We will use this value, along with absorbance of the second solution, to determine the concentration of copper. To conduct this experiment, we will be using a spectrophotometer, which is used to study how much light is absorbed by a solution and how much passes through it. We will use these values to help determine the unknown concentration of the copper in the second sample.

Background Spectrophotometry Spectophotometry is a method that measures the amount of light passing through the solution at a specific wavelength. In this method, the instrument used is a spectrophotometer and measures the percent transmittance and absorbance. The percent transmittance (%T) can be determined by the Equation 1: -

%"𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑎𝑛𝑐𝑒 = " . " ×"100% -

(1)

/

where Io is the intensity of light directed at a solution and It is the intensity of the light exiting the solution. In spectrophotometry, the wavelength of light passing through the sample can be varied. The wavelength that produces the lowest percent transmittance is the analytical wavelength. Different substances will have the lowest percent transmittance at different analytical wavelengths.

Copper in Brass - 1 !

The percent transmittance scale is linear, and its values range from only zero to one hundred. The absorbance scale of a spectrophotometer is logarithmic. Because the logarithmic scale is harder to read, we can convert percent transmittance to absorbance (A) according to Equation 2: A = 2.000 – log (%T)

(2)

Beer's Law Beer's Law describes the relationship between the absorbance and concentration of a substance in a solution (Equation 3). The variables in the equation are the concentration (c) of the substance in the solution, the pathlength (b in cm) of the light through the solution, and the absorptivity (epsilon, ε), a measure to how much of the analytical wavelength is absorbed. Absorptivity is measured in L g-1 cm-1, while molar absorptivity is measured in L mol-1 cm-1. Either absorptivity or molar absorptivity can be used in Beer's Law. If absorptivity is used, then the concentration in measured in units of g L-1. If molar absorptivity is used, then the concentration in measured in units of mol L-1 (Molarity). 𝐴 = "𝜀𝑏𝑐

(3)

A = absorbance, no units ε = absorptivity b = pathlength c = concentration

To measure absorbance, the wavelength of the spectrophotometer is set to one of the substance's analytical wavelengths. In this case, you will measure the amount of copper in a sample of brass. Therefore, you will measure the absorbance at the analytical wavelength for copper (620 nm). To determine the concentration of copper in the brass, you need to know the values for the other variables in Equation 3: You will measure absorbance (A). The pathlength (b) for most spectrophotometers is 1 cm (width of the cuvette, length light travels through solution). The other variable is molar absorptivity (ε), which is not known. To determine molar absorptivity, you will first take a piece of copper, determine the mass, and make a solution of known concentration of copper. Once you measure the absorbance at 620 nm, you will be able to calculate the molar absorptivity for copper and use that value in subsequent calculations to determine the concentration of copper in the brass solution. Copper in Brass - 2 !

With the concentration of copper in the brass solution, this can be converted to moles of copper and finally to grams of copper. The grams can be used to find the percent mass of the copper in the brass sample. Mass percent is simply calculated by taking the grams of copper that was calculated, dividing it by the mass of the brass in grams, and multiplying that value by one hundred (Equation 4). 𝑚𝑎𝑠𝑠"𝑝𝑒𝑟𝑐𝑒𝑛𝑡"𝑐𝑜𝑝𝑝𝑒𝑟 = "

89:;...