Spectrophotmetry

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The Spectrophotometric Analysis of Copper in a Copper Oxide Ore

Abstract
In this experiment, the mass percent of copper in an unknown copper oxide ore was determined by using a spectrophotometer and Beer’s Law. The mass percent of copper was found to be 5.12 0.14%. Introduction

In spectrophotometric analysis, the concentration of a solute is measured in a solution by measuring the amount of light that is absorbed by the solution in a cuvette placed in a spectrophotometer. A spectrophotometer measures the intensity of light after it is directed through and emerges from a solution. In general, the higher the concentration of an analyte, the higher the absorbance. Spectrophotometric analysis exploits Beer's Law, which predicts a linear relationship between the absorbance of the solution and the concentration of the analyte, assuming all other experimental parameters do not vary (1). In this experiment, a series of standard solutions are prepared. A standard solution is a solution in which the analyte concentration is accurately known. The absorbances of the standard solutions are measured and used to prepare a calibration curve, which is a graph showing how the absorbance with the concentration. For this experiment, the points on the calibration curve should yield a straight line (2). The slope and intercept of that line provide a relationship between absorbance and concentration: A = slope x + intercept

The unknown solution is then analyzed. The absorbance of the unknown solution, A, is then used with the slope and intercept from the calibration curve to calculate the concentration of the unknown solution, x. The absorbance varies linearly with both the cell path length and the analyte concentration. These two relationships can be combined to yield a general equation called Beer's Law (3). A = ε l c

The quantity ε is the molar absorptivity; in older literature it is sometimes called the extinction coefficient. The molar absorptivity varies with the wavelength of light used in the measurement (3). In this experiment, a stock solution was prepared that contained Cu(NO3)22.5 H2O and distilled water. Ammonia was added to the stock solution to make the complex ion copper(II) tetraamine. Ammonia acts as a Lewis base and donates electrons to copper, which acts as a Lewis acid. The reaction that occurs is as followed: Cu2+ (aq) + 4NH3 (aq) Cu (NH3)42+ (aq) (1) Experimental Methods

Cu(NO3)2.2.5 H2O(0.9029g) was weighed on a balance and then transferred to a clean 100mL volumetric flask. Distilled water was then added to the mark on the neck and the solution was mixed thoroughly. Five 25mL volumetric flasks were labeled 1-5 and different volumes of the Cu2+ stock were added to each one. 2mL of 15 M NH3 was added to each flask and then distilled water was added to the mark on the neck. The spectrophotometer was used to record the absorbance of each flask in triplicates after blanking it with distilled water between every reading. In the second part of the experiment, 0.8007g of dried copper oxide ore sample was weighed and put in a 100mL beaker with a watch glass. The beaker was warmed on a hot plate until the solution was dissolved. The solution was then transferred to a 50mL volumetric flask and distilled water was added to the mark on the neck. Three 25mL volumetric flasks were labeled 6-8 and 10mL of the copper oxide solution was transferred to each of the flasks. 4mL of 15 M NH3 was added to each of the flask and the absorbance was recorded for each flask in triplicates. Results:

Table 1: Preparation of Cu(NH3)42+ (aq) solutions
Flask #| mL Cu2+ stock solution| mL conc. NH3| Concentration Cu(NH3)42+ (M)| Average Absorbance| 1| 2.00| 2.00| 0.003104| 0.117|
2| 4.00| 2.00| 0.006208| 0.216|
3| 6.00| 2.00| 0.009312| 0.519|
4| 8.00| 2.00| 0.012416| 0.689|
5| 10.00| 2.00| 0.015520| 0.761|

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