In: Chemistry
A student prepared a set of standard nickel(II) solutions, varying in concentration, and processed them spectroscopically. The Beer’s Law curve that was obtained is shown below.
A. An unknown sample of nickel(II) was prepared in the exact same way as the standard solutions and analyzed spectroscopically. The sample produced an absorbance of 0.948 AU. What is the concentration of nickel(II) in the sample?
B Suppose the measured unknown sample was actually a dilution that had been prepared from a more concentrated sample to yield a solution that would measure within the range of the standard calibration curve. The dilution was prepared by adding 4.00 mL of the concentrated nickel(II) solution to a 10-mL volumetric flask and diluting to the mark. What is the concentration of the original nickel(II) solution?
C. Suppose the original concentrated nickel(II) solution had been prepared by dissolving a solid unknown sample, with a mass of 0.0785 g, into 500.00 mL. What is the % (wt/wt) of nickel(II) in the solid sample? (1.2)
Solution:
Beer’s law relates absorbance to both transmittance and to the concentration of the absorbing species (A = –logT = εbC).
Samples are atomized using thermal energy from either a flame or a graphite furnace. Because the width of an atom’s absorption band is so narrow, the continuum sources common for molecular absorption can not be used. Instead, a hollow cathode lamp provides the necessary line source of radiation. Atomic absorption suffers from a number of spectral and chemical interferences. The absorption or scattering of radiation from the sample’s matrix are important spectral interferences that may be minimized by background correction. Chemical interferences include the formation of nonvolatile forms of the analyte and ionization of the analyte. The former interference is minimized by using a releasing agent or a protecting agent, and an ionization suppressor helps minimize the latter interference.
Spectroscopic measurements may also involve the scattering of light by a particulate form of the analyte. In turbidimetry, the decrease in the radiation’s transmission through the sample is measured and related to the analyte’s concentration through an equation similar to Beer’s law. In nephelometry we measure the intensity of scattered radiation, which varies linearly with the analyte’s concentration.
A. Making appropriate substitutions into Beer’s law
A = 0.948 = εbC= (676 M–1 cm–1)(1 cm)C
and solving for C gives a concentration of 3.37×10-4 M.