Cliffs AP Chemistry, 3rd Edition

The ratio of Itto Iocan be helpful in identification when unknown concentrations and character-
istics are compared to known values. The lowest percent transmittance (%T) is found at the
wavelength to which the sample is most sensitive. This wavelength is known as the analytical
wavelength, λmax. Once the analytical wavelength has been determined, then it is possible to
investigate or determine: 1) concentration of the solution (c); 2) pathway of light through the
solution (b); and 3) the molar absorptivity (ε) of the solution. When multiplied together, these
three variables provide the absorbance (A) of the solution, as expressed by Beer’s law:

A = εbc

The spectrophotometer used has two scales–absorbance (log scale, 0–2) and percent transmit-
tance (linear scale, 0–100). Most readings are taken from the transmittance scale and then con-
verted to absorbance through the relationship

A = 2.000 −log (%T)

Beer’s law allows us to determine concentration since absorbance is directly proportional to it.
To negate influences due to cuvette differences, or differences between sample tubes, a refer-
ence solution is made containing all of the components except the species being analyzed.
From this reference point, accurate absorbances can then be determined. This data is then plot-
ted (absorbance vs. known concentrations) and the unknown concentration then extrapolated
from the Beer’s law plot. Factors that limit this technique include: 1) sensitivity of the instru-
ment being used — usually best between 10 and 90%T, 2) the magnitude of the molar absorp-
tivity (ε), 3) fluctuations due to pH changes, and 4) temperature changes.

Scenario: A student reacted green nickel chloride hexahydrate (NiCl 2 ⋅6 H 2 O) with NH 3. A

solid bluish purple solid was produced:

Ni^2 +(.aq green)+ 2 Cl-(aq)+nNH (^33) ]aqg"Ni NH^ hnCl 2 (,s bluish purple)
However, NH 3 in water produces a small amount of hydroxide ion:
NH (^3) ]aqg++=H O (^2) ]lgENH+- (^4) ]aqg OH]aqgKb 175 10. # -^5
So the student simultaneously produced green Ni(OH) 2
Ni()aq 2 OH()aq! Ni OH()()s
2

• 2
  +-
  resulting in an impure synthesis. To maximize the yield of the coordination compound, the
  products were separated based on their solubilities by heating the products in a small amount of
  water at 60°C. Further treatment to ensure that no Ni(OH) 2 remained involved cooling the solu-
  tion mixture to 0°C and washing the product with cold ethanol, filtering out the crystals and fi-
  nally washing them with cold, concentrated NH 3. The crystals were then dried and weighed to
  determine yield of product.
  After the coordination compound was made, it was then analyzed to determine (1) the mass
  percent of NH 3 in the compound and (2) the mass percent of Ni2+in the compound.
  Laboratory Experiments