LAB 7 Flashcards
most abundant metal in earths atmosphere
iron
- mainly found as fe2+ or fe3+
Iron is usually present….
- in surface waters as salts
- containing fe3+ that are mostly insoluble and settle out or absorbed into surfaces
The aesthetic objective (AO) for iron in Ontario drinking water is
0.3ng/L
- ministry of environment 2006
- drinking above this can cause water to taste bad and can stain laundry and plumbing fixtures, prevent growth of microorganisms, leading to deposition of slimy coating in water system
An approved method by the Ministry of the Environment for the analysis of iron in water is
- based on the measurement of light absorbed by a highly coloured Fe2+ complex
- iron ion is surrounded by various Lewis bases called ligands: many are coloured, unlike aqueous ferrous ion itself
To determine the concentration of total iron in water, all of the iron in the sample must be
- all of the iron in the sample must be reduced to its Fe2+ oxidation state prior to complexation
- accomplished through REDOX using excess reducing agent (hydroxyl amine)
eqn 2:
4 Fe3+ + 2NH2OH → 4 Fe2+ + N2O + 4 H+ + H2O
-the concentration of iron in well-aerated waters is ….
- Concentrations of iron in
Canadian surface waters are
- seldom high
- BELOW 10mg/L
Under reducing conditions, which may exist in
- exist in ground waters, lakes, reservoirs, and absence of sulphide and carbonate
- high concentrations of soluble Fe2+ can be found
- presence attributed due to weathering of rocks, minerals, acidic water drainage, etc.
The ligand used to form the coloured complex in this
experiment is
- organic compound: 1,10 phenanthroline
- stable and highlight coloured orange red with 3 molecules of the 1,10 then to 1 iron
- each nitrogen atom will form a coordinate covalent bond for a total of bonds, weak base
- the equilibrium can be shifted towards complex formation carrying out the reactions
- reaction at ph 3-4 (acetate buffer in used to maintain the pH in this range)
- plus using an excess of phenanthroline
how to determine the concentration of total iron in ‘unknown’ water
samples
- measures the absorbance of the coloured iron-phenanthroline complex using a method called spectrophotometry (or colorimetry)
The amount of light absorbed, or transmitted through a sample is measured with an instrument
called a
- spectrophotometer or a colorimeter
- a light source and monochromator that selects light within a narrow band of wavelengths
- wavelengths pass through a cuvette
- intensity of light before entering Io and after passing is If measured with a detector
- ratio of If/Io is T
- negative log of T is A
The quantitative relationship between the amount of light absorbed by a sample at a selected
wavelength (λ) and the concentration of absorbing chemical species is summarized by BEER’S LAW
wavelength = εbc
- c is the concentration in molarity
- b is the path length in cm
- epsilon is called the molar extinction coefficient or the absorptivity constant which Is unique for a given species of a particular wavelength
If the absorbance of a solution is plotted against wavelength, then the absorption spectrum
- absortopion spectrum shows minima and maxima characteristics
- in figure 2 it showed yellow light which is complimentary to purple (of the purple ion)
Spectrophotometric methods are
- methods are common quantitative analysis techniques based on the absorption of light by an analytical species
number of photons a species absorbs is directly related to the
- the number of atoms, or concentration of absorbing scopes
