CHM 114 Spectrometric Analysis of Iron: Application of Beer's Law

redox reaction

2 Fe3+ + 2 NH2OH HCl + 2 OH- = 2 Fe 2+ + N2 + 2 H+ + 4 H2O + 2 Cl-

for the blank sample, the only reagent (chemical) omitted is what?

Fe 2+

1

Fe 2+ = 0.0 NH4 AC = 5 NH2OH HCL = 5 phen = 10

2

Fe 2+ = 1.0 NH4 AC = 5 NH2OH HCl = 5 phen = 10

3

Fe 2+ = 2.0 NH4 AC = 5 NH2OH HCl = 5 phen = 10

4

Fe 2+ = 4.0 NH4 AC = 5 NH2OH HCl = 5 phen = 10

5

Fe 2+ = 8.0 NH4 AC = 5 NH2OH HCl = 5 phen = 10

6

Fe 2+ = unknown NH4 AC = 5 NH2OH HCl = 5 phen = 10

equation of the lab

Fe(H2O)6 2+ + 3 phen = Fe(phen)3 2+ + 6 H2O almost colorless + colorless = intense orange-red

low concentrations of iron in water becomes

a problem because it "settles out" as insoluble rust

some of these ions can

be used for qualitative or quantitative analysis of a particular metal ion

spectrophotometer can

be used to measure quantitatively how much light of a specific wavelength passes through your solution

in the procedure, the unknown is compared to

four standard solutions containing 0.5, 1.0, 2.0, and 4.0 ppm of iron respectively

the reaction can occur with

other metals besides Fe, but these will generally produce a different color

b

path length in cm

there are two common units of measure

percent transmittance (% T) absorbance (A)

you should NOT

plot a point of zero absorbance vs zero ppm on your graph, as this is not experimentally determined

this method is often called

colorimetric or spectrometric

3 phen

colorless

A =

log (100 / %T)

ten angstroms

1 millimicron = in a nanometer

absorbance maximum of your complex occurs around

510 nanometers

beer's law

A = ebc

on the other hand, if all of the light is absorbed, then

A = infinity and %T = 0%

If solution concentration is measured in units of molarity (M), path length of the cuvette in cm, and absorbance is a dimensionless number, then what are the units of molar absorptivity implied in the beer's law equation?

L/mol cm L/mol cm

referring to the table on page 87, in line 2, what is the final concentration of Fe 2+ in ppm of the standard prepared in that step? for this question, assume that you added exactly to volumes of all reagents as listed in the table.

M1V1 = M2V2 50(1) = (21)(M) M = 50/21 2.381 ppm

suppose that for the standard prepared on Line 4, a total of 4.12 mL of the ion stock solution was added to the volumetric flask instead of exactly 4.00 mL. What would the final concentration of iron be in ppm?

M1V1 = M2V2 50(4.12) = 24 (M) 50(4.12) / 24 8.541 ppm

soluble hydrochloride

NH2OH HCl

the reaction between iron (II) ion and three molecules of ligand requires

about 45 minutes for completion

A

absorbance

the values are volumes of reagent that are

added to make the blank, the four standards and your empty sample

this is accomplished by

adding hydroxyl amine, as the soluble hydrochloride, NH2OH HCl, to the five solutions containing iron, i.e. the four standards and the unknown

while water containing low levels of iron may appear colorless to the eye

addition of an excess of 1, 10 phenanthroline (appreciated phen) will produce a bright color

Fe(H2O)6 2+

almost colorless

the system is buffered with

ammonium acetate

it has been established that

an iron concentration as low as 0.3 ppm will eventually produce such stains

you must

calculate the actual ppm for each of the standards based on the amount of Fe 2+ you actually added from the buret

in this experiment, you will learn how to

conduct quantitative measurements of trace metals on the ppm or parts per million basis

comparing all the iron solutions against the blank will

correct for any such interference from other metals

the selection of one wavelength of light and the use of matched colorimeter tubes, called _____, produces __________

cuvettes the situation in which the absorbance is directly proportional to the concentration

the success of this experiment depends on

determining a ligand that will react specifically with iron

thus, even without measuring the molar absorptivity

direct comparison of the absorbance of your unknown to the absorbance of your standards will allow you to interpolate the value of your unknown

iron

forms several intensively colored complex ions

Fe(phen)3 2+

intense orange red

these two are

interrelated since A = log (100 / %T)

a blank reaction containing every reagent except iron

is also run

it is called colorimetric or spectrometric because

it depends on measuring the "intensity" of the color produced which is related to the concentration by Beer's law

Two greek letters, e and lambda, were introduced in this lab write up which of these is the accepted symbol for wavelength of light?

lambda

e

molar absorptivity

what does the other greek Letter represent in this experiment?

molar absorptivity

c

molar concentration

the concentrations of iron used in this experiment (from 0.5 to 4.0 ppm) are

so low that their quantitative analysis requires the use of a highly sensitive of a highly sensitive colorimeter, i.e. a spectrophotometer

in order to determine total iron present, which could be Fe 2+ or Fe 3+

the Fe 3+ must also be reduced

what would happen to absorbance (A) if the solution was diluted from 1 M to 0.25 M?

the absorbance would be lower

procedure

the reaction between the 1,10 phenanthroline and the iron (II) hexahydrate requires some time for completion this means that the preparation of the six solutions - the blank, the four standards, and the unknown - has first priority after the instrument is turned on for its 30 minute "warm up" period the following table lists some values of the six 100 mL volumetric flasks the four standards are prepared from a stock solution which contains 50 mg of Fe per liter which by definition is 50 ppm the iron solution should be measured using a buret; the other three can be measured with a graduated cylinder your unknown (X) should be completely transferred from its bottle, with the help of rinsing, to the volumetric flask add distilled water to the calibration mark of each flask mix them well to develop the characteristic orange red color of the iron (II) phenantholine complex ion allow the color to develop for 45 minutes when the instrument has completed its 30 minute "warm-up" period, set the transmittance to zero using the left knob then set the wavelength to 510 nm if you have forgotten the basic set up and operation procedures for the spectrophotometer, refer back to the "kinetics" experiment earlier in the semester there are ten angstroms, or one millimicron in a nanometer transfer, using a beaker, enough of the blank into one of the cuvettes so that the solution is just below the the B and L insignia insert this into the cell holder with the white line in line with the raised line at the front of the holder use the right hand knob to set the absorbance to zero in this manner, any absorption by materials other than the complex ion, including the cuvette, has been cancelled when the 45 minute "color developing period" has been completed, fill each of the remaining five cuvettes to the same level, with the five solutions containing iron record their absorbances in your laboratory notebook plot the values of the four standards on graph paper with the absorbances on the ordinate and the ppm of iron on the abscissa then, using a transparent plastic ruler, draw the best straight line that will fit those four points it must be noted that in very dilute solutions that line will probably not intersect the origin mark the absorbance value of the unknown on that line and drop a perpendicular from that point to the abscissa you have now established the ppm of iron in your unknown empty the volumetric flasks and rinse several times repeat the rinsing process with six cuvettes and store them, inverted, in the plastic rack

while this reaction is shown as an equilibrium reaction

the reducing agent will drive the reaction > 99.99% to the right

this is based on

the specific interaction of a ligand and metal to form a complex

if none of the light passing through the sample is absorbed

then %T = 100% and A = 0

what is the purpose of adding the hydroxyalamine hydrochloride in the solutions analyzed in this experiment?

to reduce Fe 3+

the blank sample is

used only to calibrate the spectrophotometer

you should be able to

visually guesstimate the concentration of your iron unknown by comparing the color of your unknown's solution to the color of the four standards

abcissa

x

ordinate

y