QC II LAB - Module 4: Colorimetric Determination of Iron in Ferrous Sulfate Tablets

Spectroscopy

How light interacts with matter

Spectrophotometry

Method of analysis in which the amount of electromagnetic radiation is absorbed by a sample solution is relative to the concentration of the analyte in the solution

Frequency

The number of electromagnetic waves that pass a fixed point in a second

• 1/x

Colorimetry

• Operates in the visible region (380-780 nm)

• “Visible Spectrophotometry”

• Samples that can be used in _____ include highly-colored substances

• A solution transmits a color that is complementary to the color of light it absorbs

Charge-Coupled Devices (CCD)

• The most common high performance imaging detector

• Significant in low light level conditions

• Based on a metal-oxide-semiconductor (MOS) structure

• Highly-sensitive

• Detects low intensity signals

• Has lower noise

Elements or Pixels

Individual sensors in CCDs

Photomultiplier tube

Used for detection in older types of UV/visible spectrometers

Photodiode Array (PDA)

Consists of series of photodiode detectors positioned side by side on a silicon crystal

Diode Array Instruments

• Can measure multiple wavelengths simultaneously

• Slightly less sensitive compared to CCD

Polychromator

Disperses light so that it falls on the diode array, which measures the whole range of the spectrum at once.

Transmittance

Ratio of the power of radiant energy exiting the sample, to that incident on the sample from the source

Formula:

• PT/PO

  • PT = radiant power transmitted by the sample

  • PO = radiant power from the source

Absorbance

An expression of the attenuation of radiant energy

Formula:

• -logT

• -log PT/PO

• log PO/PT

Beer-Lambert Law

E: Molar Absorptivity

b: Pathlength

c: Concentration

Principle of the Experiment

• Formation of a colored complex between ferrous ion (Fe2+) and 1, 10-phenanthroline.

  • Orange-Red Complex

• The intensity of the color is proportional to the concentration of ferrous ion in the solution.

1, 10-Phenanthroline

Chromogenic agent that forms colored complex with Ferrous ion (Fe2+)

Hydroxylamine HCl

Reducing agent that converts Fe3+ → Fe2+

Acetate Buffer (pH 4.5)

Maintains pH preventing oxidation of Fe2+

Decreases

As the Absorbance increases, the Transmittance _____

Yellow-Green

Absorbed Color:

Violet

Transmitted Color:

?

Yellow

Absorbed Color:

Blue

Transmitted Color:

?

Violet

Absorbed Color:

Green

Transmitted Color:

?

Blue

Absorbed Color:

Yellow

Transmitted Color:

?

Green-Blue

Absorbed Color:

Orange

Transmitted Color:

?

Blue-Green

Absorbed Color:

Red

Transmitted Color:

?