Spectrophotometry, Colour and Turbidity

Aim To distinguish between colour and turbidity. To know the principles behind the analytical instruments that

measure these parameters (Beer-Lamberts Law). To understand the relevance of these measurements to

Environmental Engineering. Reference Sawyer C.N., McCarty P.L., Parkin G.F.

Chemistry for Environmental Engineering

Outline Importance of Colour Importance of Turbidity

Colour Measurement Measurement of Turbidity

Spectrophotometry

Colour in Water

What is the cause True Colour (Dissolved Material)

– pH linked

Apparent Colour (Suspended Material)

Importance Aesthetic THM’s Industrial Effluent

Colour Measurement

By Eye Nessler Tubes Standard Solution (Potassium chloroplatinate)

Visual Comparison Devices Nessleriser

By Spectrophotometer Light absorbing character

Express Results in units Hazen True Colour, Apparent Colour

Spectrophotometry

Light is absorbed by species in solution

Transmittance

Transmittance T = I / I0

Optical Density or Absorbance A = log I0 / I

Sample

LAMPPhoto multiplier

I0 I

Spectrophotometry

With monochromatic light, following rules apply: Beer’s Law

A = k’c

Lambert’s Law

A = k’’ l

Beer-Lambert’s Law

A = k c l

Spectrophotometry

Colorimeter Filter produces a specific wavelength of light

Spectrophotometer Prism produces a very specific wavelength of light

Filter

Sample

LAMPPhoto multiplier

I0 I

Prism

Sample

LAMPPhoto multiplier

I0 I

Spectrophotometry

Single and Dual Beam Spectrophotometer Wavelengths between 200 - 400 nm UV

400 - 700 nm Visible

Standard Curve

Direct Measurement Natural Colour of Water (385nm)

Chromophore by Chemical Reaction numerous Anions, Cations, Organics

Importance of Turbidity

Aesthetic

Process Control Settled Secondary Effluent

– suspended cell growth

Water Supply (Sand Filtration) – Requirement for Coagulation

– Pre-Treatment

Disinfection

Lamp Photomultiplier

Burner

Gas + Sample

LightPath

Atomic Absorption Spectroscopy (AAS)

Turbidity of Water

Definition Turbidity is due to Suspended or Colloidal Particles.

Turbid samples affect light in two ways. Light may be Absorbed Light may be scattered

Refraction

InternalReflection

External Reflection

Absorbed

Turbidity - Measurement

Turbidity can be measured by: Spectrophotometer (Interference / Absorbance) Jackson Candle Turbidimeter (Interference / Absorbance) Secchi Disk (Interference / Absorbance) Nephelometry (Reflected Light)

Nephelometry

Sample

Photo multiplier

LightTrap

LAMPLens

Turbidity - Measurement

Units of Turbidity Nephelometric Turbidity Units (NTU)

Calibrate with: SiO2 Suspension (1mg/l)

Standard Formazin Suspensions

Bead Suspensions in a Gel

Precautions Sample Dilution Air Bubbles, Condensation, Stray Light Large Suspended Particles

Spectrophotometry, Colour and Turbidity

Summary

Definition and Origins of Colour, Its Measurement, and its Significance in EE.

Definition and Origins of Turbidity, Its Measurement, and its Significance in EE.

The general application of Spectrophotometry in water analysis.