47. Principle of spectrophotometry, areas of application: determination of the Kd value of the phenol red indicator

Question 1

What is spectrophotometry

Answer 1

The tool in which the material structure and / or concentration of the solutions to be tested can be detected by light absorption.

Question 2

What happen during spectrophotometry?

Answer 2

a so-called monochromatic light beam with same intensity is passed through our test substance which is contained in a small device of the same layer thickness (called a cuvette).

Question 3

The theory of spectrophotometry

The absorbed light can be quantified either by (1)___ or by (2)___ , also known as (3)_____

Answer 3

1. transmission
2. extinction
3. optical density (density: O. D.)

Question 4

The theory of spectrophotometry

Which law is used? how to apply?

Answer 4

Knowing these, using the Lambert- Beer law, the concentration of a solution of unknown concentration can be determined by the following formula (Figure 1)

  c  d = log(Io/I)

Question 5

The theory of spectrophotometry

The measurement is performed with a ___

Answer 5

spectrophotometer

Question 6

The theory of spectrophotometry

the tests be performed with (1)____ light of the appropriate (2)____

Answer 6

1. monochromatic
2. wavelength

Question 7

What is phenol red?

Answer 7

a weak acid that can act as an acid-base indicator during acid-base titration.

Question 8

Describe color of phenol red

Answer 8

While in the protonated state (acidic medium) it is yellow, its deprotonated form (alkaline medium) is red

Question 9

How to calculate deprotonated concentrations in phenolic red solutions?

Answer 9

c_protonated = c_total - c_deprotonated

Question 10

How to determine pK value of the phenol red indicator by spectrophotometry

Answer 10

• Calculate deprotonated concentrations in phenolic red solutions
  
  • c_protonated = c_total - c_deprotonated
• Determine the pK value of the phenol red solution can be determined knowing the Henderson-Hasselbalch equation.

Question 11

Determination of the pK value of the phenol red indicator by spectrophotometry

Steps

Answer 11

1. Prepares test tubes with phenol red, distilled water and buffer
2. Using photometer to determine the optical densities
3. Choose the wavelength where the O.D is the greatest
4. Find the concentration of deprotonated based on Lambert Beer Law
5. Find the pH of phenol red solution using the Henderson-Hasselbalch equation

Question 12

Determination of the pK value of the phenol red indicator by spectrophotometry

What are the materials?

Answer 12