Norfloxacin [1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-quinoline carboxylic acid] is a — Physical Chemistry — Thermodynamics Chemistry Question

Norfloxacin [1-ethyl^-6-fluoro^-1,4-dihydro^-4-oxo^-7-(1-piperazinyl^-3-quinoline carboxylic acid] is a synthetic fluoroquinolone antibacterial agent of broad spectrum characterized by high activity against many gram-positive and gram-negative bacteria. As a result of constantly increasing administration, fluoroquinolones are expected to enter environment via waste waters. These substances are capable of bringing serious harm to aquatic life even if present in marginal concentrations mainly by giving rise to drug-resistant bacteria.

Development of advanced oxidation processes leading to transformation of fluoroquinolones in water is an important task of contemporary chemistry.

Recently it was found that Norfloxacin (NF) can be oxidized by potassium permanganate in alkaline medium according to the following reaction:

[VISUAL]

According to the mass action law, the reaction rate can be expressed as:

r = k_obs * [NF]^v₁ * [MnO4^-]^v₂ * [OH^-]^v₃

where [NF], [MnO^4-], [OH^-] are the concentrations of Norfloxacin, permanganate and alkali, respectively, and v₁, v₂, v₃ are reaction orders with respect to the corresponding reagents.

In this task you will determine the reaction orders with respect to the involved reagents following the reaction progress photometrically at 525 nm, the wavelength of maximum absorbance of permanganate.

Chemicals and reagents:
- Potassium permanganate
- Sodium hydroxide
- Sodium perchlorate
- Norfloxacin

Equipment and glassware:
- Analytical balance (± 0.0001 g)
- Visible spectrophotometer (or photometer with fixed wavelength of 525 nm) equipped with thermostated cell holder
- Spectrophotometer cells with 3.5 cm optical path length (you will need to recalculate concentrations of the reagents if you use cells with different optical path length)
- Tissue to clear cell surfaces
- Thermostat
- PC (or other computing techniques) with Microsoft Excel software (English version)
- Volumetric flasks, 100 cm₃ (4 ea.)
- Graduated cylinder, 100 cm₃
- Volumetric pipettes, 1, 2, 5, and 10 cm₃
- Pipette filler
- Spatula

Procedure:

A. Preparation of stock solutions
Read the procedure to the end and calculate the concentrations of stock solutions of potassium permanganate, sodium hydroxide, sodium perchlorate, and Norfloxacin that will allow easy preparation of the reaction mixtures to be analyzed. Prepare the stock solutions of potassium permanganate, sodium hydroxide and sodium perchlorate in the volumetric flasks according to standard procedures. Norfloxacin suffers from poor solubility in water at neutral pH values, thus first dissolve it in a small amount (about 6 cm₃) of 0.3 mol dm^-3 alkali solution (in the volumetric flask) and then add water to attain the required concentration. Do not forget to account for the amount of alkali introduced with the Norfloxacin solution when preparing the reaction mixtures. Close the flasks containing the stock solutions with stoppers and place them into the thermostat pre-adjusted to 25 °C.

Note: Norfloxacin is commercially available as a pure reagent. Also, it can be found as an ingredient in medicines. In the latter case, it is recommended to check that the other ingredients do not interfere with Norfloxacin when it is being oxidized by potassium permanganate.

B. General design of the task
The experimental work consists of three parts as can be seen from the table below:
- Part 1: Concentration of KMnO₄ is varied, concentrations of all other reagents being constant (lines 1 to 5);
- Part 2: Norfloxacin (NF) concentration is varied, concentrations of all other reagents being constant (lines 6 to 10);
- Part 3: Concentration of NaOH is varied, concentrations of all other reagents being constant (lines 11 to 16).

Table of Reaction Mixtures:
Line | c(MnO4^-) * 10^4 (mol dm^-3) | c(NF) * 10^3 (mol dm^-3) | c(OH^-) * 10^2 (mol dm^-3) | c(ClO4^-) * 10^2 (mol dm^-3) | k_obs
1 | 0.4 | 1.0 | 5.0 | 5.0 |
2 | 0.8 | 1.0 | 5.0 | 5.0 |
3 | 1.0 | 1.0 | 5.0 | 5.0 |
4 | 2.0 | 1.0 | 5.0 | 5.0 |
5 | 4.0 | 1.0 | 5.0 | 5.0 |
6 | 1.0 | 0.8 | 5.0 | 5.0 |
7 | 1.0 | 1.0 | 5.0 | 5.0 |
8 | 1.0 | 4.0 | 5.0 | 5.0 |
9 | 1.0 | 6.0 | 5.0 | 5.0 |
10 | 1.0 | 8.0 | 5.0 | 5.0 |
11 | 1.0 | 1.0 | 1.0 | 5.0 |
12 | 1.0 | 1.0 | 2.0 | 5.0 |
13 | 1.0 | 1.0 | 5.0 | 5.0 |
14 | 1.0 | 1.0 | 6.0 | 5.0 |
15 | 1.0 | 1.0 | 8.0 | 5.0 |
16 | 1.0 | 1.0 | 10.0 | 5.0 |

C. Determination of the reaction orders
1. Adjust the temperature of the thermostated cell unit to 25 °C.
2. Place the flasks with the stock solutions in the thermostat and let the solutions attain the desired temperature. Check the temperatures inside the flasks from time to time with the thermometer. Always carefully wash the thermometer with water before placing it in the next flask.
3. When ready with the temperature of the solutions, use the spectrophotometer cell to prepare the mixture containing KMnO₄, NaOH, and NaClO₄ in concentrations as indicated in the corresponding line in the Table. Add the Norfloxacin solution last (note that its introduction initiates the reaction) and promptly place the cell into the thermostated cell holder.

Note: If a thermostated cell holder is not available, fulfill the task at room temperature having in mind that temperature alterations will produce only slight effect on the results of kinetic studies.

1. Immediately start recording the absorbance at the wavelength of 525 nm. Continue measuring till the absorbance remains constant (A_infinity).
2. Plot the obtained data as the dependence of log(At - A_infinity) on time (At is the reaction mixture absorbance at time t).
3. Carefully wash the cell with plenty of water, dry the cell and wipe the walls with clean tissue.
4. Use the initial straight part of the curve to calculate the observed rate constant of Norfloxacin oxidation. Write down the value in the Table.
5. Repeat steps 3-7 for all the lines of the Table.