A Practical Guide to Instrumental Analysis

Erno Pungor, G. Horvai

• Editorial: CRC Press
• Año: 2020
• ISBN:
  9781000142440
• Edición: 1
• Categoría: Physical Sciences
• Categoría: Chemistry

A Practical Guide to Instrumental Analysis covers basic methods of instrumental analysis, including electroanalytical techniques, optical techniques, atomic spectroscopy, X-ray diffraction, thermoanalytical techniques, separation techniques, and flow analytical techniques. Each chapter provides a brief theoretical introduction followed by basic and special application experiments. This book is ideal for readers who need a knowledge of special techniques in order to use instrumental methods to conduct their own analytical tasks.

Part I Electroanalytical Techniques

Chapter Potentiometry

1

Principle of the Technique

Techniques based on the measurement of potential of a sensor are termed potentiometry. In a Potentiometric type of sensor a membrane or sensor surface (indicator electrode) acts as a half-cell, generating a potential proportional to the logarithm of the analyte activity (concentration). The indicator electrode is connected directly (or through a salt bridge) with a reference electrode to form a galvanic cell. The electromotive force (EMF) or cell voltage of the galvanic cell thus obtained is measured in such a manner that no current flows through the cell. For a complete electrochemical cell the cell voltage is

${\text{E}}_{\text{cell}}={\text{E}}_{\text{ind}}-{\text{E}}_{\text{ref}}+{\text{E}}_{\text{j}}$

where E_cell is the cell voltage or the EMF, E_ind is the potential of the indicator electrode, E_ref is the potential of the reference electrode, and E_j is the liquid-junction potential.

The potential of the indicator electrode is given by the following equation:

${\text{E}}_{\text{ind}}=\text{Constant}+2.303\text{RT}/\text{zF}\log \text{a}$

where 2.303RT/zF = Nernst factor (its value at 25°C and in the case of monovalent ions is 59.16 mV), z = charge of the ion measured, and a = activity of the ion measured.

Potentiometric methods belong to two major types:

1. Direct potentiometry. Direct measurements of concentrations or activities. In this case the ion activity or ion concentration is determined by means of a calibration curve or the standard addition technique.

2. Indirect potentiometry or Potentiometric titration. The component to be determined is titrated with a suitable titrant and the indicator electrode is used to follow the changes in potential in the course of the titration.

1.1. Determination of pH in Fruit Juice

Principle of the Determination

The term pH is simply a mathematical symbol of convenience, widely accepted and firmly established for expressing the acidity of aqueous solutions. It is the negative logarithm of activity of the hydrogen ion:

$\text{pH}={\text{pa}}_{{\text{H}}^{+}}=-\log {\text{a}}_{{\text{H}}^{+}}$

where a_H⁺ = f₊[H⁺] (f₊ = single ion activity coefficient).

However, single ion activity coefficients cannot be measured directly; only the mean activity coefficients are available (f±).

Since the pH values defined by this formula cannot be determined exactly, it was necessary to introduce the practical (“operational”) pH scale, which is based on standards. The most commonly used pH scale is the NBS (National Bureau of Standards) pH scale.

For the determination of the practical pH values, the following equation can be used:

$\text{pH}(\text{x})=\text{pH}(\text{s})+(\text{Ex}-\text{Es})/2.302{\text{RTF}}^{-1}$

where pH(x) is the pH of the test solution and pH(s) is the pH of the standard.

Ex and Es are the EMF values measured in the test solution and in the standard, respectively. It has been suggested that in practical pH determinations with a glass electrode the best approach is to use two standards from the NBS scale, one below and one above the pH value of the test solution. The EMF of the cell containing test solution x (E_x):

$\text{glass electrode}\mid \text{solution}\mid \text{reference electrode}$

is measured and the EMF values E₁ and E₂ are similarly measured using the same cell containing standard solutions S₁, and S₂, respectively, having pH values pH₁, and pH₂ on either side as near as possible to the pH of x (=pHx).

$\left({\text{pH}}_{\text{x}}-{\text{pH}}_1\right)/\left({\text{pH}}_2-{\text{pH}}_1\right)=\left({\text{E}}_{\text{x}}-{\text{E}}_1\right)/\left({\text{E}}_2-{\text{E}}_1\right)$

Apparatus

mV- pH meter

Glass electrode

Reference electrode (Ag/AgCl or calomel)

Chemicals

pH buffers

Sample

Fruit juice

Procedure

Prepare the pH buffer solutions and measure the Emf values in the standards and the test solutions. Repeat the measurements on the pH scale after adjusting the pH meter to the pH value corresponding to the pH of one of the standards (e.g., pH = 7), then to the pH value of a second standard (e.g., pH = 2). Finally, the pH of the sample is measured.

Evaluation

Calculate the pH on the basis of the millivolt measurements and compare it with that obtained by direct pH measurement.

1.2. Determination of Fluoride in Toothpaste with Fluoride Ion-Selective Electrode Using Standard Addition

Principle of the Determination

The solid-state fluoride electrode has found extensive use in the determination of fluoride in a variety of materials. A total ionic strength adjustment buffer (TISAB) is used to adjust all unknowns and standards to essentially the same ionic strength; therefore, the concentration of fluoride can be measured and evaluated. The pH of the buffer is about 5, a level at which F⁻ is the predominant fluorine species. The buffer also containes complexing agent, which forms stable chelates with iron(III) and aluminum(III), thus eliminating their possible interference.

Solutions

1. TISAB. Buffer solution for 15 to 20 determinations can be prepared by mixing (with continuous stirring) 57 ml of glacial acetic acid, 58 g of NaCl, 4 g of cyclohexylamine dinitrilo-triacetic acid, and 500 ml distilled water in 1-1 beaker. Cool the content and add 6 M NaOH until a pH of 5.0 to 5.5 is reached. Dilute 1 1 with water and store in a plastic bottle.

2. Standard fluoride solution (2 × 10⁻¹ mol/l). Dry a quantity of NaF at 110°C for 2 h. Cool in a desiccator, then weigh the appropriate amount of salt into a 11 volumetric flask. (Caution! NaF is highly toxic.) Dissolve in water, dilute to the mark, mix well, and store in a plastic bottle.

Apparatus

pH meter

Solid-state fluoride electrode

Reference electrode (Ag/AgCl double junction or calomel electrode)

Magnetic stirrer

Procedure

1. Determination of the slope of the fluoride calibration curve. Prepare 100, 100-ml calibration solutions in the concentration range of 10⁻⁵ to 10⁻² mol/l fluoride containing fifty 50-ml TISAB buffer. From the calibration curve the (E vs. −log c_F−) the slope is determined.

$\text{S}=\text{ΔE}/\text{Δ}(-\log \text{c})$

2. Determination of fluoride in toothpaste. Weigh about 1 g of toothpaste into a 250-ml beaker. Add 50 m...