Editorial
Preface to the 2nd Edition, Spring 2019
In this second edition of the “Guide to Phototitration” Mettler Toledo presents a detailed manual on photometric titration, authored by Kees Mooibroek, former Senior Application Chemist in the Market Support Group Analytical Chemistry.
In addition to an in-depth overview of the fundamental principles, this Analytical Instrument Competence Guide explores an extensive collection of titration procedures for various metal ions, surfactant content determination, and turbidimetric titrations performed using Mettler-Toledo instruments.
The second edition revises, updates, and supports the original guide with all tools, procedures, and applications developed since its initial publication in 1978.
Preface to the 1st Edition, Spring 1978
This guide on phototitration methods has been prompted by two key intentions. The first was to assist operators of Mettler phototitrators in achieving optimal performance in their analyses. The second was to improve existing titration methods and to support the development of new ones. Among the assay reactions currently used in phototitration, the most important is the complexing reaction using metallochrome indicators.
Accordingly, this guide primarily focuses on this method, whose fundamentals were initially detailed by G. Schwarzenbach in 1945. The original publication of this author led to rapid development of the method, as demonstrated by the large number of articles that subsequently appeared in international analytical and chemical journals.
Today, analysts have access to a wide variety of titrating agents, indicators, and masking agents, sufficient for the assay of most cations and many ions. Not only this, but complexometry has enabled the indirect determination of numerous organic compounds.
Although visual end-point determination has potential, it is inherently disadvantaged by its precision, which relies on subjective evaluation of color changes. Errors may arise due to observer fatigue or the limited human capacity to recall and distinguish color shades reliably.
Additional inaccuracies in visual end-point determination result from difficulties in deciding when a color has reached maximum intensity and clearly recognizing color changes in colored solutions. The use of Mettler phototitrators prevents these subjective errors, allowing analyses to be conducted in open vessels in daylight using immersion probes.
In addition, Mettler phototitrators enable automation of assays and data recording. Another important feature of these titrators is that nearly all methods used for visual end-point determination can be applied.
Although significant increases in the variety of titrating agents, indicators, and masking agents available are unlikely, it should be noted that most of the fundamental data on the stability of complexes formed by these substances is still lacking. Consequently, not all of the potential combinations of these agents have yet been realized.
Further progress in these techniques can be expected, particularly through the many possibilities Mettler phototitrators offer. Users of this guide are encouraged to inform Mettler Toledo of any shortcomings encountered and to share any novel application techniques they have developed.
The author would like to express appreciation for the valuable assistance provided by colleagues in the Applications Laboratories, particularly Mrs. R. Brülhart, who conducted the majority of analyses, and Dr. P. Schauwecker, who compiled the chapter on evaluation of photometric titration curves.
Greifensee, Spring 1978
Kees Mooibroek
Photometric Titration: A Walkthrough Guide
A-1 Overview
This Guide to Phototitration is divided into two parts:
- a general section (A1 to A6), describing the fundamental principles of photometric titration,
and
- a practical section (B1 to B4), presenting a comprehensive collection of applications.
Basics
Chapter A-2-1 discusses the most important types of titration techniques suitable for photometric end-point determination. This section primarily addresses assay reactions where complexes between metal ions and organic ligands are formed, but also applies in principle to other assay reaction types.
Section A-2-2 summarizes the main categories of reactions employed as assay reactions in photometric titration. To better understand the topic, it is recommended to begin with section A-3-5, particularly A-3-5-1.
Section A-3 explains the theoretical backgrounds and instrumental aspects of phototitration. Chapter A-3-1 outlines the general principles of phototitration, followed by a historical overview of the first Mettler-Toledo Phototitrators DK18 (Scanning Phototitrator) and DK19 (Filter Phototitrator) in A-3-2. Chapter A-3-3 describes the modern approach to photometric titration, explaining the operating principle of the DP5 Phototrode™ along with selected applications.
Finally, the coupling of titration instruments and UV/VIS spectrophotometers reveals an interesting development: it enables automated UV/VIS measurements at varying pH values (A-3-4).
Complexometry finds its most significant application in the colorimetric titration assay of cations (A-3-5-1). Precipitation reactions, followed by back-titration (A-3-5-5), enable the indirect assay of a wide range of organic compounds.
Turbidimetric (heterometric) titration is considered an alternative approach when the course of the reaction is straightforward and precipitate formation is not interfered with (A-3-5-7). The turbidimetric titration of surfactants (A-3-3-7) closely resembles the colloidal titration technique.
Finally, in the two-phase titration (A-3-5-6, A-3-5-8), measurement depends on the appearance or disappearance of a specific phase, making this a physical rather than a chemical method. Examples of additional specialized methods include extractive phototitration and differential phototitration (A-3-5-9).
Section A-4 provides essential background information on evaluating photometric titration curves, while Section A-5 (pH, buffering, pH-stat technique) discusses the importance of optimum pH adjustment and methods for stabilizing the pH value in titration. Finally, section A-6 offers guidance on reagent use and the composition of calibration buffers.
Applications
Section B-2 describes the titration of selected pure substances, with each given example accompanied by an extensive bibliography. Section B-3 introduces several key assay techniques in tabular form, followed by a reference section concluding the guide.
Section B-4-1 contains general recommendations for conducting literature searches in the field of phototitration. Section B-4-2 presents literature reviews on topics not covered in sections A-3 and B-2. Literature references concern phototitration, titration with visual endpoint determination, sample preparation, and basic principles.
This information has been sourced, reviewed, and adapted from materials provided by Mettler-Toledo - Titration.
For more information on this source, please visit Mettler-Toledo - Titration.