It is important to recognize at the outset that the work of determining stability constants is incomplete unless the results are published. Publication can take the form of a technical report, a section of a thesis, or publication in a scientific journal. Certain criteria are applicable whatever the destination of the report. The purpose of including this section in the step by step guide is to ensure that when an experiment is performed note is taken of all the information that will later be required for the report. The following paper contains the IUPAC guidelines for publishing stability constant data.

IUPAC. Reprinted from: Pure and Applied Chemistry, 61 (1989) 1161

Guidelines in the Publication of Stability Constant Data

by: Dennis G. Tuck

Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4.

A cursory view of the literature will show that the manner of reporting stability constants is far from constant. Given the importance of the results in question, the frequent absence of certain critical pieces of information is regrettable. This note addresses the details which should be provided by authors. The general approach follows a document prepared by Commission V.6. of I.U.P.A.C. and subsequently adopted by a number of international journals.

The following critical information should appear in any paper reporting stability constant measurements:

1. The full name of the ligand(s) according to I.U.P.A.C. nomenclature.

2. The purity of reagents and solvents, and the procedures followed in solvent or reagent purification.

3. The composition of the solution, especially the inert electrolyte and its ionic strength, and any other relevant factors including the range of metal and ligand concentrations investigated. For mixed solvent systems the solvent composition must be defined.

4. Where appropriate, the pH range over which measurements were made and, where appropriate, the Kw value assumed.

5. The instrument(s) (e.g. pH meter, electrodes, spectrophotometer, etc.) used in the experimental studies.

6. The method used for electrode calibration, e.g. as an [H+] (concentration) probe or, as is often the case with a glass electrode, calibrated as an aH probe using standard buffers.

7 The temperature and temperature range.

8. The number of datum points recorded in a titration (or other experiment, as appropriate), and the number of replicate measurements.

9. Clear definitions of the ligand stoichiometry, HnL, and charge and of the equilibria defined by Ki, *Ki, bbi or bp,q,r:

K2: NiL + L = NiL2b2: Ni + 2L = NiL2 bp,q,r: pNi + qH + rL = NipHqLr

Use of KXMXL nomenclature is discouraged.

10. The computer program, or other method of calculation, used to derive the final results from the experimental data, and a literature reference if the program is the work of others.

11. The range of results, the standard deviation on the final result, the sources of error, and the methods used in establishing these parameters.

12. Where appropriate, evidence of reaction reversibility (from back titrations) and supplementary evidence to potentiometric data (e.g. spectroscopic, e.s.r. or n.m.r. data) to confirm stoichiometries and coordination mode inferred from the numerical analysis.

Most stability constant measurements can be reported in brief tabular form if the points above are followed (see below) and any special comments placed in the text.

Finally, a discussion of the reasons which prompted the experiments, and of the chemical information subsequently derived from the measurements, would serve to make stability constant data part of chemistry as a whole, rather than a separate topic in itself. Similarly, comparisons with previous determinations on identical or similar metal ligand systems would help to place the work in a proper context. Stability constant measurements give useful quantitative chemical information, which can often be related to other structural and preparative work, and authors should show the relevance of their results.

Suggested Summary of Experimental Parameters:

Table Summary of experimental parameters for the system copper(II) - tyrosyl-glycyl-prolyl-tyrosine

Solution composition
[TL] range/mol dm-3 0.003-0.005
[TM] range/mol dm-3 0.003-0.005
I/mol dm-3 0.10 KNO3
pH Range 2.7-10.8 (protonation)

Experimental method pH titration, calibrated in concentrations

T/C 25.0
Total number of data points
Protonation 207 (2 titrations)
Copper complexation 274 (3 titrations)

Method of calculation SUPERQUAD reference( Hyperquad)

In appropriate tables:

Protonation constants
(ligand=H3L, errors as s in last place of decimals)
lg b (HL) 10.16(1)
lg b (H2L) 19.96(1)
lg b (H3L) 27.50(2)
lg b (H4L) 30.52(3)

Stability Constants
lg b (CuL) 10.29(9)
lg b (CuHL) 19.70(1)
lg b (CuH2L) 24.36(5)
lg b (Cu2L2) 24.00(5)
lg b (Cu2H-2L2) 3.66(2)

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