Standard electrode potential

What is meant by the following:

Electrode potential for $$\ce{M^3+/M}$$

Is it standard oxidation potential (SOP) or reduction potential (SRP)? What does a high positive or negative value for SRP and SOP tells us about the reactivity of the element?

This is standard reduction potential or standard electrode potential $$E^\circ$$. More positive values refer to the strongly oxidizing agents, and more negative ones – to the strongly reducing agents. In order to convert SOP into SRP, reverse the sign. Examples and quotes from Atkins' Elements of Physical Chemistry [1, p. 209]:

\begin{align} \ce{F2 + 2 e- &-> 2 F-} \quad &E^\circ = \pu{+2.87 V}\\ \ce{Li + e- &-> Li+} \quad &E^\circ = \pu{-3.05 V} \end{align}

The standard potential, $$E$$ (Ox/Red), of a couple Ox/Red is then measured by constructing a cell in which the couple of interest forms the right-hand electrode and the standard hydrogen electrode is on the left.

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A brief comment. Standard potentials are also called standard electrode potentials and standard reduction potentials. If in an older source of data you come across a ‘standard oxidation potential’, reverse its sign and use it as a standard reduction potential.

References

1. Atkins, P. W.; De Paula, J. Elements of Physical Chemistry, 5th ed.; Oxford University Press: Oxford ; New York, 2009. ISBN 978-1-4292-1813-9.