NOVELTY - Fluid diffusion testing apparatus comprises a fluid capable of undergoing oxidation or reduction, a half-cell electrode assembly capable to receive the fluid, and a change in acidity indicator in communication with the assembly.    USE - The apparatus is used for testing a fluid diffusion assembly (claimed). It is used for evaluating individual components of a fuel cell and assemblies of at least two components of the fuel cell. It is used for visualizing the gas diffusion activity in proton exchange membrane half-cell reactions, and for focusing on parameters related to spatial visualization of gas diffusion. It may also be used to evaluate material property differences as can be seen upon comparison of material types, e.g. carbon fiber paper versus flexible graphite sheets, to predict morphology differences, e.g. micro porosity and pore size distribution, and to evaluate differences in the diffusion of gas and subsequent delivery to the catalyst layer.    ADVANTAGE - The invention accelerates the state of the art understanding of component functions within an operating fuel cell, and provides a tool to assist in rapidly commercializing the fuel cell through optimized component integration.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (a) a method of testing a fluid diffusion assembly, which comprises passing a fluid capable of undergoing oxidation or reduction through a half-cell electrode assembly to form a sample, contacting the sample with an indicator, and detecting a change in acidity in the indicator;    (b) a method for selecting a fluid permeable element for a proton exchange membrane fuel cell, which comprises passing a fluid capable of undergoing oxidation or reduction through a half-cell electrode assembly to form a sample, contacting the sample with an indicator, detecting a change in acidity in the indicator, observing at least one concentration gradient of the sample in the indicator, conducting these steps on half-cell electrode assemblies, and selecting the half-cell electrode with the most uniform concentration of the sample in the indicator;    (c) a method of visualizing gas diffusion, which comprises passing a fluid capable of undergoing oxidation or reduction through a half-cell electrode assembly to form a sample, contacting the sample with an indicator, and observing at least one concentration gradient of the sample in the indicator; and    (d) a fuel cell testing composition of matter comprising a pH sensitive component and a host material.    DESCRIPTION OF DRAWING(S) - The figure is a side schematic representation of a fluid diffusion testing apparatus.