Gravitational cells, measurement potential

To determine the E° of different cell arrangements, chemists use what are called standard reduction potentials for halfcells. A standard reduction potential is the electrical potential under standard conditions of a cell compared to the standard hydrogen electrode. The standard hydrogen electrode is a special half-cell that has been chosen as a reference to measure electrical potential. Just as sea level is a logical elevation for measuring gravitational potential,... 

The Boyle-Van t Hoff relation applies to the equilibrium situation for which the water potential is the same on either side of the two membranes surrounding a chloroplast. When T1 equals T°, net water movement across the membranes ceases, and the volume of a chloroplast is constant. (The superscript i refers to the inside of the cell or organelle and the superscript o to the outside.) If we were to measure the chloroplast volume under such conditions, the external solution would generally be at atmospheric pressure (P° =0). By Equation 2.13a (T = P — H, when the gravitational term is ignored), the water potential in the external solution is then... 

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