Effect of Concentration on Cell Potential

In a voltaic cell where all ions have a concentration of 1M, the cell potential is equal to the standard potential. For cells in which ion concentrations are greater or less than 1M, as shown below, an adjustment must be made to calculate cell potential. That adjustment is expressed by the Nemst equation  

In this equation, n is the number of moles of electrons transferred in the reaction, and x and y are the coefficients of the product and reactant ions, respectively, in the balanced half-cell reactions for the cell. 

Write the two half-reactions and the overall cell reaction for the cell shown above. 

Use Table 21-1 in your textbook to determine the standard potential of this cell. 

Calculate the cell potential for the ion concentrations shown in the cell. 

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In Chapter 20, we discussed the relationship of useful work, free energy, and the equilibrium constant. In this section, we examine this relationship in the context of electrochemical cells and see the effect of concentration on cell potential. 

The Nernst equation, expressing the effect of concentration on cell potential... 

The Nernst Equation Effect of Concentration on Half-Cell Potential (Ref 3, 6)... 

Standard Cell Potential and K Effect of Concentration on E ei Changes in Eoen During Cell Operation Concentration Cells... 

The Nernst equation has also been introduced in Chapter 20 to give a quantitative explanation of the effect of concentration on the value of electrode potential for a given half-cell. 

Tamplin et. al. (54) observed that V. cholerae and A. hydrophila cell extracts contained substances with TTX-like biological activity in tissue culture assay, counteracting the lethal effect of veratridine on ouabain-treated mouse neuroblastoma cells. Concentrations of TTX-like activity ranged from 5 to 100 ng/L of culture when compared to standard TTX. The same bacterial extracts also displaced radiolabelled STX from rat brain membrane sodium channel receptors and inhibited the compound action potential of frog sciatic nerve. However, the same extracts did not show TTX-like blocking events of sodium current when applied to rat sarcolemmal sodium channels in planar lipid bilayers. 

We will now look at the effects of Ej on thermodynamic calculations, and then decide on the various methods that can be used to minimize them. One of the most common reasons for performing a calculation with an electrochemical cell is to determine the concentration or activity of an ion. In order to carry out such a calculation, we would first construct a cell, and then, knowing the potential of the reference electrode, we would determine the half-cell potential, i.e. the electrode potential E of interest, and then apply the Nemst equation. 

The effects of digitalis on the electrical properties of the heart are a mixture of direct and autonomic actions. Direct actions on the membranes of cardiac cells follow a well-defined progression an early, brief prolongation of the action potential, followed by shortening (especially the plateau phase). The decrease in action potential duration is probably the result of increased potassium conductance that is caused by increased intracellular calcium (see Chapter 14). All these effects can be observed at therapeutic concentrations in the absence of overt toxicity (Table 13-2). 

In order to confirm the possible interaction of ethanol and crocin on NMDA receptors, we also performed whole-cell patch recording with primary cultured hippocampal neurons and measured membrane currents induced by the application of NMDA in a voltage-clamped condition. Application of 100 pM NMDA induced an inward current of 100.2 9.8 pA (n=10) at a holding potential of -60 mV. The NMDA-induced inward current was not affected by 10 pM CNQX (data not shown), but was completely abolished by 30 pM APV, supporting the fact that the response was mediated by NMDA receptors. Ethanol inhibited NMDA-induced currents in a concentration-dependent manner. Crocin (10 pM) had no effect on NMDA-induced currents by itself (data not shown), but attenuated the inhibitory effect of ethanol on NMDA-induced currents. The concentration-effect curve for ethanol was shifted to the right by the presence of crocin [22]. 

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