How Perchlorate Was Discovered on Mars

Robotic arm of Phoenix Mars Lander scoops up soil for chemical analysis on Mars. (NASA photograph courtesy S. Kounaves, Tufts University.] 

Hach of four Wet Chemistry Laboratories on the Phoenix Mars Lander shown at the opening of Chapter 6 was equipped with 23 electrochemical sensors, of which 15 were ion-selective electrodes similar to those discussed in this chapter. The robotic arm delivered soil through the sieve into the beaker compartment. Then aqueous solution was added to leach soluble salts from the soil. Sensors measured ions appearing in the liquid. 

Nobody expected perchlorate (CIO4) to be abundant on Mars, so the Wet Chemistry Lab was not designed to look for CIO4. However, the nitrate ion-selective electrode sent to Mars was 1 000 times more sensitive to CIO4 than to NO  

In Chapter 14, we learned that the voltage of an electrochemical cell is related to the concentrations of species in the cell. We saw that some cells could be divided into a reference electrode that provides a constant electric potential and an indicator electrode whose potential varies in response to analyte concentration. 

Chemically inert platinum, gold, and carbon indicator electrodes are frequently used to conduct electrons to or from species in solution. In contrast with chemically inert elements, silver participates in the reaction Ag + e Ag(5). 

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