**4.5 Partial carbon dioxide pressure (PCO2)**

Partial carbon dioxide pressure of crater lake water is the partial pressure of carbon dioxide in equilibrium with lake and saturated atmospheric pressure. PCO2 is based on the equilibria of carbon specie in solution. The PCO2 plot for the two year observation for the Pidong Lake shows zit is constantly and several orders of magnitude higher than air saturated water pressure except June, 2013 (**Figure 10**). The highest PCO2 of 4.5 X 10−2 and 2.5 X 10−2 bars were observed in March and November, 2014 respectively and correlate with highest decreases in pH from 7.38 to 6.71 in November 2014. The lowest decreases in pH were observed during lake watercolor change activities (**Table 2**). Generally, increases in pH and PCO2 were observed to be linked with decrease in pH and lake color change activities.

**Figure 9.** *Gibbs diagram for Pidong crater Lake.*

**Figure 10.** *PCO2 versus time for Pidong crater Lake.*

*Hydrogeochemistry of the Pidong Crater Lake, Jos Plateau Volcanic Province, Nigeria… DOI: http://dx.doi.org/10.5772/intechopen.99720*
