Figure E-2 Species distribution diagram showing the fraction of each species (CO₂, HCO3 and CO) as a function of pH change in water
4. Instead of using a titration procedure, the alkalinity of natural water can also be calculated if the pH and either bicarbonate or carbonate concentration is measured. The formula is:
Carbonate alkalinity (mg CaCO3/L) = 0.820 [HCO3, mg/L] +1.667 [CO, mg/L]
where the coefficients 0.082 and 1.667 are the ratios of equivalent weight of CaCO3 (100) to HCO3 (61.0) and CaCO3 to CO (30.0), respectively. The key to such calculation is species distribution diagram shown in Figure E-2, where [CO] can be calculated from given pH and [HCO3] or vice versa. Calculate the alkalinity if a groundwater sample has a pH of 8.5 and HCO concentration of 275 mg/L.
To calculate the alkalinity of the groundwater sample, we will use the given formula and the species distribution diagram in Figure E-2.
Step 1: Calculate the carbonate concentration ([CO]) from the given pH and bicarbonate concentration ([HCO3]).
Looking at Figure E-2, we can see that at pH 8.5, the fraction of CO2 is negligible and the dominant species is HCO3.
Therefore, [CO] = 0.
Step 2: Substitute the values into the formula for carbonate alkalinity.
Carbonate alkalinity (mg CaCO3/L) = 0.820 [HCO3, mg/L] +1.667 [CO, mg/L]
= 0.820 * 275 + 1.667 * 0
= 225.5 mg CaCO3/L
So, the alkalinity of the groundwater sample is 225.5 mg CaCO3/L.
To calculate the alkalinity using the given formula and the species distribution diagram, we need to determine the concentration of CO (carbon monoxide) in order to plug it into the formula.
From the species distribution diagram (Figure E-2), we can see that CO2, HCO3, and CO are the three species involved. We are given a pH of 8.5 and an HCO3 concentration of 275 mg/L.
Step 1: Calculate the concentration of CO2 (carbon dioxide).
Since CO2 is the initial species, its concentration is given by:
[CO2] = 10^(-pH) * [HCO3]
= 10^(-8.5) * 275 mg/L
= 1.77 mg/L
Step 2: Calculate the concentration of CO (carbon monoxide).
The concentration of CO can be calculated as follows:
[CO] = [HCO3] - [CO2]
= 275 mg/L - 1.77 mg/L
= 273.23 mg/L
Step 3: Calculate the alkalinity using the formula:
Using the given values for HCO3 and CO, we can now calculate the alkalinity:
Carbonate alkalinity (mg CaCO3/L) = 0.820 [HCO3, mg/L] + 1.667 [CO, mg/L]
Alkalinity = 0.820 * 275 mg/L + 1.667 * 273.23 mg/L
= 225.50 mg CaCO3/L + 455.17 mg CaCO3/L
= 680.67 mg CaCO3/L
Therefore, the alkalinity of the groundwater sample is 680.67 mg CaCO3/L.