Copper is a toxic metal routinely found in older piping systems. Water treatment

Question

Copper is a toxic metal routinely found in older piping systems. Water treatment plants must report to EPA any values of copper ions in their water that exceed 1.3 mg/L, as it is hazardous above this level. Water running through the pipes contains bicarbonate at a fixed concentration of 95 mg/L (reminder: pKa2 = 10.35 for carbonate system). Assume that copper is present in pipes as copper (II) carbonate [solid], which can disassociate into water per the equation below:    CuCO3(s) Cu2+ + CO32- Ksp = 1.4 x 10-10

If copper is naturally present in the water at a concentration of 45 ppb, calculate the pH at which the water at equilibrium with the pipes (e.g. no additional dissolution of copper into water).

Explanation / Answer

Cu concentration in water is 45 ppb. Molar mass of Cu = 63.546 g/mol.

Find out the molar concentration of Cu2+ in water:

[Cu2+] = (45 ppb)*(1 g L-1/106 ppb)*(1 mole/63.546 g) = 7.08*10-7 mol/L.

Find out the molar concentration of CO32- from the Ksp = 1.4*10-10

CuCO3 (s) <=====> Cu2+ + CO32-

Ksp = [Cu2+][CO32-]

===> 1.4*10-10 = (7.08*10-7)*[CO32-]

===> [CO32-] = 1.4*10-10/(7.08*10-7) = 1.977*10-4

The molar concentration of CO32- in the water is 1.977*10-4 mol/L.

Find out the molar concentration of bicarbonate. Molar mass of bicarbonate = 61.0168 g/mol.

[HCO3-] = (95 mg/L)*(1 g/1000 mg)*(1 mole/61.0168 g) = 1.557*10-3 mol/L.

The bicarbonate-carbonate buffering system is

HCO3- <====> H+ + CO32-

Use Henderson-Hasslebach equation to calculate the pH.

pH = pKa2 + log [CO32-]/[HCO3-] = 10.35 + log (1.977*10-4 mol/L)/(1.557*10-3 mol/L) = 10.35 + log (0.12697) = 10.35 + (-0.89628) = 9.45372 9.45

The pH of the water is 9.45 (ans).

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