Alkalinity Bicarbonate-carbonate

Alkalinity is… …the measure of the ability of a water to neutralize an acid.

Acidity Most natural waters are buffered as a result of a carbon dioxide(air)- bicarbonate (limestone – CaCO 3 ) buffer system. What is a buffer?

Buffer Mixture of an acid (or base) and its conjugate base (or acid) Think of chemical equilibrium as a see-saw: CO 2 + H 2 O ↔ H 2 CO 3 H 2 CO 3 ↔ HCO H + HCO 3 - ↔ CO H + CO 2 + H 2 O ↔ H 2 CO 3 ↔ HCO H + ↔ CO H + You need to put 2 fat kids on the see-saw!

Buffer CO 2 + H 2 O ↔ H 2 CO 3 ↔ HCO H + ↔ CO H + If you have a big heavy weight at both ends of the equilibrium, a small addition of acid or base from an outside source can’t change the pH very much. CO 3 2- CO 2

Reporting Alkalinity Alkalinity can be reported in several ways – ways which are not completely chemically accurate. Alkalinity as mg/L CaCO 3 = ml titrant * Normality of acid * 50,000 mL sample

What’s Normality? Normality is like Molarity with the stoichiometry added right in. Normality (N) = equivalent moles of solute L What’s “equivalent” mean? It means you consider the reaction.

1.5 M HCl What’s HCl? It’s an acid. What’s the relevant part of the acid? H+H+ HCl + OH -  H 2 O + Cl -

1.5 M HCl Since 1 HCl reacts with 1 OH-, there is one chemical equivalent per molecule. 1.5 mole HCl * 1 H + equivalent = 1.5 N HCl L 1 HCl HCl + OH -  H 2 O + Cl -

1.5 M H 2 SO 4 What’s H 2 SO 4 ? It’s an acid. What’s the relevant part of the acid? H+H+ H 2 SO OH -  2 H 2 O + SO 4 2-

1.5 M H 2 SO 4 Since 1 H 2 SO 4 reacts with 2 OH-, there are TWO chemical equivalents per molecule. 1.5 mole H 2 SO 4 * 2 H + equivalent = 3.0 N H 2 SO 4 L 1 H 2 SO 4 H 2 SO OH -  2 H 2 O + SO 4 2-

Moles! Moles! Moles! I titrate mL of calcium carbonate solution using a 1.5 M H 2 SO 4 solution. Equivalence (2 nd endpoint) is reached after addition of mL of acid. What is the concentration of calcium carbonate in the original sample in mg/L?

1 st thing we need? Balanced Equation CO H +  H 2 CO 3 OR CO H +  HCO 3 - HCO H +  H 2 CO 3

Moles! Moles! Moles! 1.5 moles H 2 SO 4 * L = mol H 2 SO 4 1 L mol H 2 SO 4 * 2 mol H + = mol H + 1 mol H 2 SO mol H + * 1 mol CO 3 2- = mol CO mol H mol CO mol CaCO 3 = mol CaCO 3 1 mol CO mol CaCO 3 * g * 1000 mg = mg/L L 1 mol CaCO 3 1 g

Equivalents! Equivalents! Equivalents! 3 equivalents H 2 SO 4 * L = eq H 2 SO 4 1 L eq H 2 SO 4 * = mol H mol H + * 1 mol CO 3 2- = mol CO mol H mol CO 3 2- * g * 1000 mg = mg/L L 1 mol CO g

Alkalinity as mg/L CaCO 3 = ml titrant * Normality of acid * 50,000 mL sample = mL * 3.0 N H 2 SO 4 * 50, mL =97950 mg/L The expression in the book (or lab) is just the Moles! Moles! Moles! solved for you.

A base is a base is a base If you titrate a solution with multiple bases, can you tell what reacts with what? Essentially, you have 3 different bases in the system: OH-, CO 3 2-, and HCO 3 - All 3 can be neutralized by addition of a strong acid.

H + + OH -  H 2 O (neutral) H + + CO 3 2-  HCO 3 - HCO H +  H 2 CO 3 (slightly basic) (acidic) HCO H +  H 2 CO 3 (acidic)

H + + OH -  H 2 O (neutral – EP1) H + + CO 3 2-  HCO 3 - HCO H +  H 2 CO 3 (slightly basic – EP1) (acidic – EP2) HCO H +  H 2 CO 3 (acidic – EP2)

OH- is a strong base. HCO 3 - is a weak acid If I have more OH- than HCO 3 -, it completely neutralizes it and I just have OH- If I have more HCO 3 - than OH-, then it partially neutralizes it and I detect only HCO 3 -

Example I titrate a mL water sample with M HCl. I achieve the first endpoint at 22.5 mL of HCl and the second after addition of another 27.6 mL of HCl. What can I conclude?

PossibleEP1 volEP2 volCompare CO 3 2- XXEP1 = EP2 HCO 3 - 0YEP1 = 0 OH-z0EP2 =0 CO 3 2- HCO 3 - x(x+y)EP1<EP2 EP1 not 0 CO 3 2- OH- (x+z)XEP1>EP2 EP2 not 0 HCO 3 - OH- ???????

Example I titrate a mL water sample with M HCl. I achieve the first endpoint at 22.5 mL of HCl and the second after addition of another 27.6 mL of HCl. What can I conclude? Carbonate and bicarbonate are both present. How much? M HCl * L HCl = 2.813x10 -3 mol HCl 2.813x10 -3 moles CO M HCl * ( – L HCl) = 6.375x10 -4 H x10 -4 moles HCO 3 -

Units! Units! Units! Carbonate and bicarbonate are usually measured as “mg equivalent CaCO 3 /L” So… M HCl * L HCl = 2.813x10 -3 mole H x10 -3 mole H + * 1 mol CO mol H+=2.813x10 -3 moles CO x10 -3 moles CO 3 2- * 1 mol CaCO 3 1 mol CO x10 -3 mol CaCO 3 * g = g mol CaCO g CaCO 3 * 1000 mg = mg CaCO 3 1 g mg CaCO 3 = 11,259 mg CaCO 3 /L L

What about the HCO 3 - ? CO H + = H 2 CO 3 HCO H + = H 2 CO M HCl * ( – L HCl) = 6.375x10 -4 moles HCl 6.375x10 -4 moles HCl * 1 mol CO 3 -2 = x10 -4 moles CO mol H x10 -4 moles CO 3 2- * g = g mol CaCO g CaCO 3 * 1000 mg = mg CaCO 3 1 g mg CaCO 3 = mg CaCO 3 /L L

Total Alkalinity. I titrate a mL water sample with M HCl. I achieve the first endpoint at 22.5 mL of HCl and the second after addition of another 27.6 mL of HCl. What is the total alkalinity? Assume the second endpoint is reached and it was all CaCO 3 in the sample mL mL = 50.1 mL total HCl M HCl * L = x10 -3 mol HCl x10 -3 mol H + *1 mol CaCO 3 * g * 1000 mg= mg CaCO 3 2 mol HCl mol 1 g mg CaCO 3 = 12,535 mg CaCO 3 /L L

Notice… Total alkalinity = 12,535 mg CaCO 3 /L Carbonate alkalinity = 11, 260 mg CaCO 3 /L Bicarbonate alkalinity = 1276 mg/L 11, = mg CaCO 3 /L!!!!

Example I titrate a water sample with M HCl. I achieve the first endpoint at 22.5 mL of HCl and the second after addition of another 27.6 mL of HCl. What can I conclude? Carbonate and bicarbonate are both present. Is this really true?

Example I titrate a water sample with M HCl. I achieve the first endpoint at 22.5 mL of HCl and the second after addition of another 27.6 mL of HCl. Carbonate and bicarbonate are both present. Is this really true? No – any chemical species that behaves like carbonate or like bicarbonate will look identical!!!!!!!

To be totally accurate, I should quote the levels as: “Bicarbonate and chemical equivalents” “Carbonate and chemical equivalents”

Example I titrate a mL water sample with M HCl. I achieve the first endpoint at 22.5 mL of HCl and the second after addition of another 19.6 mL of HCl. What is the total alkalinity in mg CaCO 3 /L? What can I conclude about the species present?

PossibleEP1 volEP2 volCompare CO 3 2- XXEP1 = EP2 HCO 3 - 0YEP1 = 0 OH-z0EP2 =0 CO 3 2- HCO 3 - x(x+y)EP1<EP2 EP1 not 0 CO 3 2- OH- (x+z)XEP1>EP2 EP2 not 0 HCO 3 - OH- ???????

Example I titrate a mL water sample with M HCl. I achieve the first endpoint at 22.5 mL of HCl and the second after addition of another 19.6 mL of HCl. What is the total alkalinity in mg CaCO 3 /L? What can I conclude? Carbonate and OH- are both present. BUT if I only care about total alkalinity I just ASSUME it is all CaCO 3 !!!!

Total alkalinity: 22.5 mL mL = 42.1 mL M * L = x10 -3 mol H x10 -3 mol H + * 1 mol CaCO 3 = x10 -3 mol CaCO 3 2 mol H x10 -3 mol * g * 1000 mg = mg CaCO 3 mol CaCO 3 1 g mg CaCO 3 = 5267 mg CaCO 3 /L L