Acidity and Alkalinity

1. Acidity and Alkalinity Definitions Alkalinity Measurement ----------- Task ----------- Acidity Measurement Importance

2. What are Acidity and Alkalinity • Acidity • The capacity of a water sample to neutralise added alkali. • Two Types • Mineral Acidity from Strong Acids, and from Salts of Trivalent Metal Ions H2SO4 2 H+ + SO42- FeCl3 + 3 H2O  Fe(OH)3 + 3 H+ + 3 Cl - • Carbon Dioxide Acidity (Weak Acidity) CO2 + H2O  H2CO3* H+ + HCO3-

3. What are Acidity and Alkalinity • Alkalinity • The capacity of a water sample to neutralise addedacid. • Mainly from: 1. The Carbonate - Bicarbonate buffering system. 2. The salts of weak acids • CO2 + CaCO3 + H2O Ca2+ + HCO3- • Na Acetate, Na Propionate 3. Hydroxide Ammonia

4. Equilibrium of CO2 , HCO3- , CO32- CO2 (g) 350ppm air water  CO2 (aq) + H2OH2CO3 H+ + HCO3-2H++ CO32- 10-5M pKa = 6.3 pKa = 10.3 pH4 4.55 6 7 8 8.3 9 10 pH = pKa + log ([A-] / [AH]) When  reaction is 99% to the left then pH = 6.3 + log (1/100) = 6.3 - 2 pH = 4.3

5. How is Acidity Determined • By titration with a Standard Alkali solution to a specific Endpoint. • Mineral Acidity • Indicator used is Methyl Orange (endpoint at pH 4.5 ) • Carbon Dioxide Acidity • Indicator used is Phenolphthalein (endpoint at pH 8.3 ) In practice CO2 is the major weak acid. Others are possible, eg. Acetic

6. starting pH starting pH Acidity High P endpoint (pH 8.3) P pH M endpoint (pH 4.5) M Low M = Methyl Orange P = Phenolphthalein Strong Acids Weak Acids (CO2 Acidity) Titration with N/50 NaOH

7. How is the Acidity Calculated and Reported • Titration Volume, Alkali Strength, Sample Volume • Usually Report Acidity as Calcium Carbonate Equivalent “ The Acidity to pH X = Ymg CaCO3 / litre “ or Methyl Orange Acidity / Mineral Acidity = Phenolphthalein Acidity / CO2 Acidity = If N/50 NaOH is used Acidity (mg CaCO3) = Vt x 1000/ Vs Vt = Titre volume (ml) Vs = Sample volume (ml)

8. How is Alkalinity Determined • By titration with a Standard Acid solution to a specific Endpoint. • Sample pH Values > pH 8.3 • Two Step Titration • Step 1. Indicator used is Phenolphthalein (endpoint at pH 8.3 )* • Step 2. Indicator used is Methyl Orange (endpoint at pH 4.5 )** • Step 1. neutralises alkalinity due to strong bases (OH- , CO32-) • Step2. neutralises alkalinity due to weaker bases (HCO3-) * chosen as CO32-  HCO3- reaction is complete. ** chosen as HCO3-  H2CO3 reaction is complete.

9. How is Alkalinity Determined • Sample pH Values < pH 8.3 • One Step Titration • Step 1. Indicator used is Methyl Orange (endpoint at pH 4.5 ) • only Bicarbonate present when pH < 8.3 at start • Interpretation of Data • When Sample pH value is > pH 8.3 a number of base species can potentially exist together. • 1. Carbonate and Bicarbonate pH > 8.3 • 2. Carbonate only pH > 10 • 3. Hydroxide and Carbonate pH > 10 • 4. Hydroxide only pH >>11

10. starting pH starting pH Alkalinity (5 cases) High P>1/2M P=1/2M A Titration with N/50 H2SO4 P<1/2M B P M X P endpoint (pH 8.3) pH X B Y M endpoint (pH 4.5) M = Methyl Orange P = Phenolphthalein Hydroxide Low Carbonate P = a + b M = b + P Hydroxide = a = 2P - M Carbonate = 2b = 2M - 2P P = x M = 2x + y Carbonate = 2x = 2P Bicarbonate = y = M - 2P Bicarbonate

11. How is the Alkalinity Calculated and Reported • Titration Volume, Acid Strength, Sample Volume • Usually Report Alkalinity as Calcium Carbonate Equivalent ie. mg CaCO3 / litre • For each of the 3 Classes Hydroxide Alkalinity Carbonate Alkalinity Bicarbonate Alkalinity If N/50 H2SO4 is used Alkalinity (mg CaCO3) = Vt x 1000/ Vs Vt = Titre volume (ml) Vs = Sample volume (ml)

12. Application of Acidity and Alkalinity Data • Acidity • Selection of new Water Supplies • minimise treatment costs (Lime, NaOH) • Industrial Wastewater • Metal Pickling Liquor (phosphoric acid) • Consent to Discharge • Alkalinity • Calculate safe levels of Ferric and Alum • Biological WWT Plants - gives buffering capacity • Potable - range 50 - 300 mg/l • Industrial - consent to discharge