Acidity and Alkalinity Definitions Alkalinity Measurement Task Acidity Measurement Importance

What are Acidity and Alkalinity  Acidity  The capacity of a water sample to neutralise added alkali.  Two Types  Mineral Acidityfrom Strong Acids, and from Salts of Trivalent Metal Ions H 2 SO 4  2 H + + SO 4 2- FeCl H 2 O  Fe(OH) H Cl -  Carbon Dioxide Acidity (Weak Acidity) CO 2 + H 2 O  H 2 CO 3 *  H + + HCO 3 -

 Alkalinity  The capacity of a water sample to neutralise added acid.  Mainly from: 1. The Carbonate - Bicarbonate buffering system. 2.The salts of weak acids  CO 2 + CaCO 3 + H 2 O Ca 2+ + HCO 3 -  Na Acetate, Na Propionate 3.Hydroxide Ammonia What are Acidity and Alkalinity

Equilibrium of CO 2, HCO 3 -, CO 3 2- CO 2 (aq) + H 2 O H 2 CO 3 H + + HCO 3 - 2H + + CO 3 2- CO 2 (g) pH pK a = 6.3 pK a = ppm M pH = pKa + log ([A - ] / [AH]) When  reaction is 99% to the left then pH= log (1/100) = pH= 4.3  air water

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 CO 2 is the major weak acid. Others are possible, eg. Acetic

Acidity Strong Acids Weak Acids (CO 2 Acidity) P endpoint (pH 8.3) M endpoint (pH 4.5) P M M = Methyl Orange P = Phenolphthalein Titration with N/50 NaOH pH High Low starting pH

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 = Y mg CaCO 3 / litre “ orMethyl Orange Acidity / Mineral Acidity = Phenolphthalein Acidity / CO 2 Acidity = If N/50 NaOH is used Acidity (mg CaCO 3 ) = V t x 1000/ V s V t = Titre volume (ml)V s = Sample volume (ml)

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 -, CO 3 2- )  Step2. neutralises alkalinity due to weaker bases (HCO 3 - ) * chosen as CO 3 2-  HCO 3 - reaction is complete. ** chosen as HCO 3 -  H 2 CO 3 reaction is complete.

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 BicarbonatepH > 8.3  2.Carbonate onlypH > 10  3.Hydroxide and CarbonatepH > 10  4.Hydroxide onlypH >>11

Alkalinity (5 cases) Hydroxide Carbonate Bicarbonate P endpoint (pH 8.3) M endpoint (pH 4.5) P M A B B X Y X M = Methyl Orange P = Phenolphthalein 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 Titration with N/50 H 2 SO 4 P=1/2M P>1/2M P<1/2M pH High Low starting pH

How is the Alkalinity Calculated and Reported  Titration Volume, Acid Strength, Sample Volume  Usually Report Alkalinity as Calcium Carbonate Equivalent ie.mg CaCO 3 / litre  For each of the 3 ClassesHydroxide Alkalinity Carbonate Alkalinity Bicarbonate Alkalinity If N/50 H 2 SO 4 is used Alkalinity (mg CaCO 3 ) = V t x 1000/ V s V t = Titre volume (ml)V s = Sample volume (ml)

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 mg/l  Industrial - consent to discharge