Alkalinity CE 370 Lab

Definition Alkalinity is defined as the measure of the water capacity to neutralize acids. In other words, the ability to resist the change in pH.

Alkalinity of natural waters is primarily due to three species:  OH  (hydroxyl ions)  CO 3 = (carbonate ions)  HCO 3  (bicarbonate ions)

Importance of Alkalinity Alkalinity is involved (needed) in different water and wastewater treatment processes, such as: Chemical coagulation: Calculation of the dose of the chemical coagulants depends on the Alkalinity of the water. Al 2 (SO 4 ) 3 + 6H 2 O  2Al(OH) 3 + 6H + + 3SO 4 = Water softening: Alkalinity is a major factor considered in calculating the lime and soda-ash requirements. Corrosion Control: Alkalinity is an important parameter involving corrosion control calculations.

Determination of the Different Species of Alkalinity First, we have to make two Alkalinity measurements:  Phenolphthalein Alkalinity (P): Alkalinity measured to P end point  Methyl Orange Alkalinity (M): Alkalinity measured to M end point Total Alkalinity (T) = P + M

Second, we determine the species as the following: (we assume that OH  and HCO 3  do not exist together) Possibilities:  OH  only  CO 3 = only  OH  and CO 3 =  CO 3 = and HCO 3   HCO 3  only

Then: Case 1M = 0  P = TOH  only Case 2P = M  P = ½ TCO 3 = only Case 3P > M  P > ½ TOH  and CO 3 = OH  = P – M and M = T – P Then, OH  = P – (T-P) = 2P – T And, CO 3 = = 2M = 2 (T - P) Case 4P < M  P < ½ TCO 3 = and HCO 3  CO 3 = = 2 P CO 3 = = 2 P HCO 3  = T - 2 P HCO 3  = T - 2 P Case 5P = 0  T = MHCO 3  only HCO 3  = T HCO 3  = T

Expressing the Concentration of Alkalinity Alkalinity concentration is expressed as CaCO 3 for convenience to make it possible to add and subtract. Conc. (mg/l) = N  eq. wt.  1000 Conc. of Alkalinity = N Alk  eq. wt. Alk  1000 Conc. of Alkalinity (as CaCO 3 ) = N Alk  eq. wt. CaCO3  1000 eq. wt. CaCO3 = 50 N b  V b = N a  V a

The following equation can be used to express concentration as CaCO 3 :

Purpose of the Experiment To become familiar with the concept of alkalinity and its measurement in water

Materials Burette, 25 ml Porcelain dish Magnetic stirrer and rod Beaker, 150 ml Pipetter Measuring cylinder, 100 ml pH meter 0.02N Sulphuric acid Methyl Orange indicator Phenolphthalein indicator Tap water sample Sweet water sample Synthetic samples

Procedure 1 - Indicator Method  Pipette exactly 100 ml of sample into a porcelain dish and drop in a magnetic rod.  Mount a 25 ml burette and fill it to the mark with 0.02N sulphuric acid solution.  Add 5 drops of Phenolphthalein indicator to the sample. If the solution turns pink, add acid slowly till pink color disappears. Record the volume of acid in milliliters as P.  Add 5 drops of Methyl Orange indicator to the same sampling at the end of the first titration and add 0.02N sulphuric acid slowly till orange color turns to pink. Record this volume as M. Then, T = P+M.

Procedure 2 - Potentiometric Method (pH meter) Pipette exactly 100 ml of sample into a 150 ml beaker and drop in a magnetic rod. Fill the burette with 0.02N sulfuric acid solution. If the pH of the sample is above 8.3 add 0.02N sulphuric acid slowly till pH 8.3. Record the volume of acid as P. Continue addition of acid till the pH of the sample reaches 4.5. Record volume of the acid as M. Then, T = P+M.

Calculation of Alkalinity Species ConditionOH  CO 3 = HCO 3  1. P = TT00 2. P = 1/2T02P0 3. P > 1/2T(2P-T)2(T-P)0 4. P < 1/2T02P(T-2P) 5. P = 000T