Exp 3 Alkalinity of Water

Determination of Alkalinity of water

Alkalinity of water chart

Apparatus for the determination of alkalinity of water

Observations & Calculations

Sample no. 1:   “TAP WATER”

(1)   A = 0(Phenolphthalein)                       B = 20ml (Methyl orange)
(2)   A = 0(Phenolphthalein)                       B = 20.5ml (Methyl orange)
(3)   A = 0(Phenolphthalein)                       B = 22ml (Methyl orange)
AVERAGE = 20.83ml
Methyl orange Alkalinity = (A x N x 50 x1000)/(ml of sample) = (20.83 x 0.02 x 50000)/50
= 416.6mg/l

Sample no. 2:   “Lab prepared sample”

Phenolphthalein Alkalinity                                         Methyl orange Alkalinity
(1)   A = 12.7ml,(Phenolphthalein)                       B = 8.1ml (Methyl orange)
(2)   A = 13ml,(Phenolphthalein)                          B = 11.1ml (Methyl orange)
(3)   A = 13.2ml,(Phenolphthalein)                       B = 10ml (Methyl orange)
Phenolphthalein AVERAGE = 12.7+13+13.2/3= 12.97ml

Phenolphthalein Alkalinity = (A x N x 50 x1000)/(ml of sample) = (12.97 x 0.02 x 50 x 1000)/50
= 259.4mg/l

Methyl orange AVERAGE = 8.1+11.1+10/3 = 9.73ml

Methyl orange Alkalinity = (B x N x 50 x1000)/(ml of sample) = (9.73 x 0.02 x 50000)/50
= 194.6mg/l

Total Alkalinity:

Total Alkalinity = Phenolphthalein Alkalinity + Methyl orange Alkalinity
Total Alkalinity = 259.4+194.6 =454mg/l

 Sample no. 3:   “Lab prepared sample”

(1)   A = 0,(Phenolphthalein)                       B = 14ml    (Methyl orange)
(2)   A = 0,(Phenolphthalein)                       B = 14.5ml (Methyl orange)
(3)   A = 0,(Phenolphthalein)                       B = 14.3ml (Methyl orange)
AVERAGE = 14.27ml
Methyl orange Alkalinity = (A x N x 50 x1000)/(ml of sample) = (14.27 x 0.02 x 50000)/50
= 285.4mg/l

Question Answers

1)    Why alkalinity is measured in terms of CaCo3 Eq ?

The concentration of the Ca2+ ions is greater than the concentration of any other metal ion in water. Alkalinity is expressed as the amount of CaCOthat would be consumed by that amount of acid. Common bases found in fish ponds include carbonates, bicarbonates, hydroxides, phosphates and berates. Carbonates and bicarbonates are the most common and most important components of alkalinity. Carbonate-bicarbonate alkalinity (and hardness) in surface and well waters is produced primarily through the interactions of CO2, water and limestone. Rainwater is naturally acidic because of exposure to atmospheric carbon dioxide. As rain falls to the earth, each droplet becomes saturated with CO2; and pH is lowered. Well water is pumped from large, natural underground reservoirs (aquifers) or small, localized pockets of underground water (groundwater), Typically, underground water has high CO2 concentrations, and low pH and oxygen concentrations.Carbon dioxide is high in underground water because of bacterial processes in the soils and various underground, particulate mineral formations through which water moves. As ground- or rainwaters flow over and percolate through soil and underground rock formations containing calcitic limestone (CaCO3) or dolomitic limestone [CaMg(CO3)2], the acidity produced by CO2 will dissolve limestone and form calcium and magnesium bicarbonate salts:

CaCO3 + H2O + CO2 = Ca+2 + 2HCO3-
CaMg(CO3)2 + 2H2O + 2CO2 = Ca+2 + Mg+2 + 4HCO3-
The resultant water has increased alkalinity, pH and hardness.

2)    Write applications of alkalinity measurement.

  • Control of water and waste water treatment.
  • Phytoplankton are microscopic or near microscopic, aquatic plants which are responsible for most of the  oxygen (photosynthesis) and primary productivity in ponds. By stabilizing pH at or above 6.5, alkalinity improves phytoplankton productivity (pond fertility) by increasing nutrient availability (soluble phosphate concentrations). Aquatic organisms benefit from a stable pH value in their optimal range. To maintain a fairly constant pH in a water body, a higher alkalinity is preferable. High alkalinity means that the water body has the ability to neutralize acidic pollution from rainfall or basic inputs from wastewater. A well buffered lake also means that daily fluctuations of CO2 concentrations  result in only minor changes in pH throughout the course of a day.
  • Corrosion Control

Alkalinity is an important parameter involved in  corrosion control. It must be known in order to calculate the Langelier saturation Index.    Acid Rain Diagram     Table 1 U.S. E.P.A  Concentration of lakes and ponds based on alkalinity as measured in concentration of calcium carbonate CaCo3.

3)    Why alkalinity is known as buffering capacity of water ?

Alkalinity is the water’s capacity to resist changes in pH that would make the water more acidic. This capacity is commonly known as “buffering capacity.” For example, if you add the same weak acid solution to two vials of water – both with a pH of 7, but one with no buffering power (e.g. zero alkalinity) and the other with buffering power (e.g. an alkalinity of 50 mg/l), – the pH of the zero alkalinity water will immediately drop while the pH of the buffered water will change very little or not at all.  The pH of the buffered solution would change when the buffering capacity of the solution is overloaded. Alkalinity refers to the capability of water to neutralize acid. This is really an expression of buffering capacity. A buffer is a solution to which an acid can be added without changing the concentration of available H+ ions (without changing the pH) appreciably. It essentially absorbs the excess H+ ions and protects the water body from fluctuations in pH. In most natural water bodies in the buffering system is carbonate-bicarbonate ( H2CO3, HCO3, and CO3).

4)    What is importance of alkalinity in chemical coagulation ?

Coagulation is the process in which particles in water are clumped together into larger particles, called floc.  In a well-run water treatment plant, adjustments are often necessary in order to maximize the coagulation/flocculation process.  These adjustments are a reaction to changes in the raw water entering the plant.  Coagulation will be affected by changes in the water’s pH, salt content, alkalinity, turbidity, and temperature. Alkalinity is the capacity of the water to neutralize acids, based on the water’s content of carbonate, bicarbonate, hydroxide, borate, silicate, and phosphate.  Water with a high alkalinity is preferred for coagulation since it tends to have more positively charged ions to interact with the negatively charged colloids. Alkalinity is a major item that must be considered in Coagulation. It must be present in excess of that destroyed by the acid released by the coagulant for effective and complete coagulation to occur.

5)    Write down application of alkalinity data in water softening.

Water softening is the act of reducing the dissolved calcium, magnesium, and to some degree manganese and ferrous iron ion concentration in hard water. A common water softener is sodium carbonate (Na2CO3). Alkalinity is a major item that must be considered in calculating the lime and soda ash requirements in softening of water by precipitation methods. The alkalinity of softened water is a consideration in terms of whether such waters meeting drinking standardsHard water is generally alkaline and soft water tends to be acidic. Based on this fact, in order to soften hard water one must know its ability to resist the drop in pH against the addition of acidic components to determine the amount to be added.


 Reasons to Soften
1. Reduce Soap Consumption
2. Improve Aesthetics of Water
 B. Reasons not to Soften
1. Expensive Process
2. May be less healthy
3. Hot Water Heaters last longer
Water Softening
3. Competes with health related costs


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