Alkalinity means acid neutralization capacity of water. When we add acid in water (adding H+ ions) water absorbs H+ ions without showing significant change in pH mainly it is due to carbonate, bicarbonate & hydroxide ion present in water or the mixture of two ions present in water. The possibility of OH– and HCO3– ions together is not possible since they combine together to form CO3-2 ions.
OH- + HCO3- ⇒ CO3-2 + H2O
Two types of Alkalinity present in water,
P-Alkalinity also called Phenolphthalein Alkalinity because Phenolphthalein indicator used for analysis
M-Alkalinity also called Methyl orange Alkalinity because Methyl orange indicator used for analysis
In alkalinity analysis different ions can be estimated separately by titration against standard acid solution, using selective indicators like phenolphthalein and methyl orange.
Below reaction occurs during analysis:
OH- + H+ ⇒ H2O
CO3-2 + H+ ⇒ HCO3-
HCO3- + H+ ⇒ H2O + CO2
The neutralization reaction up to phenolphthalein end point shows the completion of reactions (1) and (2) only.
The amount of acid used thus corresponds to complete neutralization of OH– plus half neutralization of CO32–.
The titration of water sample using methyl orange indicator marks the completion of the reactions (1), (2) and (3).
Based on above three reaction,
P-Alkalinity = Total Hydroxide + 1/2 Carbonate
M-Alkalinity = (Total Hydroxide + 1/2 Carbonate) +1/2 Carbonate + Total Bicarbonate
Alkalinity Summary Table
P = 0
P = 1/2M
P > 1/2M
P < 1/2M
P = M
1.Phenolphthalein alkalinity (P) = 0; that means the volume of acid used till the completion of reaction (1) and (2) is 0. This can only happen when both OH– and CO32– ions are not present in water. Alkalinity is present due to HCO3– ion only which can be determined using methyl orange indicator and called methyl orange alkalinity (M).
2. P = ½ M; indicates that only CO32– ions are present. Using phenolphthalein indicator neutralization reaches upto HCO3– but using methyl orange indicator the complete neutralization of HCO3– takes place.
3.P > ½M; implies OH– ions are also present along with CO32– ions. Upto phenolphthalein alkalinity OH– ions will be neutralized completely whereas CO32– will be neutralized upto HCO3– ion. But using methyl orange indicator HCO3– will be completely neutralized along with OH– and CO32–.
4.P < ½ M; indicates that beside CO32– ions HCO3– ions are also present. The volume of acid required for the neutralization upto phenolphthalein end point correspond half neutralization of CO32– (equation 2). Neutralization using methyl orange indicator corresponds to HCO3– obtained from CO32– and HCO3– originally present in the water sample
5.P = M; indicates only OH– ions are present.
Alkalinity of water is very important parameter in boiler water treatment program. Based on alkalinity analysis we can predict the presence of free caustic as mentioned in above table. Free caustic is responsible for caustic corrosion in boiler system.
Simple defination of hardness is the amount of calcium & magnesium ions present in water. It is divided in two parts carbonate or temporary hardness and non-carbonate or permanent hardness.
It is a major source of scale in water systems i.e.boiler water & cooling water systems. Softening is a popular method for removal of hardness from water.
Let us discuss both the types of water hardness in detail.
#1. Carbonate or Temporary Hardness
Carbonate and bicarbonate ions are responsible for this type of water hardness.It is also known as temporary hardness because it removes from water when we boil the water.
When we boil the water carbonate & bicarbonates ions are present in water decomposes & insoluble carbonate is reformed.Boiling the water causes the precipitation of calcium and magnesium carbonate so that, calcium and magnesium ions are remove from water.
Example: CaCO3, MgCO3, Ca(HCO3)2, Mg(HCO3)2
#2. Non-Carbonate or Permanent Hardness
Chloride and Sulfate (non-carbonate) ions are responsible for this type of water hardness.It is also known as permanent hardness because it isn’t remove from water by boiling it.
It is only remove from water either by softening or demineralization process.
Example: CaCl2, MgCl2, CaSO4, MgSO4
Water Hardness Scale:
As a general rule for classification of hardness, Water having hardness below 60 ppm is considered as soft water,61 ppm to 120 ppm as moderately hard; 121 ppm to 180 ppm as hard & above 181 ppm as very hard.
Water Hardness Measurement:
Analysis of Water harness divide in three parts namely total hardness,calcium hardness & magnesium hardness.
Total hardness is a sum of calcium & magnesium hardness.
Total hardness = Calcium Hardness + Magnesium Hardness
In order to determine total hardness ammonia buffer solution is added to the sample to maintain the pH of around 11. Then Eriochrome Black T indicator is added so the wine red color is developed in the sample.
After that titrate the sample against EDTA solution and sample color will change from wine red to blue. This is the end point of total hardness titration.
Determine the calcium hardness by titration sodium hydroxide solution is added to the sample to maintain pH & then murexide indicator is added. Sample color turns into pink & titrate the sample against EDTA solution this time sample color will change from pink to purple. This is the end point of calcium hardness test.
Magnesium hardness is the difference of total hardness & calcium harness value. It is determined by subtracting calcium hardness value from total hardness value.
Magnesium Hardness = Total Hardness – Calcium Hardness
Water Hardness Conversion:
In Conclusion, Water hardness is a major source of scale in water treatment system. It is very essential to remove it from water by selecting proper water treatment program.
Molarity Calculator For Concentrated Liquid Chemical
Molarity Calculator is very handy tool for science student to make reagents for analysis. You can easily calculate molarity of any concentrated acid or base liquid solution.
All you need to do is only enter the three values which is mentioned on label of reagent bottle.
Hope, This article will clear your concept about molarity formula & its calculations.