2. Determination of Types and Amount of Alkalinity in Water Sample

2. DETERMINATION OF TYPES AND AMOUNT OF ALKALINITY IN WATER SAMPLE

AIM

To determine the type and amount of alkalinity present in the given water sample. A standard solution of sodium hydroxide of strength ............ N and a link solution of hydrochloric acid are provided.

PRINCIPLE

Alkalinity is caused by the presence of hydroxide, carbonate and bicarbonate. There are five alkalinity conditions possible in a given sample of water, hydroxide only, carbonate only, bicarbonate only, combination of carbonate and hydroxide or carbonate and bicarbonate. The various alkalinities can be determined by titrating with a standard acid using phenolphthalein and methyl orange indicators successively.

1. Phenolphthalein end point

When alkaline water is titrated with acid using phenolphthalein indicator, hydroxide alkalinity is completely neutralised and carbonate alkalinity is partially neutralised.

OH^- + H⁺ → H₂O

CO^2-₃+ H⁺ → HCO^-₃

2. Methyl Orange end point

After the phenolphthalein end point, methyl orange indicator is added and titrated with acid. Bicarbonate neutralisation occurs.

HCO^-₃ + H⁺ → CO₂ + H₂O

From the two titre values the different alkalinities are calculated.

When,

P = M, → hydroxide alkalinity

2P = M, → carbonate alkalinity

P = 0, → bicarbonate alkalinity

Calculations

I. If the data satisfies the condition P > 1/2 M

(i) Volume of Hcl required for [OH-] alkalinity = 2[P] - [M]

= 2 × ……… - ………

= ……….. ml

(ii) Volume of HCl required for [CO^2-₃) alkalinity = 2[M] - 2 [P]

= 2 × ……… - 2× ………

= ……….. ml

(iii) HCO₃^- is not present.

Note :

1000 cc of lN HCl = 1 gm equivalent of CaCO₃

1000 cc of lN HCl = 50 gm of CaCO₃

1. Calculation of OH^- alkalinity

Volume of HCl V₁ = ……….. ml

Strength of HCl N₁ = ……….. N

Volume of water sample V₂ = 20 ml

Strength of water sample (OH^- alkalinity) N₂ = ?

According to the law of volumetric analysis

Alkalinity values are expressed in terms of milligrams per litre as calcium carbonate.

TABLE - 1

Titre values and different alkalinities

i.e., OH alkalinity interms of CaCO₃ equivalent = …….. N × 50 × 1000ppm

Alkalinity due to OH^- ions = ............. ppm.

2. Calculation of CO^2-₃ alkalinity

Volume of HCl V₁ = .................. ml

Strength of HCl N₁ = .................. N

Volume of water sample V₂ = 20 ml

Strength of water sample (CO^2-₃ alkalinity) } N₂ = ?

According to the law of volumetric analysis

Alkalinity due to CO^2-₃ ions = ............. ppm.

 

PROCEDURE

 

TITRATION – I

Standardisation of HCI

The burette is washed well with water and rinsed with the given hydrochloric acid solution. It is then filled with the same upto zero mark. 20 ml of the standard sodium hydroxide solution is pipetted out in a clean conical flask. 2-3 drops of phenolphthalein indicator is added, the colour turn to pink colour. It is then titrated against the hydrochloric acid taken in the burette. The end point is disappearance of pink colour. The titration is repeated to get concordant values. From the titre values, the normality of HCl is calculated.

 

TITRATION - II

(With Phenolphthalein indicator)

20 ml of the water sample is pipetted out in a clean conical flask. A drop of phenolphthalein indicator is added. Pink colour is observed. This solution is titrated against the standard HCl, already taken in the burette, until pink colour is disappeared. The end point is noted. This titre value corresponds to phenolphthalein end point (P).

 

TITRATION - III

(With Methyl orange indicator)

Few drops of methyl orange indicator is added to the same solution after the phenolphthalein end point. The titration is continued until the solution becomes red orange. The total titre value is noted. This titre value corresponds to methyl orange end point (M). The titration is repeated for concordant values.

From the titre values the amount of each alkalinity present in given water sample is calculated.

Calculations

II. If the data satisfies the condition P < 1/2 M

(i) Volume of HCl required for [CO^2-₃) alkalinity = 2 [P]

= 2 × ………

= ……….. ml

(ii) Volume of HCl required for [HCO^-₃ ) alkalinity = [M] – 2[P]

= …….. – [2 × ............... ]

= ........... ml

 

1. Calculation of CO^2-₃ alkalinity

Volume of HCl V₁ = .................. ml

Strength of HCl N₁ = .......... N

Volume of water sample V₂ = 20 ml

Strength of water sample (CO^2-₃ alkalinity) N₂ = ?

According to the law of volumetric analysis

V₁N₁ = V₂N₂

N₂ = V₁N₁ / V₂

i.e., CO^2-₃  alkalinity interms of CaCO₃ equivalent = …..... N × 50 × 1000 ppm

Alkalinity due to CO^2-₃ ions = ........... ppm. |

 

2. Calculation of HCO^-₃ alkalinity

Volume of HCl V₁ = .................. ml

Strength of HCl N₁ = ................ N

Volume of water sample V₂ = 20 ml

Strength of water sample (HCO^-₃  alkalinity) } N₂ = ?

According to the law of volumetric analysis

i.e., HCO₃^- alkalinity interms of CaCO₃ equivalent = N × 50 × 1000 ppm

Alkalinity due to HCO^-₃  ions = ........ ppm.

 

RESULTS

I. Water sample-I contains the following alkalinity

(i) Hydroxide alkalinity (OH^-) = ........ ppm.

(ii) Carbonate alkalinity (CO²₃ ) = ……… ppm.

(iii) Total alkalinity (OH^- + CO²₃) = .......... ppm.

 

I. Water sample-II contains the following alkalinity

(i) Carbonate alkalinity (CO²₃ ) = …………. ppm.

(ii) Bicarbonate alkalinity (HCO₃^-) = …………. ppm

(iii) Total alkalinity ((CO²₃ + HCO₃^-)  = …………. ppm

 

Structure of disodium salt of EDTA (Fig.I)

Step I: STANDARDISATION OF EDTA

TITRATION – I

Standard Hard water Vs. EDTA

Calculation

Volume of standard hard water = 20 ml

Volume of EDTA solution consumed, V₁ = 28.0 ml

1 ml of standard hard water contains 1 mg of calcium carbonate (CaCO₃)

20 ml of standard hard water contains 20 mg of CaCO₃

20 ml of standard hard water consumes V₁ ml of EDTA

ie., V₁ ml of EDTA solution = 20 mg of CaCO₃

: 1 ml of EDTA solution = 20 / V₁  mg of CaCO₃ equivalent.