EXPERIMENT ON DETERMINATION OF ALKALINITY OF A WATER SAMPLE TEST

  TITLE OF EXPERIMENT:    

ALKALINITY TEST

AIM AND OBJECTIVE:

To determine the alkalinity of a given water sample.

THEORY:

Alkalinity is a measure of the capability of water to absorb H+ ions without significant change of pH. In other words, alkalinity is a measure of the acid buffering capacity of water. The determination of alkalinity of water is necessary for controlling the corrosion, to calculate the amount of lime and soda needed for water softening; in conditioning the boiler feed water, etc.

Alkalinity of a sample of water is due to the presence of OH– (hydroxide ion), HCO3– (bicarbonate ion) and CO₃^2- (carbonate ion) 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^– + HCO^3-– CO₃^2- + H₂O

PRINCIPLE: – Alkalinity of water means the total content of those substances in it which causes an increased

OH- ion concentration up on dissociation or due to hydrolysis. The alkalinity of water is attributed to the presence of (i) Caustic alkalinity (Due to OH- and CO₃²-)

(ii) Temporary hardness (Due to HCO₃-)

Alkalinity is a measure of ability of water to neutralize the acids

Determination of alkalinity

OH-, CO32- and HCO3- can be estimated separately by titration against standard acid using phenolphthalein and methyl orange as indicators

The determination is based on the following reactions

(i) OH^– + H⁺ —H₂O

(ii) CO₃^2- + H —HCO₃^–

(iii) HCO₃– + H⁺— H₂O + CO₂

The titration of water sample against a standard acid up to phenolphthalein end point (P) marks the completion of reaction (i) and (ii) only. This amount of acid used thus corresponds to OH- plus one half of the normal CO₃^2- present On the other hand, titration of the water sample against a standard acid to methyl orange end point (M) marks the completion of reaction (i), (ii) and (iii). Hence the total amount of acid used represent the total alkalinity.

Thus,

P = OH^– + ½ CO₃^2-

M = OH^– + CO₃^2- + HCO3^–

METHODOLOGY:

APPARATUS AND MATERIALS

• Burette
• Pipette
• Conical flask
• Beakers
• Funnel and
• Dropper
• Burette stand
• Clamp

CHEMICALS REQUIRED AND MATERIALS

• phenolphthalein
• methyl orange indicator
• Distilled Water
• Standard sulphuric acid
• Ethyl alcohol

PROCEDURE

1. Firstly, the pH value for both water samples was measured.
2. 100ml of the first sample AB was measured into a conical flask
3. Then 2-3 drops of phenolphthalein was added to the conical flask
4. When there was no color change was observed, 2-3 drops of methyl orange was added to it
5. The burette was the filled with the sulphuric acid, noted the initial reading and began titrating it against the water sample AB
6. The titration continued until there was a colour change observed in the water sample, which indicates the end point
7. The titration was stopped, noting the final reading and the water sample was then discarded.
8. The average titre value was then calculated and the result recorded using this formula:

9. The phenolphthalein alkalinity was calculated using this formula:

• The whole Procedure was the repeated for sample AC.
  

DATA 

FIRST READINGS:

FOR AB

VOLUMES
	INITIAL	FINAL	ACTUAL VALUE
AB (ml)	100.00	100.00	100.00
Acid (ml)	0.50	9.50	9.00

FOR AC

VOLUMES
	INITIAL	FINAL	ACTUAL VALUE
AC (ml)	100.00	100.00	100.00
Acid (ml)	9.50	13.00	3.50

SECOND READINGS

FOR AB

VOLUMES
	INITIAL	FINAL	ACTUAL VALUE
AB (ml)	100.00	100.00	100.00
Acid (ml)	13.00	20.00	7.00

FOR AC

VOLUMES
	INITIAL	FINAL	ACTUAL VALUE
AC (ml)	100.00	100.00	100.00
Acid (ml)	20.00	24.70	4.70

Average ml of acid used:

For AB = (9 + 7 ) / 2 = 8.00ml

For AC = (3.5 + 4.7) /2 = 4.10ml

Ppm AB as alkalinity as CaCO₃ =  { (ml of acid) X 1000 } / ml of sample

                                                =  8000/ 100 = 80

Ppm AC as alkalinity as CaCO₃ =  { (ml of acid) X 1000 } / ml of sample

                                                = 4100/100 = 41

PRECAUTION

• All the glass apparatus was washed thoroughly with distilled water before use
• Before use, the burette and pipette was rinsed properly.
• It was ensured that the burette has no leakage.
• The conical flask was placed on white paper to identify the color change at the end point.

OBSERVATION

During the experiment, it was observed that after adding the phenolphthalein indicator to both water samples, there was no form of change in their colour. Methyl orange changed their colour to yellow, after titration with the standard sulphuric acid, their end point colour was pink. But the average titre value of the water sample AB was relatively higher than that of the water sample AC.

CONCLUSION

At the end of the experiment and from the analysis of the results obtained, the following conclusion can be made;

• Sample AB=61.00 ppm (61.00 mg/L) CaCO₃
• Sample AC= 40.00 ppm (40.00 mg/L) CaCO₃
• Total Alkalinity =101 mg/L CaCO₃
• The Phenolphthalein Alkalinity was zero, this implies that the alkalinity due to Hydroxyl and Carbonate ions is zero hence, making the Alkalinity due to Bicarbonate equal to the Total Alkalinity for both water samples.

                                         

RECOMMENDATION

Alkalinity is not considered detrimental to humans but is generally associated with pH values, hardness and excess dissolved solids. High alkalinity waters may also have a distinctly flat, unpleasant taste. Based on the experiment carried out above, it was found that the alkalinity of the samples is 61.00 ppm (61.00 mg/L) and 40.00 ppm (40.00 mg/L). As per the provisional code, alkalinity should not exceed 200 ppm (200 mg/L) for potable water. For the fresh water alkalinity ranges from 20 – 100 ppm (20 – 100 mg/L). The Alkalinity of the given samples are within the limits specified in the standards.

REFERENCES

1. 0 EXPERIMENT ON DETERMINATION OF ALKALINITY OF WATER http://vlab.amrita.edu/?sub=2&brch=193&sim=1549&cnt=1
2. http://nitmeghalaya.in/nitm_web/fp/chem_dept/documents/chem_course/Expt%201-Alkalinity%20of%20water%20sample.pdf
3. Experiment-3 Determination of Alkalinity from the Effluent water.

https://jayeshkparmar.files.wordpress.com/2015/02/practical-31.pdf

4. EXPERIMENT-2 AIM: – To determine the alkalinity of a given waterggnindia.dronacharya.info/apsdept/downloads/…/experiment2-18052012.pdf