top-rated free essay

Colorimetric Determination of pH

By Carleen-Almira Sep 29, 2013 1403 Words
Experiment #6: Colorimetric Determination of pH
Almira, Faerie Carleen Lucile L.
Gallardo, Charlotte O.
Group #6, Chemistry 18.1, MHEG1, Ma’am Arlou Angeles
September 23, 2013

I. Abstract
The acidity of the four unknown solutions were determined with the use of colorimetry using McIlvaine’s buffer solutions varying in proportion of its constituents (disodium phosphate and citric acid). These buffer solutions were subjected to the addition of corresponding pH indicators and the variation of colors depending on its pH level was used as standards. At the end of the experiment, the colors of the unknowns were compared with the standard buffer solutions and then calculated  to obtain an estimation of their pH or acidity. Experimental results show that Solution A, B, C, and D, have pH levels of 4.9, 5.2, 4.2,and 4.9 respectively. Keywords: McIlvaine buffers, colorimetry, pH indicators, Henderson-Hasselbalch equation

II. Introduction
Colorimetry is a quantitative chemical analysis with the use of standard colors and the determination of the spectral absorbance of a solution. This is essential in determining the acidity or basicity of a substance using acid-base indicators (also known as pH indicators). These indicators are usually weak acids or weak bases that, when added in solutions, helps detect changes in pH presented visually through color change. Common pH indicators are shown in Table 1.

Table 1. Common pH indicators used in Colorimetric determination of pH. Indicator
Lower pH Color
pH Range
Higher pH Color
Thymol blue
Red
1.2 - 2.8
Yellow
Bromophenol blue
Yellow
3.0 - 4.6
Purple
Chlorophenol red
Yellow
4.8 - 6.4
Violet
Bromothymol blue
Yellow
6.0 - 7.6
Blue
Phenol red
Yellow
6.8 - 8.4
Red

In this experiment, the colorimetric comparison was done using McIlvaine buffers which contain a citrate and a phosphate, using citric acid and Na2HPO4 stock solutions, to measure pH of an unknown in a wide range. Buffer solutions are solutions that contain an acid-base pair in reasonable concentrations. McIlvaine’s Buffer solutions can resist changes in pH because they contain both an acidic species to react with the OH- ion, and a basic species to react with the H3O+ ion. This is an example of a Common Ion Effect, a special case of Le Chatelier’s Principle that occurs when a given ion is added to an equilibrium mixture, and the position of equilibrium shifts away from forming more of it. HA + H2O H3O+ + A-

Addition of more H3O+, shifts the equilibrium to the left forming more HA.

B- + H2O OH- + HB
Addition of more OH-, shift the equilibrium to the left forming more B-.

At the end of the experiment, the pH of the unknown solution was obtained through colorimetric analysis and using the Henderson-Hasselbach equation stating pH = pKa + log [conj. base]/[acid]. The ionization constant of a weak acid was also calculated using the equation: [H3O+][A-]

Ka = ____________
[HA]

III. Experimental
PART A.
The McIlvaine’s buffer solution was prepared using 0.2 M Na2HPO4 and 0.1 M citric acid with certain amounts to produce certain pH levels appropriate for each indicator:

Table 2: 5 drops of methyl orange were used to obtain standard colors of the prepared buffer solution in different proportions.

Methyl orange
pH
mL Na2HPO4
mL Citric Acid
2.8
1.58
8.42
3.0
2.05
7.95
3.2
2.47
7.53
3.4
2.85
7.15
3.6
3.22
6.78
3.8
3.55
6.45
4.0
3.25
6.15
4.2
4.14
5.86
4.4
4.41
5.59
Table 3: 5 drops of bromophenol blue were used to obtain standard colors of the prepared buffer solution in different proportions.

Bromophenol blue
pH
mL Na2HPO4
mL Citric Acid
3.0
2.05
7.95
3.2
2.47
7.53
3.4
2.85
7.15
3.6
3.22
6.78
3.8
3.55
6.45
4.0
3.86
6.15
4.2
4.14
5.86
4.4
4.41
5.59
4.6
4.67
5.33

Table 4: 5 drops of bromocresol green were used to obtain standard colors of the prepared buffer solution in different proportions.

Bromocresol green
pH
mL Na2HPO4
mL Citric Acid
3.8
3.55
6.45
4.0
3.25
6.15
4.2
4.14
5.86
4.4
4.41
5.59
4.6
4.67
5.33
4.8
4.93
5.07
5.0
5.15
4.85
5.2
5.20
4.80
5.4
5.58
4.42

Table 5: 5 drops of chlorophenol were used to obtain standard colors of the prepared buffer solution in different proportions.

Chlorophenol red
pH
mL Na2HPO4
mL Citric Acid
4.8
4.93
5.07
5.0
5.15
4.85
5.2
5.20
4.80
5.4
5.58
4.42
5.6
5.80
4.20
5.8
6.05
3.95
6.0
6.31
3.69
6.2
6.61
3.39
6.4
6.92
3.08

Table 6: 5 drops of bromothymol blue were used to obtain standard colors of the prepared buffer solution in different proportions.

Bromothymol blue
pH
mL Na2HPO4
mL Citric Acid
6.0
6.31
3.69
6.2
6.61
3.39
6.4
6.92
3.08
6.6
7.34
2.66
6.8
7.72
2.28
7.0
8.24
1.76
7.2
8.69
1.31
7.4
9.08
0.92
7.6
9.37
0.63

Table 7: 5 drops of phenol red were used to obtain standard colors of the prepared buffer solution in different proportions.

Phenol red
pH
mL Na2HPO4
mL Citric Acid
6.4
6.92
3.08
6.6
7.34
2.66
6.8
7.72
2.28

Tables 2-7 represents the indicator used with the corresponding pH of the Mcilvaine buffers. The indicators only covers approximate pH ranges.

PART B.
Following amounts of distilled H2O, 0.1 M CH3COOH, and 0.1 M NaCH3COO were mixed in four different test tubes (duplicated for two sets of indicators) and labeled as follows:

Table 8. Composition of the Unkown pH Solutions.

d H2O
0.1 M HOAc
0.1 M NaOAc
Indicator 1
Indicator 2
A
0 mL
10 mL
0 mL
Bromophenol blue
Methyl orange
B
8 mL
1 mL
1 mL
Chlorophenol red
Bromocresol green
C
8.9 mL
1 mL
0.1 mL
Bromophenol Blue
Methyl orange
D
8.9 mL
0.1 mL
1 mL
Bromothymol blue
Phenol red

Upon addition of two drops of designated indicators for every test tube, the resulting colors of the unknown were compared to that of the standard buffer solutions from part A to obtain the acidity for each. Each of the two results from unknown solutions A, B, C, and D were averaged to obtain estimated acidity.

IV. Results

Table 9. Observed and Calculated pH of Unknown pH Solutions. Sol’n
Observed pH
Ave. pH
Calculated
pH
A
6.4
(bromophenol blue)
3.4
(methyl orange)
4.9
4.88
B
5.2
(chlorophenol red)
5.2
(bromocresol green)
5.2

4.74
C
4.0
(bromophenol blue)
4.4
(methyl orange)
4.2
3.74
D
4.8
(bromothymol blue)
5.0
(phenol red)
4.9

5,74

Above is a table showing the obtained and calculated pH values of Solutions A to D.

V. Discussion
Buffer solutions can be composed with a weak acid and its conjugate base pair and vice versa—a weak base and its conjugate acid. The solution in equilibrium is able to resist drastic changes in pH. When a strong acid is added to the buffer, the acid will be neutralized with by the conjugate base of the buffer. The equilibrium then would push forward because of the Le Chatelier’s principle and common ion effect thus the final pH level will not be drastically affected. The common ion in the experiment is OAc-. McIlvaine’s Buffer System is composed of two stock solutions (0.2 M disodium phosphate and 0. 1 M citric acid) combined in such volumes to cover a range of pH 2.8- 7.6 (originally 2.2 to 8.0). Two different indicators were used in each of the unknown pH solutions. Acid-base indicators should be used appropriately according range for pH change for better accuracy in determining pH of the unknown. Solutions B to D differ between the volumes of HOAc and NaOAc added. Solution B has the same HOAc and NaOAc volumes. Solution C has more volume of HOAc than NaOAc and the vice-versa for Solution D.

Henderson- Hasselbalch equation:
[A-]
pH= pKa + log _______
[HA]

The equation above is used to measure pH of buffered solutions. The unknown pH of solutions has the chemical reaction below:

CH3COOH(aq) + H2O  H3O+(aq) + CH3COO-(aq)

The Henderson-Hasselbalch equation was used to determine the pH of solutions B to D. The equation relates the concentration of the weak acid or base to its corresponding salt. It is derived from the equilibrium constant expression.

[H3O+][CH3COO-]
Ka= ____________________
[CH3COOH]

[CH3COO-]
pKa= pH - log _______________
[CH3COOH]

[CH3COO-]
pH= pKa + log ______________
[CH3COOH]

Calculated pH (solutions B-D):
To compute MHOAc and MNaOAc :
M1V1=M2V2

Where:
M1 = initial concentration of HOAc/NaOAc
V1 = volume of the HOAc/NaOAc used
M2 = concentration of HOAc/NaOAc in final solution
V2 = volume of final solution (10 mL)

M1V1
MHOAc = _______
V2

M1V1
MNaOAc= _______
V2

Ka= 1.8 x 10-5

Solution B: (8 mL H2O, 1 mL 0.1 M CH3COOH, 1 mL 0.1 M NaCH3COO) MHOAc= 0.01 M
MNaOAc= 0.01 M

[0.01 M]
pH = - log (1.8 x 10-5) + log ___________
[0.01 M]

pH= 4.74

Solution C: (8.9 mL H2O, 1 mL 0.1 M CH3COOH, 0.1 mL 0.1 M NaCH3COO) MHOAc= 0.01 M
MNaOAc= 0.001 M
[0.001 M]
pH = - log (1.8 x 10-5) + log ___________
[0.01 M]
pH= 3.74

Solution D: (8.9 mL H2O, 0.1 mL 0.1 M CH3COOH, 1 mL 0.1 M NaCH3COO) MHOAc= 0.001 M
MNaOAc= 0.01 M
[0.01 M]
pH = - log (1.8 x 10-5) + log ___________
[0.001 M]
pH=5.74

The experimental ionization constant of acetic acid can be computed as follows: Observed pH of 0.01 M HOAc: 4.9
[H3O+]:
pH= - log [H3O+]
4.9= - log [H3O+]
[H3O+]=10-4.9
[H3O+]= 1.26x10-5 M
Ka of HOAc= 1.8 x 10-5
[H+][OAc-]
Ka= _____________
[HOAc]
HOAc  H+ + OAc- I 0.01 0 0 C -1.26x10-5 M 1.26x10-5 M 1.26x10-5 M
_________________________________________________________________________________________________________________________________________________________ E 0.01- 1.26x10-5 M 1.26x10-5 M 1.26x10-5 M

[1.26x10-5 M]2
Ka= ________________________ = 1.59 x 10-8
[0.01 - 1.26x10-5 M]

[H3O+] of 0.01 M HOAc: 1.26x10-5 M
Calculated Ka of HOAc: 1.59 x 10-8

The experimental result showed that increasing acidity of the solutions is in the order of solution B

Cite This Document

Related Documents

  • pH Titration

    ...Experiment 20: pH Titration: Phosphoric Acid in Cola Drinks Post-lab Assignment or Report The post-lab report for this experiment is due at the beginning of the following lab period. Student notes for the lab will be available on the lab T-Square site. Learning Objectives Students will be able to... • Use a known mass of solid acid to d...

    Read More
  • PH lab

    ...been presented. First the PH measurements, Dr. honey dew does not explain how the PH device was calibrated or if the blood sample was added or mixed with anything prior to the measuring of the pH (like water, etc.). In regards to the absorption spectroscopy, the method by which the concentration was obtained is questionable. Was the concentrat...

    Read More
  • Determination

    ...Determination is a positive emotion that involves persevering towards a difficult goal in spite of obstacles.[1][2] Determination occurs prior to goal attainment and serves to motivate behavior that will help achieve one’s goal. Empirical research suggests that people consider determination to be an emotion; in other words, determinat...

    Read More
  • Determination

    ...Determination In “The Glass Castle” we see many different possibilities pertaining to Maslow’s Hierarchy of Needs. We see the father, Rex and one of the daughters, Maureen, who fall victim to Maslow. Then there’s Jeanette and the mother, Rose Mary who have both seemed to achieve self-actualization regardless of their hardships. Traditio...

    Read More
  • Ph Meter

    ...A pH meter is an electronic instrument used to measure the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading. The prob...

    Read More
  • Ph Determination of Solution

    ...Lung cancer kills more people in one year than all criminal and accidental deaths combined. These statistics are shocking, but the good news is that people are now well-informed about the risks connected to lung cancer. They know that their risk of contracting this terrible disease decreases if they either stop smoking or do not smoke at all. Un...

    Read More
  • pH and its meaning

    ...pH pH means Power of Hydrogen or Potential of Hydrogen. pH is a scale of acidity from 0 to 14. It tells how acidic or alkaline a substance is. More acidic solutions have lower pH. More alkaline solutions have higher pH. Substances which are not acidic or alkaline i.e neutral usually have a pH of around 7. Acids have a pH less than ...

    Read More
  • Measuring Ph

    ...MEASURING pH Date of Lab: 02/09/13 Date Report submitted: 02/26/13 Purpose of the experiment By doing this experiment, we were trying to learn how to measure a pH with a pH meter to determine whether a solution is weakly, moderately or strongly acidic or basic; but also to compare the different data founded to see how the addition of ce...

    Read More

Discover the Best Free Essays on StudyMode

Conquer writer's block once and for all.

High Quality Essays

Our library contains thousands of carefully selected free research papers and essays.

Popular Topics

No matter the topic you're researching, chances are we have it covered.