Chem Lab Titration

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Chemistry Laboratory Report

Titration of Monoprotic Acid

11 June 2014

Letitia Sarah

061302856

Team member: Masami Hazu

Laboratory in Chemistry

Nagoya University


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Preparation prior to the experiment

1. Consult reference books to check the value of the dissociation constant pKa of the weak acid that you will use inyour experiment.According to Quantitative Chemical Analysis, pKa of acetic acid (CH3COOH) at standard condition is 4.756

2. Confirm the exact concentration of the KOH standard solution to be used for the titration (it is labeled on the bottleand it is specific to each bench) and write it down in your notebook.The exact concentration of the KOH standard solution is 0.0995 M.

3. Calibrate pH meter by measuring the pH values of the buffer solutions with known pH of 4, 7, 9, respectively andcorrect for the deviations in the nominal values as measured by your pH meter.4. Set up burette to perpendicular to the bench and remove the air bubbles trapped in the tip. Dispense the KOH

standard solution from the reservoir into the burette until the meniscus reaches near zero on the burette. Read theposition of the meniscus up to one tenth of the minimum scale division on the burette

4.1.1 Titration of the monoprotic acidsExperiment 1.1.a Titration of hydrochloric acid

Objective: to determine the precise concentration of the HCl acidic solution by titrating it with a basic solution, KOHof known concentration

Methods

1. Dispense 10 mL of 0.1 M HCl into 200 mL beaker using volumetric pipette2.

Add 90 mL of water and 2/3 drops of bromothymol blue3. Put a magnetic stir bar into the beaker. Stir the solution slowly

4. Dip the glass of electrode of the pH meter into the solution. Make sure the electrode does not touch both the beakerand the stir bar.

5. Add 0.5-1.5 mL of 0.1 M KOH while keeping the pH meter on6. Read the dispensed volume at the meniscus. Record the amount of KOH solution added and the pH of the solution

10 s after the KOH was added7. Repeat the 2 previous steps until total KOH added reaches 9 mL8. Add 0.05-0.1 mL of KOH at a time while watching the color of the solution9. Repeat the addition of small amount of KOH until the color of indicator does not change10.When pH starts to stabilize, increase the volume of KOH added to 1 mL11.Repeat the previous step until the pH is greater than 11.12.

After experiment is finished, wash beaker, stir bar, and glass electrode with deionized water.

ObservationQuantity of KOH (mL) 0 0.70 1.50 2.10 2.80 3.60 4.40 5.20 5.80

pH 1.77 1.78 1.81 1.84 1.88 1.92 1.99 2.06 2.11

Quantity of KOH (mL) 6.80 7.90 8.20 8.50 8.60 8.70 8.80 9.00 9.10

pH 2.22 2.42 2.51 2.58 2.63 2.68 2.73 2.8 2.87

Quantity of KOH (mL) 9.15 9.20 9.30 9.40 9.50 9.60 9.70 9.80 9.85

pH 2.92 2.99 3.06 3.14 3.29 3.43 3.71 4.16 4.71

Quantity of KOH (mL) 9.95 10.00 10.10 11.20

pH 5.3 6.16 6.96 11.1

Table 1 Experiment 1: Sample results for pH at various points in the titration of 10.0 mL HCl with 0.0995 M KOH

Quantity of KOH (mL) 0 0.8 1.8 2.65 3.3 4 4.8 5.7 6.5

pH 1.77 1.78 1.83 1.87 1.91 1.96 2.02 2.1 2.2

Quantity of KOH (mL) 7.2 8.1 8.8 9 9.1 9.15 9.35 9.4 9.5

pH 2.3 2.47 2.71 2.81 2.87 2.93 3.07 3.14 3.26

Quantity of KOH (mL) 9.6 9.8 9.85 9.9 9.95 10 10.15 10.2 10.22

pH 3.47 4.53 4.87 5.13 5.49 5.85 7.64 8.48 9.13

Quantity of KOH (mL) 10.3 10.9 11.1

pH 9.83 10.82 11.06

Table 2 Experiment 2: Sample results for pH at various points in the titration of 10.0 mL HCl with 0.0995 M KOH


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Figure 1 Titration Curve of HCl by KOHNote:

Blue rectangle : pH range of bromothymol blue (6.0-7.6)

Sun : equivalence point

Discussion

In this experiment, 0.0995 M KOH was added to analyte, approximately 0.1 M HCl. The experiment was done twotimes and the result was presented in table 1 and 2. During the experiment, indicator bromothymol blue was used.

Bromothymol blue, which is a pair of weak acid and its conjugate base , has pH range from 6.0-7.6. For pH which isless than 6.0, the indicator will turn the solution yellow. While for pH which is greater than 7.6, the indicator will turnthe solution blue. From both experiments, the solution turned completely blue when KOH added was 10 mL. However,the pH of the sample at this point for the first experiment was 6.16, while in the second experiment was 5.85. Thischange in color indicates the end point of titration. End point is the point at which the indicator changes color of the

solution. In this experiment, the indicator turned the solution into blue even though the pH meter showed pH at thatpoint was less than 7.6. This might be because the range of indicator slightly is affected by temperature. Pkind is

influenced by change in temperature.Nevertheless, end point which could be observed does not indicate the exact volume needed to neutralize the reaction.The exact amount necessary for stoichiometric reaction with the analyte can be only determined from the equivalence

point from the graph. By using graphical method, the equivalence point occurs when 10.1 mL of 0.0995 KOH is addedand the pH is 7.

Questions1. Determine the accurate concentration of hydrochloric acid and its dissociation constant pKa based on the

results of your experiment

By using the equivalent point from graphical approach which was at the volume of 10.1 mL,

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From the calculation, the concentration of HCl is approximately close to the concentration data of the solution,which was 0.1 M. By using the same geometric method, the mid point of the curve was approximately when

the volume of KOH was 5 mL (pH = 2.0). Assuming that at the mid point pH = pKa, pKaof HCl will be 2.However this value is not possible because pKaof HCl theoretically less than 1 and the value is very small.Therefore, we can not determine pKaof HCl with this method.

2. Based on the concentration of hydrochloric acid determined in 1, determine the pH at each of the followingstages: I before the addition of the KOH standard solution, ii at the half-neutralization point, iii just before the


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neutralization point, iv at the neutralization point, v just after the neutralization point, and vi at the completionof the titration. Mark these pH values on your titration curve and compare these estimated data with yourexperimental results.

(i) Because the solution was diluted with water, the concentration of the diluted HCl before the addition ofKOH was:

[] []

pH of Experimental result: 1.77

(ii) At the half neutralization point, 50% of HCl had been neutralized and [] [].[] [] , [] [] [] []

[]

pH of experimental result : 2.06

(iii) Assume that just before the neutralization point, about 10 mL of KOH is added. Therefore,

HCl + KOH KCl + H2O

1.00 0.995

-0.995 -0.995 0.9950 0.995

0.005 0.995 0.995

Initial mole of HCl = 0.100 x 10.0 = 1.00 mmol

Initial mole of KOH = 0.0995 x 10.0 = 0.995 mmol

pH= -log [H+]= -log [] = 4.30

pH of experimental result : 6.005 (mean value of 5.85 and 6.16)

(iv) The equation to determine concentration of proton at the neutralization point can be obtained from

[] .[]

pH of experimental result : 6.96

(v) Assume that just after the neutralization point, about 10.15 mL KOH was added

HCl + KOH KCl + H2O

1.00 1.01

-1.00 -1.00 1.00 1.00

0 0.01 1.00 1.00

Initial mole of HCl = 0.100 x 10.0 = 1.00 mmol


pOH= -log [OH-]= -log [

] = 4.00pH = 14-4 = 10.00

pH of experimental result: 7.64

(vi) Assume at the end of titration, volume of KOH added was 11.15 mL (mean value of 11.10 mL and 11.20

mL)

HCl + KOH KCl + H2O

1.00 1.11

-1.00 -1.00 1.00 1.00

0 0.11 1.00 1.00

Initial mole of HCl = 0.100 x 10 = 1.00 mmol


pOH= -log [OH-]= -log [

] = 2.96pH = 11.04

pH of experimental result: 11.08 (mean value of 11.1 and 11.06)


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3. List other indicators used in titration of acids and bases with their chemical structures and the pH ranges whenthe indicators change color.

Indicators pH range Color range Structures

Thymol blue 1.2 - 2.8 Red-yellow

Methyl orange 3.2-4.4 Red-yellow

Bromocresol green 3.8-5.4 Yellow-blue

Methyl red 4.8-6.0 Yellow-red

Experiment 1.1 b Titration of acetic acid

Objective: to determine the precise concentration of the acetic acid solution by titrating it with a basic solution,KOH of known concentration

Method

1. Dispense 10.0 mL of 0.1 M acetic acid solution into a 200 mL beaker2. Add 90.0 mL of water and 2 drops of phenolphthalein3. Repeat step 3-12 in the experiment 1.1 a for this titrationObservation

Quantity of KOH (mL) 0 0.5 1 1.4 1.7 2.1 2.7 2.9 3.3

pH 3.07 3.27 3.49 3.63 3.74 3.86 3.95 4.04 4.11

Quantity of KOH (mL) 3.7 4 4.3 4.9 5.6 5.9 6.5 7 7.4

pH 4.18 4.25 4.3 4.4 4.53 4.58 4.71 4.81 4.9

Quantity of KOH (mL) 8 8.5 8.6 8.8 9 9.2 9.25 9.35 9.4

pH 5.05 5.21 5.26 5.35 5.46 5.56 5.62 5.68 5.76

Quantity of KOH (mL) 9.5 9.55 9.6 9.65 9.7 9.8 9.85 9.9 10

pH 5.83 5.94 6.01 6.08 6.15 6.25 6.53 7.09 7.56

Quantity of KOH (mL) 10.05 10.15 10.2 10.5 10.8 11

pH 8.46 9.15 9.75 10.31 10.81 10.96

Table 3 Experiment 2: Sample results for pH at various points in the titration of 10.0 mL CH3COOH with 0.0995 M

KOHQuantity of KOH (mL) 0 0.6 1 1.55 2.1 3 3.6 3.9 4.6

pH 3.06 3.32 3.52 3.71 3.85 4.07 4.2 4.25 4.38Quantity of KOH (mL) 5.2 5.7 6.3 7 7.6 8.1 8.3 8.5 8.8

pH 4.48 4.57 4.68 4.81 4.96 5.08 5.16 5.24 5.36

Quantity of KOH (mL) 8.9 9 9.05 9.15 9.3 9.35 9.45 9.55 9.6

pH 5.41 5.45 5.51 5.6 5.67 5.71 5.75 5.89 6.01

Quantity of KOH (mL) 9.65 9.7 9.75 9.8 9.9 9.95 10 10.02 10.1

pH 6.08 6.15 6.24 6.35 6.63 6.84 7.24 7.85 8.35

Quantity of KOH (mL) 10.15 10.4 10.7 11

pH 9.26 10.16 10.7 11.02

Table 4 Experiment 2: Sample results for pH at various points in the titration of 10.0 mL CH3COOH with 0.0995 M KOH


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Figure 2 Titration Curve of CH3COOH by KOH

Discussion

In this experiment, the same concentration of KOH (0.0995 M) was used for the titration. The experiment was alsodone 2 times and the values of titration were presented in table 2.

The indicator used in the titration was phenolphthalein and has pH range between 8.2 10.0 and will change colorfrom colorless to deep pink. In figure 13, the pH range of phenolphthalein was represented by purple rectangle.From the observation, the color of the solution started to change into pale pink when the volume of KOH added was10.0 mL (pH = 7.56) in the first experiment and 10.0 mL (pH=7.24) in the second experiment, determining the end

point of the titration. The solution then continued to turn into deep pink. By using geometric method, the equivalent

point of the titration was when the volume of KOH approximately 9.90 mL and the pH of the solution was 7.8.

Question1. Using the experimental data, estimate the concentration of acetic acid solution used in the experiment and the

value of the dissociation constant pKa of acetic acid

By using the equivalent point from geometrical approach which was at the volume of 9.90 mL,

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From the calculations, the concentration of CH3COOH is approximately close to the concentration data of the

solution, which was 0.1 M.According to the laboratory manual, the value of pKawill be the same with pH at the half neutralization point.By using geometric approach, the half neutralization point is when the volume of KOH added was 4.95 mL and

thus the pH at this point is 4.6. Therefore the pKaof acetic acid was 4.6.

2. Based on the concentration of hydrochloric acid determined in 1, determine the pH at each of the followingstages: I before the addition of the KOH standard solution, ii at the half-neutralization point, iii just before theneutralization point, iv at the neutralization point, v just after the neutralization point, and vi at the completion

of the titration. Mark these pH values on your titration curve and compare these estimated data with yourexperimental results.(i)Because the solution was diluted with water, the concentration of the diluted CH3COOH before the addition

of KOH was:

[]

[] ( )


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pH of experimental result: 3.065 (mean value of 3.06 and 3.07)(ii) At half neutralization point, pH = pKa. Therefore,

pH of experimental result: 4.4

(iii) Assume that at the point before equivalent point, 9.8 mL of KOH was addedCH3COOH + OH

- H2O + CH3COO-

0.990 0.975-0.975 -0.975 +0.975 +0.975

0.0150 0.975 0.975Moles of initial OH- = Moles of KOH = 0.0995 x 9.8 = 0.975 mmolMoles of CH3COOH = 0.0990 x 10 = 0.990 mol

pH= pKa + log []= 4.74 + 1.816 = 6.56pH of experimental result: 6.30 (mean value from experiment I and II result)(iv) At equivalence point

[]

pH of experimental result: 7.8(v) Assume that at the point just after the equivalent point, 10 mL of KOH was added.CH3COOH + OH

- H2O + CH3COO-

0.990 0.995-0.990 -0.990 0.990 0.9900.005 0.990 0.990

Moles of initial OH- = Moles of KOH = 0.0995 x 10.0 = 0.995 mmolMoles of CH3COOH = 0.0990 x 10 = 0.990 mmol

pH = -log [OH-]= -log [] = 4.301

pOH = 9.699pH of experimental result: 7.4 (mean value from both experiments)(vi) At the completion of titration, 11 mL of KOH was addedCH3COOH + OH

- H2O + CH3COO-

0.990 1.095-0.990 -0.990 0.990 0.990

0.105 0.990 0.990Moles of initial OH- = Moles of KOH = 0.0995 x 11.0 = 1.095mmolMoles of CH3COOH = 0.0990 x 10.0 = 0.990 mmol

pH = -log [OH-]= -log [] = 2.98

pOH = 11.02

pOH of experimental result: 10.99 (mean value from both experiments)

Conclusion

The accurate concentration of HCl and CH3COOH was obtained from the titration with 0.0995 M KOH. From thetitration, the concentration of HCl was 0.1000 and the concentration of CH3COOH was 0.0990 M.

There are several possible errors in this experiment which caused the pH at some points in the experiment differ fromthe theoretical pH:

Parallax error when reading the burette scale Time used to observe pH meter is sometimes too short because at some points of the titration, the value of pHkeeps increasing even after observation time is over

Observation error when identifying the color change at end point. Another source of error regarding end pointmight happen when the color of the solution still changes when flask is shaken but that point has already beenregarded as end point.


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Reference

Harris, Daniel C. Quantitative Chemical Analysis.2007. New York: W.H Freeman and Company.

Murata, Shizuaki; Fumi Urano; and Masahiro Yoshimura. Chemistry Laboratory Manual. 2013. Nagoya: Institute ofLiberal Arts and Sciences, Nagoya University.

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