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Volumetric Analysis

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Experiment

A Volumetric
Analysis
A titrimetric analysis requires the careful addition of titrant.

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• To prepare and standardize a sodium hydroxide solution
• To determine the molar concentration of a strong acid

Objectives

The following techniques are used in the Experimental Procedure

Techniques

2

4

16b

16a
90

80

5

16c

6

13c

15a

15b

!

A chemical analysis that is performed primarily with the aid of volumetric glassware (e.g., pipets, burets, volumetric flasks) is called volumetric analysis. For a volumetric analysis procedure, a known quantity or a carefully measured amount of one substance reacts with a to-be-determined amount of another substance with the reaction occurring in aqueous solution.

The volumes of all solutions are carefully measured with volumetric glassware. The known amount of the substance for an analysis is generally measured and available in two ways:

1. As a primary standard: A precise mass (and thus, moles) of a solid substance is measured on a balance, dissolved in water, and then reacted with the substance being analyzed. 2. As a standard solution: A measured number of moles of substance is present in a measured volume of solution—a solution of known concentration, generally expressed as the molar concentration (or molarity) of the substance.

A measured volume of the standard solution then reacts with the substance being analyzed.

Introduction

Primary standard: a substance that
has a known high degree of purity, a
relatively large molar mass, is
nonhygroscopic, and reacts in a
predictable way
Standard solution: a solution having a
very well known concentration of a
solute

The reaction of the known substance with the substance to be analyzed, occurring in aqueous solution, is conducted by a titration procedure.
The titration procedure requires a buret to dispense a liquid, called the titrant, into a flask containing the analyte (Figure 9.1a). The titrant may be a solution of known or unknown concentration. The analyte may be a solution whose volume is measured with a pipet or it may be a dissolved solid with a very accurately measured mass. For the acid–base titration studied in this experiment, the titrant is a sodium hydroxide solution and the analyte is an acid. Experiment 9

109

Figure 9.1 (a) Titrant in the buret is dispensed into the analyte until (b) the indicator changes color at its endpoint.
Stoichiometric amounts: amounts corresponding to the mole ratio of the balanced equation Acid–base indicator: a substance having an acidic structure with a different color than its basic structure pH: the negative logarithm of the molar concentration of H3Oϩ, pH ϭ Ϫlog[H3Oϩ]

Standardization of a Sodium Hydroxide Solution Hygroscopic: able to absorb water vapor readily

COOH
_

+
COO K
potassium hydrogen phthlate

A reaction is complete when stoichiometric amounts of the reacting substances are combined. In a titration this is the stoichiometric point.1 In this experiment the stoichiometric point for the strong acid–strong base titration is detected using a phenolphthalein indicator. Phenolphthalein is colorless in an acidic solution but red (or pink) in a basic solution. The point in the titration at which the phenolphthalein changes color is called the endpoint of the indicator (Figure 9.1b). Indicators are selected so that the stoichiometric point in the titration coincides (at approximately the same pH) with the endpoint of the indicator.

Solid sodium hydroxide is very hygroscopic; therefore its mass cannot be measured accurately to prepare a solution with a well-known molar concentration (a primary standard solution). To prepare a NaOH solution with an exact molar concentration, it must be standardized with an acid that is a primary standard. In Part A of this experiment, dry potassium hydrogen phthalate, KHC8H4O4, is used as the primary acid standard for determining the molar concentration of a sodium hydroxide solution. Potassium hydrogen phthalate is a white, crystalline, acidic solid. It has the properties of a primary standard because of its high purity, relatively high molar mass, and because it is only very slightly hygroscopic. The amount of KHC8H4O4 used for the analysis is calculated from its measured mass and molar mass (204.44 g/mol): mass (g) KHC8H4O4 ϫ

mol KHC8H4O4
ϭ mol KHC8H4O4
204.44 g KHC8H4O4

(9.1)

From the balanced equation for the reaction, one mole of KHC8H4O4 reacts with one mole of NaOH according to the net ionic equation:
HC8H4O4Ϫ(aq) ϩ OHϪ(aq) l H2O(l) ϩ C8H4O42Ϫ(aq)
1

(9.2)

The stoichiometric point is also called the equivalence point, indicating the point at which stoichiometrically equivalent quantities of the reacting substances are combined.

110

A Volumetric Analysis

In the experimental procedure an accurately measured mass of dry potassium hydrogen phthalate is dissolved in deionized water. A prepared NaOH solution is then dispensed from a buret into the KHC8H4O4 solution until the stoichiometric point is reached, signaled by the colorless to pink change of the phenolphthalein indicator. At this point the dispensed volume of NaOH is noted and recorded. The molar concentration of the NaOH solution is calculated using Equation 9.2 and

molar concentration (M) of NaOH (mol/L) ϭ

mol NaOH
L of NaOH solution

(9.3)

Once the molar concentration of the sodium hydroxide is calculated, the solution is said to be “standardized” and the sodium hydroxide solution is called a secondary standard solution. In Part B, an unknown molar concentration of an acid solution is determined. The standardized NaOH solution is used to titrate an accurately measured volume of the acid to the stoichiometric point. By knowing the volume and molar concentration of the NaOH, the number of moles of NaOH used for the analysis is

volume (L) ϫ molar concentration (mol/L) ϭ mol NaOH

Molar Concentration of an
Acid Solution

(9.4)

From the stoichiometry of the reaction (your instructor will inform you of the acid type, HA or H2A), the moles of acid neutralized in the reaction can be calculated. From the moles of the acid that react and its measured volume, the molar concentration of the acid is calculated: molar concentration of the acid (mol/L) ϭ mol acid volume of acid (L)

(9.5)

Procedure Overview: A NaOH solution is prepared with an approximate concentration. A more accurate molar concentration of the NaOH solution (as the titrant) is determined using dry potassium hydrogen phthalate as a primary standard. The NaOH solution, now a secondary standard solution, is then used to determine the molar concentration of an acid solution.

Experimental
Procedure

You are to complete at least three “good” trials (Ϯ1% reproducibility) in standardizing the NaOH solution. Prepare three clean 125-mL or 250-mL Erlenmeyer flasks for the titration.
You will need to use approximately one liter of boiled, deionized water for this experiment. Start preparing that first.

A. The Standardization of a
Sodium Hydroxide Solution

1. Prepare the Stock NaOH Solution.2 One week before the scheduled laboratory period, dissolve about 4 g of NaOH (pellets or flakes) (Caution: NaOH is very corrosive—do not allow skin contact. Wash hands thoroughly with water.) in 5 mL of deionized water in a 150-mm rubber-stoppered test tube. Thoroughly mix and allow the solution to stand for the precipitation of sodium carbonate, Na2CO3.3

13c

2

!

2

Check with your laboratory instructor to see if the NaOH solution is prepared for Part A.1 (and/or Part A.3) and to see if the KHC8H4O4 is dried for Part A.2.
3
Carbon dioxide, CO2, from the atmosphere is an acidic anhydride (meaning that when CO2 dissolves in water, it forms an acidic solution). The acid CO2 reacts with the base NaOH to form the less soluble salt, Na2CO3.

CO2(g) ϩ 2 NaOH(aq) l Na2CO3(s) ϩ H2O(l)

Experiment 9

111

6

15a

5

2. Dry the Primary Standard Acid. Dry 2–3 g of KHC8H4O4 at 110C for several hours in a constant temperature drying oven. Cool the sample in a desiccator. 3. Prepare the Diluted NaOH Solution. Decant about 4 mL of the NaOH solution prepared in Part A.1 into a 500-mL polyethylene bottle (Figure 9.2). (Caution: Concentrated NaOH solution is extremely corrosive and will cause severe skin removal!) Dilute to 500 mL with previously boiled,4 deionized water. Cap the polyethylene bottle to prevent the absorption of CO2. Swirl the solution and label the bottle. Calculate an approximate molar concentration of your diluted NaOH solution. 4. Prepare the Primary Standard Acid. Calculate the mass of KHC8H4O4 that will require about 15–20 mL of your diluted NaOH solution to reach the stoichiometric point. Show the calculations on the Report Sheet. Measure this mass (Ϯ0.001 g) of KHC8H4O4 on weighing paper (Figure 9.3) and transfer it to a labeled Erlenmeyer flask. Similarly, prepare all three samples while you are occupying the balance. Dissolve the KHC8H4O4 in about 50 mL of previously boiled, deionized water and add 2 drops of phenolphthalein.

15b

!
6

Figure 9.2 A 500-mL
polyethylene bottle for the NaOH
solution.
2

16c

Figure 9.3 Weighing paper
for the KHC8H4O4 measurements.

5. Prepare a Clean Buret. Wash a 50-mL buret and a funnel thoroughly with soap and water using a long buret brush. Flush the buret with tap water and rinse several times with deionized water. Rinse the buret with three 5-mL portions of the diluted NaOH solution, making certain that the solution wets the entire inner surface. Drain each rinse through the buret tip. Discard each rinse in the “Waste Bases” container. Have the instructor approve your buret and titration setup before continuing.

6. Fill the Buret. Using a clean funnel, fill the buret with the NaOH solution.5 After 10–15 seconds, read the volume by viewing the bottom of the meniscus with the aid of a black line drawn on a white card (the buret can be removed from the stand or moved up or down in the buret clamp to make this reading; you need not stand on a lab stool to read the meniscus). Record this initial volume according to the guideline in Technique 16A.2, “using all certain digits (from the labeled calibration marks on the glassware) plus one uncertain digit (the last digit which is the best estimate between the calibration marks).” Place a sheet of white paper beneath the Erlenmeyer flask.

4

16a
90

80

4

Boiling the water removes traces of CO2 that would react with the sodium hydroxide in solution. Be certain all air bubbles are removed from the buret tip.

5

112

A Volumetric Analysis

7. Titrate the Primary Standard Acid. Slowly add the NaOH titrant to the first acid sample prepared in Part A.4. Swirl the flask (with the proper hand6) after each addition. Initially, add the NaOH solution in 1- to 2-mL increments. As the stoichiometric point nears, the color fade of the indicator occurs more slowly. Occasionally rinse the wall of the flask with (previously boiled, deionized) water from your wash bottle. Continue addition of the NaOH titrant until the endpoint is reached. The endpoint in the titration should be within one-half drop of a slight pink color (see opening photo). The color should persist for 30 seconds. Read (Figure 9.4) and record the final volume of NaOH in the buret. 8. Repeat the Analysis with the Remaining Standard Acid Samples. Refill the buret and repeat the titration at least two more times with varying, but accurately known, masses of KHC8H4O4.

9. Do the Calculations. Calculate the molar concentration of the diluted NaOH solution. The molar concentrations of the NaOH solution from the three analyses should be within Ϯ1%. Place a corresponding label on the 500-mL polyethylene bottle.

Disposal: Dispose of the neutralized solutions in the Erlenmeyer flasks in the “Waste Acids” container.

Three samples of the acid having an unknown concentration are to be analyzed. Ask your instructor for the acid type of your unknown (i.e., HA or H2A). Prepare three clean 125- or 250-mL Erlenmeyer flasks for this determination. 1. Prepare the Acid Samples of Unknown Concentration. In an Erlenmeyer flask, pipet 25.00 mL of the acid solution. Add 2 drops of phenolphthalein. 2. Fill the Buret and Titrate. Refill the buret with the (now) standardized NaOH solution and, after 10–15 seconds, read and record the initial volume. Refer to Parts A.6 and A.7. Titrate the acid sample to the phenolphthalein endpoint. After 10–15 seconds, read and record the final volume of titrant.

3. Repeat. Similarly titrate the other samples of the acid solution. 4. Save. Save your standardized NaOH solution in the tightly capped 500-mL polyethylene bottle for Experiments 10, 26, 27, and/or 28. Consult with your instructor. 5. Calculations. Calculate the average molar concentration of your acid unknown.

Disposal: Dispose of the neutralized solutions in the “Waste Acids” container. Consult with your instructor.

CLEANUP: Rinse the buret and pipet several times with tap water and discard through the tip into the sink. Rinse twice with deionized water. Similarly clean the Erlenmeyer flasks. Check and clean the balance area. All solids should be discarded in the “Waste Solid Acids” container.

16c

Figure 9.4 Read the volume of
titrant with a black background.
4

B. Molar Concentration of
an Acid Solution

16b

2

16c

16a
90

80

4

2

6

Check Technique 16C.3 of this laboratory manual.

Experiment 9

113

NOTES

114

AND

CALCULATIONS

A Volumetric Analysis

Experiment 9 Prelaboratory Assignment
A Volumetric Analysis
Date __________ Lab Sec. ______ Name ____________________________________________ Desk No. __________ 1. a. Define the analyte in a titration.

b. Is the indicator generally added to the titrant or the analyte in a titration?

2. a. What is the primary standard used in this experiment? Define a primary standard.

b. What is the secondary standard used in this experiment? Define secondary standard.

3. Distinguish between a stoichiometric point and an endpoint in an acid–base titration.

4. a. When rinsing a buret after cleaning it with soap and water, should the rinse be dispensed through the buret tip or the top opening of the buret? Explain.

b. In preparing the buret for titration (Experimental Procedure, Part A.5), the final rinse is with the NaOH titrant rather than with deionized water. Explain.

c. How is a “half-drop” of titrant dispensed from a buret?

Experiment 9

115

5. In Part A.1, a 4-g mass of NaOH is dissolved in 5 mL of water. In Part A.3, a 4-mL aliquot of this solution is diluted to 500 mL of solution. What is the approximate molar concentration of NaOH in the diluted solution? Enter this information on your Report Sheet.

6. a. A 0.397-g sample of potassium hydrogen phthalate, KHC8H4O4 (molar mass ϭ 204.44 g/mol) is dissolved with 50 mL of deionized water in a 125-mL Erlenmeyer flask. The sample is titrated to the phenolphthalein endpoint with 16.22 mL of a sodium hydroxide solution. What is the molar concentration of the NaOH solution?

b. A 25.00-mL aliquot of a nitric acid solution of unknown concentration is pipetted into a 125-mL Erlenmeyer flask and 2 drops of phenolphthalein are added. The above sodium hydroxide solution (the titrant) is used to titrate the nitric acid solution (the analyte). If 12.75 mL of the titrant is dispensed from a buret in causing a color change of the phenolphthalein, what is the molar concentration of the nitric acid solution?

116

A Volumetric Analysis

Experiment 9 Report Sheet
A Volumetric Analysis
Date __________ Lab Sec. ______ Name ____________________________________________ Desk No. __________ Maintain at least three significant figures when recording data and performing calculations. A. Standardization of a Sodium Hydroxide Solution

Approximate molar concentration of diluted NaOH solution (Part A.3). Show calculations.

Approximate mass of KHC8H4O4 for the standardization of the NaOH solution (Part A.4). Show calculations.

Trial 1
1. Tared mass of KHC8H4O4 (g)

Trial 2

Trial 3

_______________

_______________

_______________

2. Molar mass of KHC8H4O4
3. Moles of KHC8H4O4 (mol)
Titration apparatus approval

204.44 g/mol
_______________

_______________

_______________

______________________

4. Buret reading of NaOH, initial (mL)

_______________

_______________

_______________

5. Buret reading of NaOH, final (mL)

_______________

_______________

_______________

6. Volume of NaOH dispensed (mL)

_______________

_______________

_______________

7. Molar concentration of NaOH (mol/L)

_______________

_______________

_______________

8. Average molar concentration of NaOH (mol/L)

______________________

*Show calculation for Trial 1.

Experiment 9

117

B. Molar Concentration of an Acid Solution
Acid type: __________ Unknown No. __________
Balanced equation for neutralization of acid with NaOH.

Sample 1

Sample 2

Sample 3

25.0

25.0

25.0

2. Buret reading of NaOH, initial (mL)

_______________

_______________

_______________

3. Buret reading of NaOH, final (mL)

_______________

_______________

_______________

4. Volume of NaOH dispensed (mL)

_______________

_______________

_______________

1. Volume of acid solution (mL)

5. Molar concentration of NaOH (mol/L), Part A

______________________

6. Moles of NaOH dispensed (mol)

_______________

_______________

_______________

7. Molar concentration of acid solution (mol/L)

_______________

_______________

_______________

8. Average molar concentration of acid solution (mol/L)

______________________

*Show calculations for Sample 1.

Laboratory Questions
Circle the questions that have been assigned.
1. Part A.2. Pure potassium hydrogen phthalate is used for the standardization of the sodium hydroxide solution. Suppose that the potassium hydrogen phthalate is not completely dry. Will the reported molar concentration of the sodium hydroxide solution be too high, too low, or unaffected because of the moistness of the potassium hydrogen phthalate? Explain.

2. Part A.4. The potassium hydrogen phthalate primary standard is dissolved in 25 mL instead of the suggested 50 mL of deionized water. How will this volume change affect the calculated molar concentration of the NaOH solution? Explain. 3. Part A.4. Phenolphthalein is a weak organic acid, being colorless in an acidic solution and pink (because of the color of its conjugate base) in a basic solution. The Experimental Procedure suggests the addition of 2 drops of phenolphthalein for the standardization of the sodium hydroxide solution. Explain why the analysis will be less accurate with the addition of a larger amount, e.g., 20 drops, of phenolphthalein. 4. Part A.7. A drop of the NaOH titrant adheres to the side of the buret (because of a dirty buret) between the initial and final readings for the titration. How does this technique error affect the reported molar concentration of the NaOH solution? Explain. 5. Part B.2. The wall of the Erlenmeyer flask is occasionally rinsed with water from the wash bottle (see Part A.7) during the analysis of the acid solution. How does this affect the reported molar concentration of the acid solution? Explain. 6. Part B.2. An air bubble initially entrapped in the buret tip is passed from the buret during the titration. Will the reported molar concentration of the acid be reported too high, too low, or unchanged as a result? Explain. 7. Part B.4. The polyethylene bottle containing the standardized NaOH solution is not tightly capped for subsequent experiments. How does this affect the molar concentration of the NaOH solution? Explain.

Cite this Volumetric Analysis

Volumetric Analysis. (2016, Jul 29). Retrieved from https://graduateway.com/volumetric-analysis/

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