DETERMINATION OF COMPOSITION OF A CARBONATE – …



DETERMINATION OF COMPOSITION OF A CARBONATE – BICARBONATE MIXTURE: METHOD OF ALIQUOT AND pH TITRATION

BACKGROUND

Crude sodium carbonate, called soda ash, is commonly used as a commercial neutralizing agent. The titration with standard acid to the bromocresol green end point gives the total carbonate content. The results are usually expressed as the weight percent of sodium carbonate in the crude sample. Since the samples are frequently non-homogeneous, the method of aliquot portions is employed. In this experiment, the same basic principle is used to find the percent composition of a mixture of sodium carbonate, sodium bicarbonate in unknown sample. This is done in two ways: acid-base titration using two different indicators and the pH titration using a pH meter. You are then asked to compare the two methods.

PROCEDURE

PART I: Standardization of HCl

Follow the attached procedure for the standardization of HCl.

PART II: Analysis of a mixture of carbonate and bicarbonate

Follow attached procedures (from Harris web site).

OR,

PART II: Method of aliquots in the determination of percent sodium carbonate in an unknown mixture

Weigh precisely into a clean 250-ml beaker a 5.0 g sample of the dried (110 oC for one hour followed by cooling in a desiccator) unknown. Dissolve the sample in about 125 ml of distilled water. Place a clean funnel in a 250 ml volumetric flask and transfer the solution from the beaker to the flask. Rinse the beaker, add the rinsate to the flask, and finally dilute to the calibration mark with distilled water. Mix the contents of the volumetric flask by inversion and shaking.

Pipet a 25 ml aliquot into a 125 mL Erlenmeyer flask and add 3-5 drops of bromocresol green. Titrate with standard hydrochloride acid (prepared in part I) until the indicator turns from blue to green. Then boil the solution to expel CO2. The solution should return to blue color. Carefully add HCl from the buret until the solution turns green again. Repeat the titration with at least two other 25 ml aliquots. At the end of the titration clean the volumetric flak thoroughly. An alkaline solution should never be left in a volumetric flask for a long period of time (why?). Transfer the solution into a plastic bottle for later use.

Report the mass percentage of sodium carbonate (Na2CO3) in the unknown. A precision of 3 to 5 parts per thousand is not unusual for this analysis.

PART III. Analysis of a mixture of carbonate and bicarbonate by pH Titration.

The unknown soda ash is titrated with standard HCl using a potentiometric (pH) end point measured with a pH meter and combination electrode. The indicator (bromocresol green) end point results are compared with potentiometric end point results to observe the clarity and variability of various techniques. The relevant reactions and end point changes are as follows:

CO32- + H+ ( HCO3- (First end point, pH = 8.3)

HCO3- + H+ ( H2O + CO2 (Second end point, pH = 4.5)

The purpose of the first titration is to locate approximately each of the two equivalence points in the titration of carbonate ion with HCl. The second, or final, titration is used to determine the second equivalence point in the titration.

Manual Titration Procedure

First, calibrate the pH meter using two standard pH buffers (pH 4.00 and pH 7.00). Transfer using a volumetric pipet a 25.00 mL aliquot of the sample soda ash solution to a 250 mL beaker containing a magnetic stirring bar. Add approximately 25 mL of distilled water (Of which the exact volume is not important. Why?) to the beaker. Place the beaker on a stirrer, immerse the pH electrode, and start the stirrer being careful not to touch the electrode to the stirring bar. Titrate with standard HCl, taking both pH and mV readings every 2.00 mL. Plot a curve (using any spreadsheet software) of pH versus volume of HCl added and locate the approximate end points (both of them) on the graph.

For the final titration, after transferring a 25.00 mL sample aliquot from the original sample solution and diluting with 25 mL of deionized water, take pH readings every 5 mL to within 3 mL of the second equivalence point (including both sides of equivalence point). Then take readings every 0.50-mL interval. Near the equivalence point, take readings as quickly as possible because the pH will tend to drift as carbon dioxide escapes from the solution. Enter your data into a spreadsheet and set up data entry as mL HCl added in one-column and mV readings in the other.

Repeat the titration focusing at the second end point. From 0.9Ve2 to Ve2, add HCl in 0.5 mL increments.

Automated Titration

Pipet 5.0 mL of the unknown solution and add 25.0 mL of water in the titration beaker. Prime the pump with the standard HCl. Measure pump flow rate by delivering a known volume of HCl in a 10.0 mL measuring cylinder for a fixed time. Assemble the titration setup and ready for the program run. Ask your instructor to help you in this step. The computer controlled pH measurement system must be started the same time the pump is on. Record the pH until it drops to a stable acidic range. The real time titration graph shows the two end points and the end of the titration. The titration data- pH vs. time is recorded in XL format. You must convert the time to volume of HCl added by the flow rate. A typical auto-titration plot is shown in the following figure.

[pic]

Gran Plot

[pic]

1

A Gran plot will be used to graphically determine the equivalence points from the titration data. This will be discussed in class. Other descriptions of plotting potentiometric end points are provided in the textbook. Be sure and review these pages before you plot your data according to the Gran function. Use the titration data at preceding and up to the equivalence point (i.e., 0.9Ve to Ve) for the Gran plot. The acid dissociation equilibrium equation for the protonation of carbonate and bicarbonate ions can be used to derive the Gran function below (read Harris web book!).

Where, VH = volume of HCL added; Ka1 = acid dissociation constant for first dissociation of carbonic acid; ( = activity coefficients; Ve2 = second equivalence volume; HA and A are the conjugate acid base pairs, respectively. The above equation is given for the second end point region. The fist equivalence point can be obtained similarly by using the data near the first end point region. Which Ka value did you get from this region? Convince yourself by writing appropriate equilibrium expression. For this experiment you can assume the activity ratio as unity.

A plot of the Gran plot of VH x 10pH vs. VH for titration volumes near and up to the equivalence point provides a straight line which is extrapolated to the x-axis to find Ve. You should only consider the straight line portion to find the slope and the intercept. Calculate Ka from the slope of the Gran plot. A typical Gran plot is shown below.

|[pic] | |

| |The Gran plot shown here was done with the above |

| |titration data in the region of first infection |

| |point. This region should be selected such that it |

| |covers data points from the foot to the peak of the |

| |second derivative (as your guide). Then chose the |

| |linear portion to extrapolate volume on the x-axis |

| |as shown by the straight line. Alternatively, you |

| |can use ‘trend line’ to calculate the volume only on|

| |the straight-line portions of the data. You can find|

| |the Ka1 of carbonic acid from the slope which is = |

| |-1/Ka1. |

DATA AND RESULTS

1. This is a composite lab. A complete lab report should be submitted with the following information.

2. Present your data for the standardization of HCl. You also need to use the NaOH previously standardized.

3. Present your data for the indicator based titration of the mixture. Use the attached example calculation to calculate the percent composition. If a group is using the same unknown, then you must present all the data of the group and calculate the average, relative standard deviation and (95% . Make sure to have all the raw data from your group members.

4. pH titration: Obtain the pH titration XL file. Plot the pH-volume, the first derivative, and the Gran plots. Find the exact titration inflection points and the volume of HCl from the Gran plots. You must do two Gran plots: one for the first inflection region and the other for the second inflection region.

5. Also, obtain the Ka value from the Gran plot. Compare the Ka1 and Ka2 with those of the literature.

6. Calculate the percent composition (and errors) of the mixture by using the inflection volumes obtained from Gran plots. Example calculations are shown in the following pages.

7. Finally, compare and contrast the two methods as an analytical tool for this analysis.

QUESTIONS

(Submit answers to the following questions with your lab report)

1. Explain why is it necessary to expel CO2 near the end of the titration?

2. The alkalinity of natural waters is usually controlled by OH-, CO32-, and HCO3-, which may be present singularly or in combination. Titrating a 100.0 mL sample to a pH of 8.3 requires 18.70 mL of a 0.0281 M solution of HCl. A second 100.0 mL aliquot requires 48.20 mL of the same titrant to reach pH of 4.5. Calculate the concentrations of CO32- and HCO3- in ppm. (Alkalinity is a measure of water's ability to neutralize acid)

NOTES

❑ With appropriate planning of the work, the sample should be placed in the oven overnight for proper drying.

❑ Be extremely careful in the weighing and sample transfer operations because any error here has a grave consequence. The results of the subsequent determination will be incorrect even if the results of repeated titrations of aliquots of the sample solution agree well.

Calculation Procedure

TITRATION OF A MIXTURE OF CARBONATE AND BICARBONATE

Part II. Titration using visual indicators

Find the volume of HCl for Phenolphthalein end point: Vp

Let MHCl = molarity of HCl and MOH = molarity of NaOH

Volume of NaOH added = Va

mols of bicarbonate = Va x MOH - Vp x MHCl

[HCO3-] = mols of HCO3- / Volume of aliquot

Calculate average, standard deviation and (95%

Find volume of HCl required for end point with Bromocresol Green (BG): Vb

The BG end point indicates the complete conversion of carbonate and bicarbonate to carbonic acid (pH about 4-5). We know

mols of CO32- / mols of HCl = ½ mols of HCl = 2 mols of CO32-

Total mols of HCl used = (Vb x MHCl) = mols HCl used for HCO3- + mols HCl used for CO32-

Vb x MHCl = mols of bicarbonate + 2 . mols of carbonate

Since mols HCO3- is known from the previous calculation, mols of CO32- can be calculated.

[CO32- ] = mols of CO32- / Volume of aliquot

Calculate average, standard deviation and (95%

Calculate alkalinity, Alkalinity = 2 [CO32- ] + [HCO3-] molarity

Calculate error in alkalinity based on standard deviations of [CO32- ] and [HCO3-]

(Do error propagation)

Calculate wt% of CO32- and HCO3-

Find mols of CO32- and HCO3- in original sample from molarity

Find gms of CO32- and HCO3- in original sample from mols

Find wt% of CO32- and HCO3- in original sample,

= (g found x 100) / g sample taken

Example Calculation

You are given a mixture containing NaHCO3 and Na2CO3. You took 3.5000 g of the sample and dissolved to 250.0 mL in a volumetric flask with deionized water. A 25.0 mL aliquot of the sample required 36.0 mL of standard 0.100 M HCl to reach the Bromocresol green end point. Another 25.0 mL aliquot was mixed with 50.0 mL of standard 0.100 M NaOH. Then the solution is treated with BaCl2 to precipitate carbonate as BaCO3(s) and immediately titrated with standard 0.100 M HCl to phenolphthalein end point. The volume of HCl required was 40.0 mL. Calculate the wt% of NaHCO3 and Na2CO3 in the mixture.

Vp = 40.0 mL

Va = 50.0 mL

mmols of bicarbonate = 50.0 x 0.100 - 40.0 x 0.100 = 1.00 mmols

mg of NaHCO3 = 1.00 mmol x 84 mg/mmol = 84.0 mg (in 25.0 mL aliquot)

[HCO3-] = 1.00/ 25.0 = 0.04M

Vb = 36.00 mL

36.00 x 0.1000 = 2 * mmols of carbonate + mmols of bicarbonate

mmols of carbonate = 0.5 (3.6 - 1.00) = 1.30

mg of Na2CO3 = mmols of carbonate x MW of Na2CO3

= 1.300 mmol x 106 mg/mmol = 137.8 mg (in 25.0 mL aliquot)

[CO32-] = 1.300 mmols / 25.00 mL = 0.052 M

Wt% Na2CO3 = 137.8*100/350.0 = 39.4% Wt% NaHCO3 = 84.0*100/350 = 24.0 %

Part III: pH titration of CO32- and HCO3- mixture

Use the final pH titration data for the following calculation

• Plot mV vs. Volume of HCl added

• Take first derivative of mV-mL data and find the volume of HCl at two inflection points: V3 and V4

• Do Gran plot: Find precise V4, Ka1 and their errors from linear regression. Compare Ka1 with that of literature value ( Ka1 = 4.45 x 10-7 for H2CO3 == H+ + HCO3- )

• Use V3 from derivative and V4 from Gran plot to calculate CO32- and HCO3- as follows:

• mols of CO32- = V3 x MHCl

• mols of HCO3- = (V4 - 2V3) x MHCl

• Calculate the molarity of CO32- and HCO3- in the original solution: mols/volume of aliquot used. Also, calculate the alkalinity. Compare the results of the two methods.

• Calculate the wt% of CO32- and HCO3- in the original mixture. Compare the results with that of the indicator titration method.

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