Determination of H3PO4 and NaH2PO4 in a Mixture
The purpose of this experiment is to:
o improve proficiency with acid-base titrimetry, preparing standard solutions, potentiometric measurements, and graphical analysis of titration curves.
o perform a determination of the composition of a binary mixture of phosphates.
o illustrate the use of titrimetry for quantitative studies of complex acid-base equilibria, utilizing ionic strengths and activity coefficients to determine equilibrium constants.
o prepare buffer solutions of specified pH using activity coefficients
Synopsis of the Analytical Problem
A solution containing both H3PO4 and NaH2PO4 is titrated with a standard NaOH solution. The titration curve (pH vs vol. of titrant) is determined using a pH meter, with the inflection points being used as the equivalence points (see Fig. 1).
Figure 1. Schematic pH titration curve of diprotic acid with strong base.
NOTE: Fig. 1 shows the titration curve starting with H3PO4. You will start with a mixture of H3PO4 and NaH2PO4. This is equivalent to beginning the titration somewhere before the first equiv. pt.
The weight-volume percentage (g/100 mL solution) of H3PO4 and of NaH2PO4 will be calculated and reported, as well as experimental values for KA1 and KA2. You will also report directions for preparing a phosphate buffer solution of pH 7.00.
Significance of the Phoshphate Titration
Many materials, for example biological fluids and environmental water samples, contain mixtures of polyprotic acids or bases, and produce very complex equilibria. pH titration data for these samples can provide quantitative determinations as well as insight to the complex chemistry involved. This experiment examines a simple example of these complex systems, and introduces the fundamental principles and techniques needed to study the more complex systems.
For specific procedures below, refer to Text: Skoog, West, Holler, Analytical Chemistry. An Introduction, Saunders Publishers, 6th Ed., 1994.
HCl Preparation (optional).
Measure into the small graduated cylinder sufficient con. HCl to prepare one liter of 0.1M solution. Transfer the HCl and about one liter of distilled water to a glass stoppered bottle. Mix thoroughly and label. (This solution will only be needed for back-titration during standardization of NaOH.)
Prepare about one liter of 0.1M NaOH by adding the proper volume of 50% w/w NaOH (density » 1.48 g/ml) to boiled, distilled water. Store in a polyethylene bottle. (The residue in the bottom of the bottle of saturated NaOH is mostly Na2CO3; carefully pipet the solution so that no Na2CO3 gets into your solution.) See your textbook (p. 217) for an illustration of a storage bottle constructed in a way to prevent CO2 from being absorbed by the solution; although a bottle of this type would be required for long term storage, but is not necessary or practical for this experiment.
Comparison of HCl with NaOH (optional).
See textbook (p. 572) for directions.
Standardization of NaOH with potassium acid phthalate (KHP).
See textbook (p. 573) for directions.
Analysis of Unknown.
Unknown solution. Obtain the unknown from the instructor in a labeled, clean, 500 mL volumetric flask. Dilute to the calibration mark with cool, boiled, distilled water and mix thoroughly. Pipet a 50 mL aliquot of unknown into a 250 mL beaker. (Permit the pipet to drain about 15 seconds, and touch off the drop at the tip. Remember, the volume remaining in the tip is not to be blown or rinsed into the beaker.)
Calibration. (The calibration procedures for the pH meter and pH electrode are in Appendix 7.) The pH meter and the pH electrode used must be calibrated before use. The pH meter is calibrated by determining its response to two known buffer solutions (use pH 7.0 and 4.0 solutions). The response factor of the electrode used (a combination glass(pH) & reference electrode) must also be determined and recorded in the pH electrode(s) calibration record book.
Sample titration. After calibrating the pH electrode with the buffer solutions, rinse the electrode(s) with distilled water, and insert in your sample. Stirring may be done either manually or with a magnetic stirrer and stir bar (obtained from the service center). However stirring is done, be especially careful of the fragile, expensive electrode(s).
Prepare a buret and fill with standard NaOH solution. Read the initial volume and pH; for this titration, it is desirable to start with the initial volume at 0.00 mL. Have a data table prepared in your lab notebook with one column for volume readings and another for corresponding pH readings. Add the NaOH to the stirred sample solution while watching the pH meter; add the base somewhat rapidly at first (reading the volumes after each increase of about 0.3 pH units change). Take volume and pH readings after each addition of base. The meter may be read as soon as the solutions are mixed (10-15 seconds). As the first equivalence point is approached (about pH 3.5) add NaOH more slowly (0.1 to 0.2 mL portions), as the pH changes rapidly with added base. After pH 5, the NaOH can be added more rapidly, and from pH 5.5 to 7.5 several milliliters can be added between readings. Above pH 7.5 add titrant slowly (0.1 to 0.2 mL portions) as the second equivalence point is approached. Stop the titration at about pH 10.5. Titrate three samples. (NOTE: It will usually save the analyst time if a fourth sample is first titrated rapidly to determine the approximate volume and pH at which end points occur.)
Data analysis. Using laboratory computer spread-sheet software, produce plots of the volume-pH data for each determination. (See posted example. A separate hand-out is available from the lab instructor providing instructions for producing these plots using ExcelÔ spread-sheet software.)
Using the volumes read from the graph, calculate the weight-volume percent for both H3PO4 and NaH2PO4. Calculate KA1 and KA2 for H3PO4 from your data. Make use of activity coefficients in your calculations.
(1) Wt/vol % H3PO4 and NaH2PO4, with standard deviation, relative and absolute range,
and confidence limits.
(2) KA1 and KA2 for H3PO4, using activity coefficients
(3) Directions for preparing a phosphate buffer, pH 7.00, approximately 0.05M (again using
activity coefficients. (This should be turned in as a separate report sheet.)
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