Photometric TitrationIntroduction and Theory

Read pages 802-4 for additional background information

For some titrations, the equivalence point can be detected by following absorption changes of theanalyte. A primary experimental concern in a photometric titration is the selection of a proper analytical

wavelength. In these titrations at least three components can absorb light: (1) the analyte, (2) the titrant,and (3) the titration product or products. If possible, a wavelength at which only one component absorbs

is selected. The selected chromophore must follow Beer's Law (A = εbc) at this wavelength.

According to Beer's Law, when the concentration of the absorbing species changes, a linear

change in absorbance is observed. In a photometric titration, the chromophore concentration changes upto the equivalence point. In this region, a graph of the absorbance versus the volume of titrant should

yield a straight line.Once the equivalence point is reached, continued titration should not affect the chromphore

concentration. Thus, a graph of the absorbance versus the volume of titrant beyond the equivalence

point should also yield a straight line. Over the entire experiment, a plot of absorbance (or %Transmittance) versus volume of titrant should generate two straight lines. The lines obtained before

and after the equivalence point will not have the same slope. The intersection of these lines occurs at the

equivalence point. Greatest accuracy can be obtained in a photometric titration when the angle of thelines formed by the absorbance vs. concentration plot is acute. In this case the change in the direction of

the line is very pronounced and it gives a good endpoint reading.The selected wavelength may still not be a good one if the absorbance readings are very high,

that is, %T readings that are very low (between 0 and 20%). Here large errors in absorbance

measurement could reduce the accuracy that can be obtained in such a titration. Beer's Law is bestfollowed at wavelengths where absorbance changes slowly with wavelength, where the solutions'

concentrations are not too high, and where factors that could change the chemistry are held constant. Toaddress the latter concern, a buffer is used in this experiment.

In this experiment an unknown solution of Cu(NO3)2 and Ca(NO3)2 is titrated with

ethylenediaminetetraacetic acid- disodium salt (EDTA). The % Transmittance changes are followed at755 nm. (Absorbance could be monitored, but % Trans. yields larger numerical changes.) At this

wavelength the copper-EDTA complex and the copper solution have the greatest % Transmittancedifference. The other molecules in the titration either do not absorb or do so weakly at 755 nm. (neither

Ca+2 nor CaY2- absorb strongly at any wavelength between 300 and 800 nm.)

Procedure

I. Titration Apparatus: Reaction -Pumping Flask and Spectroscopic Cell

Side view Top view sample cell

The titration apparatus consists of a reaction and pumping flask connected to a test tube that acts

as the sample tube for the spectrophotometer. The reaction solution is circulated through the sampletube by the pumping action of the rotating magnetic stirring bar. [Note: all magnetic stirrers do not

rotate the magnet in the direction indicated above. You must use one that rotates the magnet as

indicated.]

In order for pumping to occur, the entire assembly must be filled with solution with no air

bubbles. To fill the system with water, remove the stopper from the sample tube. Fill the sample tubewith water and replace the stopper. Add about 50 mL of water to the reaction flask and clamp the flask

above the magnetic stirrer. While holding the sample tube over a beaker to catch the water, gently

remove the stopper from the tube and let air escape from the tubing coming from the reaction flask.When the lines are filled with water, replace the stopper and seal the system. Adjust the water level in

the reaction flask to the 50 mL line. At this level the system contains approximately 70 mL of water.

Check to make sure that the stopper is tightly fitted to the spectroscopic cell and that the system does notleak.

II. Analysis

Turn on the spectrophotometer and set the wavelength to 755 nm. Zero the % Transmittance

reading to 0.000 with the sample tube in the light path.Using volumetric pipets, transfer 10.0 mL of your Cu-Ca unknown solution and 25.0 mL of

ammonia buffer into the reaction flask. Keep a record of the volume changes of the solution. Turn onthe magnetic stirrer and see that the blue solution circulates and is uniformly distributed. Note the time

it takes for the spectrophotometer to attain a steady %T. This is the maximum mixing time. Record the

initial and all other absorbance values to the nearest 0.1 %T.Rinse and fill the (expensive) 10 mL buret with 0.250 M EDTA solution. Add 0.5 mL of EDTA

to the reaction flask. Allow through mixing and read the %T. Continue adding 0.5 mL increments of

EDTA and reading the %T until a total of 10.00 ml of EDTA has been added. Repeat the above titration

for a replicate analysis.When the titrations are complete, remove the sample tube from the spectrophotometer, clean the

buret and sample pumping system. The reaction flask is fragile! Please be careful!

Treatment of Results

Plot the %T vs. volume (mL) of 0.250 M EDTA data obtained in the photometric titration. Youshould obtain three straight lines, which give two intersections corresponding to a copper equivalence

point and a calcium equivalence point.Calculate the number of moles of Ca(II) and of Cu(II) in the unknown solution. (Note: While no

direct statement has been made regarding which inflection point corresponds to Cu(II), and which to

Ca(II), this should be evident form the discussion above and the shape of the titration curve. Be sure

you associate the correct volume with the proper metal ion.)

Reference: Rehm, Bodin, Connors, and Higuchi Anal. Chem. 1959, 31, 483.

Questions (show calculations as needed)1. If a solution contained only Ca(II), how you would modify this titration with EDTA so as to

determine the number of moles calcium? (5 points)

2. Even though this experiment follows % Transmittance, it was pointed out that adherence to Beer'sLaw is necessary. Dilution of the sample by adding titrant may produce sufficient deviation in theabsorbance so as to require correction.a) Using your data, calculate the absorbance at your Cu2+ and Ca2+ endpoints.

(A = -log T; %T = T x 100) (3 points)

b) If Vi = initial volume of solution and Vt = volume of titrant added, which factor Vi/(Vi + Vt) or(Vi + Vt)/Vi, should you use to correct the absorbance for dilution? (3 points)

c) How may a photometric titration be carried out so that we can neglect the dilution caused byadding the titrant? (3 points)

d) Using the appropriate conversion factor, calculate the percent error in absorbance (due todilution) at your Cu+2 endpoint? At your Ca+2 endpoint? (Use 70.0 mL water, 10.0 mL unknownand 25.0 mL buffer in your calculation. (3 points)

e) Would a plot of the corrected absorbance vs. volume of titrant be advisable in the titration justperformed in lab? (3 points)

Extra Credit (5 points)In view of the fact that copper forms a more stable EDTA complex (log K = 18.7) than calcium(log K = 10.7), explain the order of titration found in this experiment.