Experiment: Fading of Phenolphthalein in Alkaline Solution

Computer-Interfaced Experiments - Absorbance Measurement

Kinetics
Fading of Phenolphthalein in Alkaline Solution
Objectives: Test for a Pseudo-First-Order Behaviour, Determination of the Half-Life and the Rate Constant

Peter Keusch

Datalogging and data analysis using the Program CHEMEX and the
Analog-Digital-Converter CHEMBOX IBK electronic + informatic
IBK electronic + informatic

German version

Chemicals:
phenolphthalein solution, 1 % in ethanol (Merck)
1 M NaOH

Apparatus and glass wares:
volumetric pipet 2 mL

micropipette

diode photometer IBK

plastic cuvettes 10 · 10 · 45 mm (Sarstedt)

disposal container

Theoretical background:

Phenolphthalein is colorless in acid solution (pH < 8), red in slight alkaline solution (9 < pH < l3) and again colorless in strong alkaline solution (pH > 14)

When phenolphthalein is added to an alkaline solution it first undergoes a rapid and successive conversion into mono and then diphenylate ion which then rearranges to give a pink colored quinoid species (dianion). The two phenol rings of the dianion are incorporated into a planar resonance form. The dianion reacts slowly with hydroxide ions to form the non-resonant (colorless) carbinol form (trianion). The addition of hydroxide to the central carbon of the triphenylmethane structure results in a disruption of the conjugatedp system.

Fig. 1: Reaction of phenolphthalein with NaOH

The kinetics of the "fading" reaction can be conveniently studied by following the decolorization of the reacting mixture using a photometer. The change in concentration of (P ^2-) is monitored by measuring the chsnge in absorbance as a function of time.
The reaction obeys the rate law of a second order reaction
rate = k [ P ^2- ] [ OH ^- ]
If the reaction is carried out under conditions where the concentration of phenolphthalein is very small compared to OH ^- (see experimental procedure below), thus, during the reaction, it is safe to assume that OH ^- concentration remains essentially constant.
The rate law will reduce to the form rate = k' [ P ^2- ]
where k' is the pseudo-first-order rate constant incorporating both the "true" second-order rate constant k and the eyperimentally constant [ OH ^- ]:

k' = k [ OH^- ]

Kinetic equations (Download PDF file)

Calibration of the photometer and matching of the program CHEMEX

The recorder output of the photometer is connected to the input 'Sensor1' of the CHEMBOX.

Calibration of the photometer

Via the menu item Options - Calibration the dialog for the appropriate input is to activate. The calibration procedure is carried out as shown in Figure 2 Bromination of reactive Aromatics.

The wavelength is set to 470 nm. The green button of the photometer is pressed (Fig. 4). Nothing should be in the sample compartment
10 mL of alcoholic phenolphthalein solution are added to a cuvette, filled with 2 ml of 0.1 M aqueous NaOH. (The mixture must be freshly prepared). After being shaken, the cuvette is placed into the sample compartment. 0,0V is entered into the field of Ref1 and the button Set of Ref1 is clicked. The cuvette is removed.

Now 1V is entered into the field of Ref2. A cuvette containing a decolorized reaction solution (2 mL of 1 M aqueous NaOH, 10 mL alcoholic phenolphthalein solution) is inserted in the sample compartment and and the button Set of Ref2 is clicked.
The boxes of both buttons (Ref1, Ref2) must show a green check mark. If this mark does not appear, the calibration must be repeated.

Setup - Channel-Linking

Under the menu item View the Setup dialog is activated.

Linking

Fig. 2: Matching of the program
The calibrated signal of the photometer is indicated by K1. K1 · 100 is for the transmittance T. The other channels use likewise the signal K1, convert this however to absorbance A or to -lnA or to 1/A.

Experimental procedure:

Fig. 3: Experiment set-up and recording of the measurement

The experiment is performed at room temperature. A cuvette is filled with 2 mL of 1 M aqueous NaOH. Using a micropipette 10 mL of the alcoholic phenolphthalein solution are added and the cuvette is shaken. If necessary the outside of the cuvette is wiped to dry. After the cuvette has been positioned in the sample compartment of the photometer (l = 470 nm) and the cover closed, the sensing software is started. The measuring interval is 1 second.

The change in transmittance, in absorbance, in -lnA and in 1/A are displayed simultaneously on the measuring screen (Fig.3, 4).

The in-situ determination of the reaction rate on the basis of a continuous logging of photometrical data is allowed in fast reactions (see temperature constancy).

Fig. 4: Real-time plots
red: T = transmittance blue: A = absorbance violet: -lnA green: 1/A

Testing for the order of the reaction is illustrated on the screen (Fig. 4). The change in absorbance corresponds to the change in concentration. Hence, the plot of absorbance A against time t is a zero order kinetics plot, the plot of -lnA vs time corresponds to a first order plot and the plot of 1/A vs time is a second order kinetics plot. A straight-line plot is only obtained for -lnA vs t (violet graph). The reaction is first order.

Fig. 5: Plot of absorbance versus time determination of half-life

When dealing with first-order reactions, the use of a half-life rather than a rate constant is often convenient. Half-life is the time that it takes for 1/2 of the reactant to disappear. Appropriate cursor-positioning allows the determination of the half-life t _1/2 on the screen (Fig. 5 mouse cursor: nw-resize, title bar). The half-life is completely independent of the initial concentration or amount chosen as the "starting point". In the figure above two successive half-life periods of 25.1 s are illustrated. The constancy of the half-life indicates a first order reaction.

According to

equation

the pseudo-first order rate constant at room temperature is found: k' = 0.0276 [ s ^-1 ]

Reference:
Computer-Interfaced Experiments Kinetics: Fading of Triphenylmethane Dyes - Pseudo First Order Reaction

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