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Computer-Interfaced Experiments - Absorbance Measurement Reaction of Methyl Orange with Tin(II) Chloride - Pseudo First Order Reaction Peter Keusch |
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Analog-Digital-Converter CHEMBOX IBK electronic + informatic IBK electronic + informatic |
0.2 M SnCl2 solution: 25 g of SnCl2 · 2 H2O are dissolved in 21.8 mL of conc. HCl. The solution is made up to 1 L with dist. water.
Hazards and safety precautions:
Safety goggles and gloves must be worn. The experiment should be performed in a well ventilated room. The SnCl2 solution should be prepared under a fume ! Theoretical background: SnCl2 reduces methyl orange in strongly acidic solution to form p-amino benzene sulfonic acid and p-dimethyl-amino aniline 
Fig. 1: Fading of methyl orange with Sn 2+Hence the reaction can be monitored by measuring the methyl orange absorbance as a function of time. Under the given conditions (see procedure - large excess of one of the reactants) the reaction is pseudo first order. Kinetic equations (Download PDF file)
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20 mL of a 0.001 molar aqueous solution of methylorange are pipetted into a beaker. 20 mL of
0.2 M SnCl2 solution are transferred into a further beaker. The two beakers are placed in a water bath, in which a contact
thermometer and a thermometer with a resolution of 0.1 °C are immersed (Fig. 2). A reaction temperature below
the room temperature is obtained and maintained by careful addition of ice or cold water to the water bath. After thermal
equilibrium has been reached (15 minutes) the reaction temperature is read.
2 mL of the thermostated SnCl2 solution is pipetted into a cuvette. 1 mL of the thermostated solution of methylorange is added. The cuvette covered with a slide plate is inverted 2-3 times to ensure a proper mixing. If necessary the outside of the cuvette is wiped to dry. Immediately the cuvette is placed into the sample compartment of the photometer (l = 470 nm) and the sensing software is started. The measuring interval is 1 second. The absorbance maximum of methyl orange is 410 nm. The used diode photometer IBK provides light emitting diodes with the wavelengths 660 nm (red), 565 nm (green) and 470 nm (blue). The change in transmittance is displayed simultaneously on the measuring screen. The in-situ determination of the reaction rate on the basis of a continuous logging of photometrical data is allowed in rapid reactions (small change in temperature during te reaction). |
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