1、 Purpose requirements

1. MasteringMicrocoulometric sulfur titratorThe principle and method of using chemical indicators to indicate the endpoint.

2. Apply Faraday's law to calculate the concentration of an unknown substance.

2、 Principle

In an acidic medium, 0.1mol/L KI is electrolyzed on a Pt anode to produce "titrant" I2 to "titrate" S2O32-, with starch indicating the endpoint. Calculate the concentration of Na2S2O3 based on the Coulomb number consumed by electrolysis.

Specifically, it is to oxidize iodine ions to iodine on the platinum anode, and then react with S2O3 in the test solution. The electrode reaction of the working electrode pair is: anode: 2I=I2+2e cathode: 2H+2e=H2

Titration reaction: I2+2S2O32-2-+S4O6+2I, the endpoint can be indicated by starch indicator. When S2O3 is completely oxidized from 2-2 to S4O6, excess iodine will precipitate on the indicator electrode, resulting in starch blue in the solution and stopping electrolysis. According to the amount of electricity consumed during the electrolysis of iodine, the Na2S2O3 content in the solution can be calculated using Faraday's law.

3、 Instruments and reagents

　　Microcoulometric sulfur titratorOne set, two platinum electrodes.

Reagent 0.1mol/L KI solution: Weigh 1.7g of KI and dissolve it in 100ml of distilled water for later use; Unknown Na2S2O3 solution;

0.1% starch solution.

4、 Experimental steps

(1) Connect the circuit, that is, connect the Pt working electrode to the positive electrode of the coulometer, connect the Pt auxiliary electrode to the negative electrode, and install it in a glass sleeve. Add 5mL of 0.1mol/L KI solution to the electrolytic cell, add a stirrer, insert 2 Pt electrodes and add an appropriate amount of distilled water to immerse the electrodes perfectly. Also add an appropriate amount of KI solution to the glass sleeve. Add 3ml of 0.1% starch to indicate the endpoint. If the solution turns blue, perform experimental step (2); If the blue color remains unchanged, proceed with experimental step (3) directly.

(2) Zero adjustment of solution

The purpose of this step is to remove I2 that may have been oxidized during the placement of KI solution, and to add a few drops of dilute Na2S2O3 to remove the blue color of the solution. Set the electrolysis current to 10mA, start the electrolysis, and the endpoint is when the solution turns blue. At this time, the instrument reading is the millicoulomb number consumed (no need to record, zero adjustment clears the original impurities in the solution), and reset to zero.

(3) Measurement: Take 5ml of the test solution (determined by the amount of Na2S2O3 in the test solution, with V within the range of 0.1-10ml), place it in an electrolytic cup, and start electrolysis. Electrolysis to the endpoint, record the number of millicoulombs consumed.

(4) Retest twice

(5) Turn off the mixer and clean the electrolytic cup and electrodes.

5、 Attention:

1. The polarity of the electrode must not be connected incorrectly. If it is connected incorrectly, the electrode must be carefully cleaned.

2. KI solution should be placed in the protective tube to immerse the Pt electrode.

3. The test solution must be accurately removed each time.