Experimental determination of trace arsenic by coulometric titration

Summary

This experiment is from the official website of College of Chemistry, Qingdao University of Science and Technology.

Operation method

Experimental determination of trace arsenic by coulometric titration

Principle

Coulometric titration is a fairly accurate and sensitive analytical method based on the controlled electrolysis of an electric current and can be used for microanalysis and the determination of trace substances. The "titrant", which reacts quantitatively with the substance to be measured, is produced inside the test solution by electrolysis at a constant current. The end point of the coulometric titration is indicated by an indicator or electrochemical method. The amount of "titrant" consumed in the reaction is calculated according to Faraday's law, and thus the arsenic content is calculated. This experiment uses a double platinum sheet electrode electrolysis at a constant current, in the platinum anode KI in the I- can be oxidized to I2. in the anode in the cathode in the anode precipitation of I2 is an oxidant, can oxidize the solution of As (III), the chemical reaction: titration endpoints can be used to indicate the method of starch, i.e., produce excess iodine, can be made to the starch solution appeared blue. blue color. The endpoint can also be indicated by the current-rise method (dead-stop method), in which the endpoint is indicated by a sudden jump in the current. The amount of I2 consumed in the titration can be calculated from the amount of electricity consumed in the electrolysis of I2, and the amount of electricity Q can be obtained from the constant current I during electrolysis and the electrolysis time t: Q = I × t (amperes × seconds) In this experiment, the amount of electricity can be read directly from the digital tube of the KLT-1 general-purpose coulometer. The content of arsenic can be obtained by the following formula: where M is the atomic weight of arsenic 74.92, n is the number of electron transfer of arsenic. the reaction between I2 and AsO33- is reversible, when the acidity is more than 4 mol/L, the reaction is quantitatively carried out to the left, that is, the oxidation of I- by H2AsO4; when the pH>9, the reaction of I2 occurs, which affects the stoichiometric relationship of reaction. When pH>9, I2 undergoes radicalization, thus affecting the stoichiometric relationship of the reaction. Therefore, in this experiment, NaH2PO4-NaOH buffer system is used to maintain the pH of the electrolyte between 7 and 8, so that the reaction proceeds quantitatively to the right. That is, I2 quantitatively oxidizes H3AsO3, and the dissolved oxygen in the water can also oxidize I- to I2, which makes the result low. Therefore, in titrations with high standardization requirements, deoxygenation is necessary. In order to avoid the reducing effect of H2 generated on the cathode, an isolation device should be used.

Materials and Instruments

Phosphate buffer solution 0.2 mol L Potassium iodide solution Arsenic standard solution
KLT-1 Universal Coulometer 10 mL Cylinder 0.5 mL 5 mL Pipette

Move

1. Instruments and reagents

KLT-1 universal coulometer; 10 mL measuring cylinder; 0.5 mL and 5 mL pipettes.

Reagents:

(1) phosphate buffer solution: weigh 7.8 g NaH2PO4-2H2O and 2 g NaOH, dissolve with deionized water and dilute to 250 mL (0.2 mol/L NaH2PO4; 0.2 mol/L NaOH).

(2) 0.2 mol/L potassium iodide solution: weigh 8.3 g of KI and dissolve in 250 mL of deionized water.

(3) Arsenic standard solution: weigh 0.6600 g As2O3 accurately, wet with a small amount of deionized water, add NaOH solution and stir to dissolve, dilute to 80~90 mL, neutralize the solution with a small amount of H3PO4 until the solution is close to pH 7, then transfer it to 100 mL volumetric flask and dilute it to the scale, shake well. The concentration of this solution is arsenic 5.00 mg/mL, and can be further diluted to 500 μg/mL when used.

2. Experimental procedure

(1) Adjust the universal coulometer;

(2) Turn on the power switch to warm up for half an hour;

(3) Take 10 mL of 0.2 mol/L KI, 10 mL of 0.2 mol/L phosphate buffer solution, put it in the electrolytic cell, add 20 mL of distilled water, add arsenic-containing water samples 5.00 mL, and submerge the electrodes in the solution.

(4) End point indication selection current - rise

(5) Press the electrolysis button, the light goes out and electrolysis starts. The millicoulombs starts to be recorded on the digital tube.

(6) When electrolysis is complete, note the number of millicoulombs consumed.

(7) Add 5.00 mL of arsenic-containing water sample to the electrolyte and do another electrolysis to get the second millicoulomb, and repeat this 4 times to get 4 millicoulombs.

(8) Round off the first data, take the average of the last three, calculate the amount of As in the water sample. As mg/mL, or As2O3 mg/mL.

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