9. Procedure
9.1 Extraction
9.1.1 Weigh about 40 g of sample, to the nearest 0.1 g, into a 250 mL beaker and heat on a water bath or in an oven to 65 more or less 5°C. Heat 40 more or less 1 mL of xylene to the same temperature and add slowly to the sample while stirring constantly until dissolution is complete. Transfer the solution quantitatively to the extraction apparatus, rinsing the beaker with two separate portions of 15 more or less 1 mL of hot xylene and adding these rinsings also to the extraction apparatus.
9.1.2 While the solution is still hot, add 25 more or less 1 mL of ethanol or isopropyl alcohol and 15 more or less 1 mL of acetone, using these portions for further rinsing of the beaker. Switch on the heating element of the extraction apparatus to full heat until boiling begins, then adjust the rheostat to regulate the heat to maintain boiling at a vigorous rate, but not at such a rate to cause bumping in the flask or to cause the condenser to flood. Allow to reflux for 2 min after the liquid starts boiling (see 8.1). Switch off the heater. When boiling ceases, add 125 more or less 1 mL of water and again bring the liquid to the boil and reflux for a further 15 min.
9.1.3 Switch off the heater, and allow the two phases to separate for 5 to 10 min. Draw off the aqueous phase, filtering through a filter paper into a conical flask of suitable capacity, stopper the flask, and retain the contents for the total halide determination as described in 9.2 and 9.3.
9.2 Removal of Sulfur-Containing Compounds - Pipet 50.0 mL of the aqueous extract (see 9.1.3) into a beaker, and add 5 mL of 5 mol/L nitric acid (see 6.7). Cover the beaker with a watch-glass, and bring the contents to the boil. Test the vapors periodically for hydrogen sulphide with lead acetate paper and continue boiling for a further 5 min after a negative result has been obtained. Allow the contents of the beaker to cool and quantitatively transfer to a 250-mL titration vessel, rinsing the beaker with water. (Warning - Hydrogen sulfide presents a health risk if incorrectly handled. Avoid inhalation. Extract escaping gas by working in a fume cupboard.)
9.3 Salt Determination
9.3.1 Pipet 10.0 mL of the 1 mmol/L sodium chloride solution (see 6.10; see Note 2) in the titration vessel (see 9.2). Adjust the volume of the sample solution in the vessel to 150 mL with acetone (see Note 3). Add approx. 0.5 g of barium nitrate crystals. Stir to dissolve the barium nitrate.
NOTE 2 - To obtain a titration end point even for small amounts of chloride, a fixed amount of sodium chloride is added to the blank and sample solution.
NOTE 3 - Acetone is added to lower the solubility of the silver chloride precipitate.
9.3.2 Fill the burette with 0.01 mol/L silver nitrate solution, place the beaker on a magnetic stirrer, and immerse the electrodes in the sample solution. Immerse the tip of the burette approximately 25 mm below the liquid surface, and adjust the magnetic stirrer to produce vigorous stirring without spattering.
9.3.3 Titrate as follows:
9.3.3.1 When applying manual titration, record the initial burette reading and the pH/millivolt meter reading. Titrate with standard silver nitrate solution, adding the titrant in small portions. After each addition, wait until a constant potential has been established and record the burette and meter readings (see Note 4). In regions between inflections where the potential change is small for each increment of silver nitrate used, add volumes as large as 0.5 mL. When the rate of change of potential becomes greater than 5 mV per 0.1 mL, use 0.1 mL increments of silver nitrate solution. Construct a graph by plotting the meter readings versus the volumes of standard silver nitrate solution used in the titration.
NOTE 4 - If silver halides are precipitated on the silver electrode, tap the electrode gently to dislodge the clinging precipitate and ensure that an equilibrium has been reached before taking a meter reading.
9.3.3.2 When using an automatic recording titrator, titrate with standard silver nitrate solution, adding fixed increments.
NOTE 5 - It is essential that incremental titration is applied, as described in 9.3.3, to ensure that complete precipitation occurs between additions.
9.3.3.3 After each titration, clean the electrodes with water (see also 8.2)
9.4 Curve Interpretation
9.4.1 The end-point of the titration is found as the most positive value of the steepest portion of the observed inflection of the titration curve.
9.4.2 The exact location of the inflection point is dependent on the chloride concentration, the electrodes used, the nature of the titration medium, and the concentration of the silver nitrate solution applied. To establish the value of the cell potential at the inflection point, prepare a typical solution containing 0.001 to 1 mol of chloride, titrate this solution as described in 9.3, and determine the inflection point.
9.5 Blank Determination - Prepare a blank solution by pipetting 10.0 mL of 1 mmol/L sodium chloride solution (see 6.10) in a titration vessel. Add 50 mL demineralized water, 5 mL of 5 mol/L nitric acid (see 6.7) and approximately 0.5 g of barium nitrate crystals (see 6.5). Adjust the volume of the sample solution in the vessel to 150 to 175 mL with acetone. Stir to dissolve the barium nitrate. Titrate the solution as described in 9.3.
