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Measurement of Sulfide of Hazardous Waste sample

 Measurement of Sulfide of Hazardous Waste sample


ô PURPOSE:

                    To describe the Laboratory standard operating procedure for measurement of Sulfide of Hazardous waste samples.

                     

ô SCOPE:

                    Applicable to the Laboratory for analysis of Hazardous waste samples where measurement of     Sulfide is required.

 

ô RESPONSIBILITY:

Lab chemist

:

Analysis of sample for measurement of Sulfide

Technical Manager

:

Review of activity

Quality Manager

:

Implementation and compliance of SOP.

                     

ô PROCEDURE:

1.     Principle:

·       The separation of sulfide from the sample matrix is accomplished by suspending the sample in concentrated hydrochloric acid by vigorous agitation.  Tin(II) chloride is added to prevent oxidation of sulfide to sulfur by the metal ion [as in copper(II)], or by dissolved oxygen in the reagents.  The prepared sample is distilled under acidic conditions at 100°C under a stream of nitrogen.  Hydrogen sulfide gas is released from the sample and collected in gas scrubbing bottles containing Zinc(II) and a strong acetate buffer.  Zinc sulfide precipitates out.

2.     Instrument and Equipment:

      As shown in figure





3.     Reagents:

·       Zinc acetate solution for sample preservation (2N): Dissolve 220 g of zinc acetate dihydrate in 500mL of reagent water.

·       Sodium hydroxide (1N): Dissolve 40 g of NaOH in reagent water and dilute to 1 liter.

·        Formaldehyde (37% solution), CH2O. 

·       Zinc acetate sodium acetate buffer solution: Dissolve 100 g sodium acetate, NaC2H3O2, and 11 g zinc acetate dihydrate in 800mL of reagent water.  Add 1mL concentrated hydrochloric acid and dilute to 1 liter.  The resulting pH should be 6.8.

·       Hydrochloric acid (9.8N), HCl: Place 200mL of reagent water in a 1-liter beaker.  Slowly add concentrated HCl to bring the total volume to 1 liter.

·        Nitrogen gas Cylinder

·       Tin(II) chloride, SnCl2, granular.

·       Sodium hydroxide (6N), NaOH: Dissolve 240 g of sodium hydroxide in 1 liter of reagent water.

·       Hydrochloric acid (6N), HCl: Place 51mL of reagent water in a 100mL Class A volumetric flask.  Slowly add concentrated HCl to bring the total volume to 100mL.

 

 

For Titrimetric Method:

·       Sodium thiosulphate (0.025N): Dissolve 3.9528g Na2S2O3 in distilled water. Add 0.4 g NaOH & dilute it to one liter. Standardize against 0.025N K2Cr2O7.

·       Starch Solution: Dissolve 2 g soluble starch and 2 g salicylic acid, C7H6O3, as a preservative, in 100mL hot water.

·       Iodine Solution (0.025N): Dissolve 25 g of potassium iodide, KI, in 700mL of water in a 1-liter volumetric flask.  Add 3.2 g of iodine, I2.  Allow to dissolve.  Dilute to 1 liter and standardize against 0.025N Na2S2O3.

·       6N HCL Solution: Add 500ml Conc. HCL to 500ml distilled water

4.   Test Method:

            Interferences:

·       Aqueous samples must be taken with a minimum of aeration to avoid volatilization of sulfide or reaction with oxygen, which oxidizes sulfide to sulfur compounds that are not detected.

·       Reduced sulfur compounds, such as sulfite and hydrosulfite, decompose in acid, and may form sulfur dioxide,  which may be carried over to the zinc acetate gas scrubbing bottles and subsequently react with the iodine  The addition of formaldehyde into the zinc acetate gas scrubbing bottles removes this interference

Sample Preservation:

·       All aqueous samples are preserved by adding four drops of 2N zinc acetate/100ml & then adjusting its pH above 9.0 by adding 6N sodium hydroxide solution. If high conc. of sulfide is present then add zinc acetate till complete precipitation of sulfide.

·       For solid samples, fill the surface of the solid with 2N zinc acetate until moistened & preserve at 4oC.

Distillation:

·       Weigh the sample depending upon the concentration of sulfide In case of aqueous sample take 50ml volume. Volume of water should be added such as to maintain the normality of HCl to 6.5N.

·       Add 5g SnCl2 into the distillation flask. Use upto 50ml distilled water for rinsing.

·       Assemble the distillation apparatus as in Figure 1.  Place 100 + 2.0mL of zinc acetate/sodium acetate buffer solution and 5.0 + 0.1mL of 37% formaldehyde in each gas scrubbing bottle.  Tighten the pinch clamps on the distillation flask joints

·       Add 100 ± 1.0mL of 9.8N HCl to the dropping funnel.  Connect the nitrogen line to the top of the funnel and turn the nitrogen on to pressurize the dropping funnel headspace.

·       Set the nitrogen flow at 25mL/min.  The nitrogen in the gas scrubbing bottles should bubble at about five bubbles per second.  Purge the oxygen from the system for about 15 minutes.

·       Turn on the magnetic stirrer.  Set the stirring bar to spin as fast as possible.  The fluid should form a vortex.  If not, the distillation will exhibit poor recovery.  Add all of the HCL from the dropping funnel to the flask.

·       Heat the water bath to the boiling point (100 oC).  The sample may or may not be boiling.  Allow the purged distillation to proceed for 90 minutes at 100 oC.  Shut off nitrogen supply.  Turn off heat. Then find out the concentration of sulfide by iodometric titration method.

By Titrametric Procedure:

·       Pipette a known amount of standardized 0.025N iodine solution.  Add enough water to bring the volume to 100mL.  The volume of standardized iodine solution should be about 65 mL for samples with 50 mg of sulfide.

·       Add 2mL of 6N HCl to the iodine.

·       Pipette solution from both of the gas scrubbing bottle into the flask, keeping the end of the pipette below the surface of the iodine solution.  If at any point in transferring the zinc acetate solution or rinsing the bottles, the amber color of the iodine disappears or fades to yellow, additional 0.025N iodine must be added. Record the total volume of standardized 0.025N iodine solution added.

·       Prepare a rinse solution by taking known amount of standardized 0.025N iodine solution, 1mL of 6N HCl, and water to rinse the remaining white precipitate (zinc sulfide) from the gas scrubbing bottles into the flask.  There should be no visible traces of precipitate after rinsing.

·       Rinse any remaining traces of iodine from the gas scrubbing bottles with water, and transfer the rinses to the flask.

·       Titrate the solution in the flask with standard 0.025N sodium thiosulfate solution until the amber color fades to yellow.  Add enough starch indicator for the solution to turn dark blue and titrate until the blue disappears.  Record the volume of titrant used.

 

a.     Calculation:

       i.          By Titrimetric Method:

            Sulfide mg/kg   =     [ (A x B ) -  (C x D)] x 16000

       

                        Weight of sample in gm

 

            Where    A = Volume of Iodine

                          B = Normality of Iodine

                          C = Volume of Na2S2O3

                          D = Normality of Na2S2O3



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