Determination of Sulfide in Water

This method is applicable in essentially all waters including most industrial wastewaters. 

Equipment Required

1. pH/ION analyzer or pH meter with millivolt scale
2. Sulfide combination ion selective electrode 

Reagents

1. Sulfide anti-oxidant buffer (SAOB)
2. Sodium sulfide standards

Standard Preparation

Sulfide Anti-Oxidant Buffer (SAOB)

 

To approximately 600 ml of distilled water in a 1000 ml beaker, add 200 ml of 10M NaOH (or 80g reagent grade pellets), 35 grams of ascorbic acid and 67 grams of disodium EDTA. Stir until everything dissolves and transfer the solution to a 1000 ml volumetric flask. Dilute to the mark with distilled water.

 

Freshly prepared SAOB, when stored in a tightly closed bottle, has a shelf life of approximately two weeks, if opened frequently. When oxidized, the solution turns dark brown and should be discarded. Light brown solutions are still usable.

 

Sodium Sulfide Standards

Sulfide standardizing solutions are prepared from reagent grade sodium sulfide hydrate, Na₂S.9H₂O. It is advised that all sulfide solution preparation and measurement be performed in a hood, to avoid breathing noxious fumes.

Precise standards cannot be prepared by weighing the salt because of the large and variable hydration of water. Instead, prepare saturated Na₂S solution by adding approximately 100g of the Na₂S.9H₂O hydrate to approximately 100 ml of water, shake well, close securely, and allow to stand at least overnight.

To prepare a stock sulfide solution, pipette 1 ml of the saturated solution described above into 50 ml of SAOB and dilute to 100 ml volume.

The concentration of stock sulfide solution must be determined by electrode titration before constructing a calibration curve. Use a titrant of known concentration of either 0.1M lead perchlorate or 0.1M cadmium nitrate. 

Take 50 ml of stock sulfide solution, add this to 25 ml SAOB and 25 ml H₂O, and titrate using the sulfide electrode, and either of the titrants mentioned previously.

Calculation

The concentration of stock sulfide solution in mg/liter (C) is given by:

C= (ml of lead or cadmium solution) x 64.

Each day, prepare four calibration standards using 100 ml volumetric flasks as follows:

 

“A”       5.00 ml of “sulfide stock”

                        45 ml of SAOB (use granulated cylinder)

                        water to volume

“B”       1.00 ml of “sulfide stock”

            50 ml of SAOB

            water to volume

“C”       2.00 ml of calibration standard “A”

            50 ml of SAOB

            water to volume

“D”       1.00 ml of calibration standard “A”

            50 ml of SAOB

            water to volume

The concentration of the calibrating standard is calculated from the concentration of the sulfide stock, as determined by titration.

If the stock concentration is C mg/liter, then the calibration standards have the following concentrations: 

            “A”=      0.05C

            “B”=      0.01C

            “C”=     0.001C

            “D”=     0.0005C

Sample Preparation

Samples are treated prior to analysis with sulfide anti-oxidant buffer.

Samples should be taken with a minimum of aeration to avoid air oxidation of sulfide or loss of volatile hydrogen sulfide from the sample. Samples can be preserved by adding 0.2 ml (4 drops) of 2M zinc acetate (equivalent to 128 mg/liter S⁼) and 0.05 ml (1 drop) of 6M sodium hydroxide to a 100 ml bottle, filling it completely with the sample. Close it with no air bubbles trapped under the stopper. If the concentration of sulfide is greater than about 100 mg/liter, the amounts of both reagents should be increased.

Method

Prepare a calibration curve by immersing the electrode pair in each of the calibration standards, beginning with the weakest one, and record the stable millivolt potential reading developed by each one. Construct a graph using this data, using semi-log paper and placing the sulfide concentration scale on the logarithmic scale and the mV reading on the linear scale.

Pipette the sample into an equal volume of SAOB, stir thoroughly without vortex, and allow to stand for 3-5 minutes. Place the electrodes in the solution; record the stable electrode potential and determine the sulfide concentration of the sample from the calibration curve.

Between samples, rinse the electrodes with de-ionized water, blot dry and immerse them in a “blank” solution of 50 ml SAOB plus 50 ml de-ionized water.

Expected Range

Samples containing 0.1 to 3200 mg/liter of sulfide may be analyzed by this method. The concentration range may be extended by dilution of an appropriate aliquot.

 

Reference

Annual Book of ASTM Standards, Part 31.