Porewater Sulfide Analysis Protocol

Updated 7-28-2016 by S. Widney

Sample Collection in the Field:

  1. Dilute samples 1:1 with sulfide anti-oxidant buffer (SAOB) (mixed with ascorbic acid) as they are collected, to prevent the oxidation of sulfide. Add 20 mL SAOB to a 50ml centrifuge tube, then add 20 mL of sample to prepared tube.

Reagents Needed for Lab Analysis:

  1. Milli-Q water in a squeeze bottle
  2. Sodium sulfide (stored in fridge)
  3. SAOB (one 475 mL bottle)
  4. 0.4061 g lead perchlorate (to make 10 mL 0.1 M lead perchlorate)
  5. Optimum B solution for electrode
  6. Will also need clean beakers (at least 5) and graduated cylinders, pipette tips, disposable glass pipettes

Standard Solutions:

  1. (Instruction Manual, p.5) Prepare a stock solution of saturated sodium sulfide by dissolving 10 g Na2S . 9H2O in 5 mL distilled, deaerated water and 5 mL SAOB (do in anaerobic chamber if possible). Measure Na2S . 9H2O with a non-metal spatula.Stir well and let stand overnight. Store in a tightly stoppered bottle in the hood or the refrigerator for longterm storage.
  2. Na2S is stored in the fridge. Use an Erlenmeyer flask (not volumetric) to avoid spillage.
  3. Prepare Orion Silver/Sulfide Combination Electrode according to instructions on p.6-8 in Instruction Manual. Fill electrode with Optimum B filling solution. Connect electrode to pH meter and adjust the meter to measure mV. Before measuring, check the slope of the electrode according to instructions on page 9 in Instruction Manual (will need to prepare 100 ppm standard for this step).
  4. Prepare a sulfide standard weekly by pipetting 1 ml of the stock solution into a 100mL volumetric flask. Add 50ml SAOB and dilute to volume with Milli-Q.
  5. [s1]Determine the exact concentration by titrating 10ml of standard with 0.1 M lead perchlorate.
  6. To make 0.1 M lead perchlorate, dissolve 0.4061g lead perchlorate in 10ml distilled H20.
  7. Pipette 10ml of the weekly standard into a small beaker and insert the electrode. Add lead perchlorate solution in 0.25ml increments (using automatic pipetter). Record the reading in mV after each addition. Reading will drop from around –920mV to around

–730mV at some point. Graph these data in Excel to see the drop.

Use the following equation to determine the concentration of the standard:

C = 3206(Vt/Vs)

where:

C = concentration (ppm sulfide)

Vt = volume of titrant at end point (ie. ml of lead perchlorate added at the point where the slope of the curve is greatest. Use the average of the values on either side of the drop: (2.25+2.5)/2 = 2.375)

Vs = volume of standard (10ml)

In this case, C = 3206(2.375/10) = 761.4 ppm

  1. by serial dilution of the weekly standard (~1000ppm, specific ppm will vary depending on initial concentration of weekly standard, determined with the equation above). See Standard Preparation section below for detailed instructions.

Standard Preparation (serial dilution):

1000 ppm = 25 mL 1000 ppm stock (weekly standard) + 25 mL SAOB

100 ppm = 2.5 mL 1000 ppm +22.5 mL water + 25 mL SAOB

10 ppm = 2.5 mL 100 ppm + 22.5 mL water + 25 mL SAOB

1 ppm = 2.5 mL 10 ppm +22.5 mL water + 25 mL SAOB

0.1ppm = 2.5 mL 1 ppm + 22.5 mL water +25 mL SAOB

Prepare standards in beakers. Add 25 mL SAOB to all standards AFTER the serial dilution has been completed.

Sulfide Measurement:

  1. All standards and samples should be at the same temperature for precise measurement. Remove samples from freezer and allow them to reach room temperature before measuring sulfide.

Standards:

  1. Rinse electrode with distilled water, blot dry and place into the beaker containing the most dilute standard. When a stable reading is displayed, record the mV value and corresponding standard concentration.
  2. Repeat step 2 for all standards in order of increasing concentration (ppm). Create a calibration curve by graphing the mV values against concentration in Excel and add a logarithmic trendline to get the equation for the line (see below). Positive mV values may be used for easier computation without changing the resulting concentrations (this would give the equation y = 12.959Ln(x) + 821.89).

Solve the equation for x: x = exp((y-821.89)/12.959).

Samples:

  1. Rinse electrode with distilled water, blot dry and place into the 50mlcentrifuge tube containing the first sample. Do not add more SAOB before measuring, since samples have already been preserved with SAOB. When a stable reading is displayed, record the mV value. Use the equation for x obtained in step 4 to determine the concentration of each sample. Rinse electrode with distilled water between each sample.After measuring all samples, re-measure a few for QC.

Waste Disposal:All sodium sulfide and lead perchlorate liquid waste should be disposed of in waste bottles in the hood, and collected by IU Environmental Health and Safety. See Sulfide Analysis Template for hazardous waste components (update as necessary if components change significantly). Put paper towels, etc in a container to prevent fumes

Storage of Orion Silver/Sulfide Combination Electrode:

The Optimum B solution in the Orion Model 9616 Combination Silver/Sulfide Electrode should not be allowed to evaporate, causing crystallization.

For short periods of time (up to one week): Store the electrode in distilled water.

For storage longer than one week: Drain the electrode; flush the inside with distilled water and store dry with the cap on to protect the sensing element. To drain the electrode, hold by the cap and push the plastic cap down onto the glass tube (filling solution will drain out of the bottom). Flush several times with distilled water, then let dry before putting away.

Notes: Standard preparation, measurement of standard curve, and measurement of samples can be done all in the same day (will likely take most of the day). Make the stock solution the day before, and proceed with Step 2 the next day. Make sure you set samples out to warm up the night before you want to measure them.

  • Preparing the stock takes ~1 hour
  • Weekly standard preparation and titration takes ~2 hours
  • Serial dilution of standards and standard measurement takes ~1.5 hours
  • Measuring 31 samples takes ~2-3 hours

Methods adapted from Orion Model 9416 Silver/Sulfide Half-Cell and Model 9616 Sure-Flow Combination Silver/Sulfide Electrodes Instruction Manual pp 5, 8, 39-40 (Copyright 1997, Orion Research, Inc.)

1

[s1]Possibly could replace these steps with dissolving 1 gram NaS in 50 mL SAOB in a 100 mL volumetric & dilute to volume (no need to make stock solution)