Introduction
The Sulphide Ion-Selective Electrode has a solid-state crystal membrane. The electrode is designed for the detection of sulphide ions (S^-2 ) in aqueous solutions and is suitable for use in in both field and laboratory applications.  The Sulphide Ion is a bivalent anion.
One mole of (S-2) has a mass of 32.064 grams; 1000 ppm is 0.0312M
Dissolve 7.491g sodium sulphide nonahydrate (Na2S.9H2O) in 1 litre water.

Physical Specifications
Length of body excluding gold contact = 130 mm
Length of body including gold contact = 140 mm
Diameter of body = 8 mm
DC resistance at 25°C < 2.5 MOhm
Minimum feasible sample volume = 5 ml

Chemical / Operational Specifications
Preconditioning / Standard solution : Approx. 1000 ppm S-- as Na2S
(See below - Standard Solutions and Reagents)
Preconditioning time : 5 minutes
Optimum pH range : pH 13 to pH 14
Temperature range : 0 to 80°C
Recommended ISAB : 10M NaOH (2% v:v) or SAOB (Sulphide Anti-Oxidant Buffer)Recommended reference electrode : Double junction (ELIT 002 or 003)
Reference outer filling solution : 0.1M KNO3 or 0.1M CH3COOLi
Electrode slope at 25°C : 26 ± 3 mV/decade
Concentration range : 0.003 to 3,200 ppm (9x10-8 to 0.1 Molar)
Response time : < 10 seconds
Defined as time to complete 90% of the change in potential after immersion in the new solution.
Time for stable reading after immersion : < 1 to > 5 minutes
Depending on concentration, use of ISAB, nature of sample and stabilisation time of liquid junction potential of reference electrode.
Potential drift (in 1000 ppm) < 3 mV/day (8 hours)
Measured at constant temperature and with ISE and Reference Electrode continually immersed.

Interference:
Ag+ and Hg²+ have very high interference and can only be tolerated in very low concentrations relative to the S - ideally they should be absent.

Procedure for determining the concentration of SULPHIDE (S^-2) in WATER

Ion-Selective Electrode for sulphide ion (ELIT 8225 crystal membrane)

Reference electrode: double junction potassium nitrate (ELIT 002).

Dual electrode head (ELIT 201)

Standard solution: Approx. 1000 ppm S as Na₂S - see below.

Buffer solution (ISAB): 10 Molar NaOH or SAOB (Suphide Anti Oxidant Buffer).

ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.

100 or 150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10ml pipettes.

Standard Solutions and Reagents

It must be noted that precise stock sulphide standards are difficult to make and keep because of the high and variable water of crystallisation and the ease of oxidation. Thus, for the highest possible precision and reliability, it is recomended that sulphide electrode calibration solutions are made fresh by the analyst. Because of the uncertainty about the water of crystalisation and the extent of sulphur loss, it is necessary to make an approximate concentration solution first (see below) and mix it with SAOB or NaOH. This solution should then be calibrated by titration with a suitable reagent just before diluting it to make the calibration solutions for the electrodes. Preferably this should all be done in a well ventilated area or a fume cupboard to avoid noxious fumes. However, it must be stressed that this procedure may not be necessary for many applications where only the order of magnitude of the S concentration is required.

If one assumes that the molecular formula is correct, with exactly 9H2O, then a 1000ppm S solution can be made by dissolving 7.5g of Na2S in pure water and adding to a 1000ml flask containing 500ml SAOB OR 20ml 10M NaOH, then making up to the mark.

Before making the calibration standards, take 50 ml of this solution, and titrate it against, for example, 0.1 Molar lead perchlorate or cadmium nitrate using the sulphide electrode. Plot a graph of measured electrode potential against volume of titrant. As the titrant is added, the S concentration will fall and the mV reading will increase.  The end point will be when there is a big jump in mV for only a very small extra addition of titrant. This can be explained as follows: if the starting solution is really around 1000ppm then adding titrant to reduce this to 100ppm will cause an increase in signal of about 26mV (the electrode slope). A further reduction to 10ppm will require much less titrant but will also cause a ~26mV increase. So, the last few drops of titrant to mop up the remaining S will reduce the concentration again by more than an order of magnitude and hence cause a further big increase in mV.
See: Typical example of Standard Titration

The ppm concentration of the stock solution is the volume of titrant used (in ml) multiplied by 64. The basis of the calculation is that every 1 ml of titrant contains 0.0001 moles of Pb (or Cd) and this is equivalent to removing 0.0001 moles of S.  Mol Wt of S is 32g. So total S removed is mls x 3.2 mg.  Thus the ppm (mg/L) concentration in the titrated 50 mls of solution is that figure multiplied by 20.

Before use, the electrodes must be calibrated by measuring a series of known standard solutions, made by serial dilution of the stock standard solution. For a full calibration, prepare 100ml of solutions containing in the region of 1000, 100, 10, 1, and 0.1ppm S (i.e. decade differences in concentration to facilitate checking the electrode slope - but note that for sulphide standards these are unlikely to be whole round numbers!). If the approximate range of concentrations of the samples is known, and this is within the specified linear range of the ISE, then it is only necessary to make two solutions which span this range: e.g. if samples are known to lie between, say, 50 and 150ppm then you should use standards of 20 and 200ppm. Before calibration, the standard solutions must be mixed 1:1 with SAOB or 2:100 with 10M NaOH to help prevent oxidation and ensure the correct pH for the analysis.

Follow the instructions in the General Operating Instructions to measure these standard solutions and prepare a calibration graph.

SAOB
This can be prepared by adding 200ml 10N NaOH (or 80g reagent grade pellets) to approximately 600ml de-ionised water - slowly to avoid over-heating and spitting. Then add 35g ascorbic acid and 67g disodium EDTA and stir until all has dissolved. Transfer to a 1000ml flask, dilute to volume with water and shake well. This should last for at least two weeks in a tightly stoppered bottle - but the colour will gradually darken and the solution should be discarded when dark brown.

Sample Preparation:

For best results, and/or if there will be a delay between sampling and analysis, samples should be taken with a minimum of aeration to avoid oxidation and loss of H2S. They can be preserved in the sampling area by adding to a 100ml bottle 0.2mls (4 drops) of 2N zinc acetate and 0.05 mls (1 drop) of 6N sodium hydroxide for each expected 100 ppm Sulphide and filling completely with sample water, with no air bubbles trapped - these added volumes should not produce a significant error in the final concentration. Before analysis, mix samples 1:1 with SAOB (which will liberate any S complexed with zinc). In this case, be sure that calibration standards have also been mixed with SAOB. Alternatively, if SAOB is not available, do not preserve with Zn solution but be sure to mix each sample and standard 2:100 with 10M NaOH buffer solution and stir well before measurement.

Follow the instructions in the electrode operating instructions to measure a series of samples and record the results. Briefly, it is important to note that the electrodes must be washed and dried between each sample, to avoid cross contamination, and sufficient time must be allowed (2 or 3 minutes), before taking a reading after immersion, to permit the electrode signal to reach a stable value. For the highest precision, frequent recalibration is recommended (see operating instructions).

The results will be displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples then these figures will not need adjusting because they will all be affected by the same dilution factor.

Note high and narrow pH range (13 to 14).

WARNING: 10M NaOH is a very caustic solution and should be handled with care.

Special Method for in-situ samples.

If it is impractical or undesirable to take subsamples and treat with buffer to raise the pH then it is possible to measure samples in-situ at a lower pH as long as the calibration standards have the same pH as the samples. A method has been published for measuring sulphide directly in pore waters of soft sediments by a team from Birkbeck College, London: "The Influence of Sulphide on the Distribution of Higher Plants in Salt Marshes by: A. Ingold and D. C. Havill. Journal of Ecology, Vol. 72, No. 3 (Nov., 1984), pp. 1043-1054.

Although full analytical details are not given, it is stated that the electrodes were gently pushed directly into the sediment. First the pH was measured. Then the standard solutions were adjusted to this pH (presumably taking care to not significantly change the concentration of the standards or at least be able to recalculate the concentration) and the electrodes were calibrated before use. The authors comment that " The results obtained were very similar to those obtained by other analytical techniques (Beaton and Burns, 1968)" but there is no record of the actual precision and accuracy achieved by this method.