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

Reference electrode: double junction Lithium Acetate (ELIT 003n).

Dual electrode head (ELIT 201)

Standard solution: 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.

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

Standard Solutions and Reagents

One of the problems with S measurements is that precise stock sulphide standards are difficult to make and keep because of the high and variable water of crystalisation and the ease of oxidation. Thus, for the highest possible precision and accuracy, it is recommended that sulphide calibration solutions are made fresh by the analyst.
Nevertheless, proprietry 1000ppm S standards are available from many chemical suppliers which are accurate enough for most applications.

If the highest accuracy is required then it is necessary to make an approximate concentration solution first and mix it with SAOB or NaOH. This solution must 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.
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 these are unlikely to be whole round numbers!) If the approximate range of concentrations of the samples is known, and this is within the linear range of the ISE, then it is only necessary to make two solutions (preferably a decade apart) which span this range. 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 - but note that whichever is used the samples must be treated the same.

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.

Because of the high pH range required for the S electrode it is normally not used for in-situ measurements. However, 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 "gently pushing the electrodes directly into the sediment" :- "The Influence of Sulphide on the Distribution of Higher Plants in Salt Marshes" by: A. Ingold and D. C. Havill, Birkbeck College, London. Journal of Ecology, Vol. 72, No. 3 pp. 1043-1054, Nov. 1984.

Although full details of procedures are not given it is clear that the method involves first measuring the pH in the sediment then adjusting the standard solutions to the same pH (presumably taking care to avod significantly changing the concentration of the standards or at least be able to recalculate the new concentration). The sulphide electrode was then calibrated with these standard solutions 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.

Technical Specifications
for the Sulphide Ion-Selective Electrode (ELIT 8225)

Click here to download a printer-friendly (pdf) Specification Sheet.