⚗️ Dissolved sulfide in water and wastewater — filtration through 0.45 µm, preservation with ascorbate, stripping with nitrogen at pH 4 and determination with methylene blue at 665 nm

Physicochemistry ISO 10530

Overview

Sulfides in water and wastewater are troublesome for three reasons at once: they are toxic to aquatic organisms, on acidification they release hydrogen sulfide — a gas deadly to sewer staff — and they are responsible for the sulfate corrosion of concrete sewers. The analytical trouble is different: sulfide is unstable. The standard itself states that because of the instability of dissolved sulfide and because the filtration of an unpreserved sample is an integral part of the method, it was not possible to carry out classical interlaboratory trials with samples sent out — instead a comparative trial was performed in a single laboratory, by operators from different institutes, on a solution with a sulfide concentration of 1 mg/l. That one sentence from the clause on precision says more about this method than the whole procedure: the sample cannot be put off until later.

Scope and limits of application. The method is intended for determining dissolved sulfide in the range of mass concentrations from 0.04 mg/l to 1.5 mg/l; higher concentrations are determined by reducing the volume of the sample used and by dilution. It applies to wastewater and natural waters which can be filtered. This limitation is hard: waters which cannot be filtered in the manner described in clause 6 of the standard must not be analysed by this method — for them one determines the sulfide readily released at pH 4, that is what ISO 13358:1997 describes (Dosage des sulfures aisément libérables). With the procedure as laid down, the determination of the part of the sulfide coming from polysulfides is incomplete. There is no interference from: cyanides up to 2 mg/l, iodides up to 20 mg/l, thiosulfates up to 900 mg/l, thiocyanates up to 900 mg/l and sulfites up to 700 mg/l; nor from carbon disulfide below 10 mg/l and ethanethiol (formerly: ethyl mercaptan) below 1 mg/l.

Which standard is the right one for a Polish laboratory. The national candidate, the series PN-C-04566 „Woda i ścieki — Badania zawartości siarki i jej związków" [Water and wastewater — Tests for the content of sulfur and its compounds], is not suitable as a reference document: in the PKN catalogue all its sheets have the status "Withdrawn". This also applies to all the sheets devoted to sulfide itself — PN-C-04566-02:1982 (colorimetric method with thiofluorescein and o-hydroxymercuribenzoic acid), PN-C-04566-03:1982 (thiomercurimetric method), PN-C-04566-04:1974 (undissociated hydrogen sulfide), PN-C-04566-05:1981 (ion-selective electrode) and PN-C-04566-13:1985 (mercurimetric method with preliminary concentration) — for none of them does the catalogue name a superseding standard. For example the card of sheet -03:1982 (titration with the salt of o-hydroxymercuribenzoic acid against dithizone, the sum of hydrogen sulfide and soluble sulfides above 0.04 mg/l) gives the date of publication 20 April 1982 and the date of withdrawal 16 January 2006, and there is no "Superseded by" field at all — the sheet disappeared from the PN collection without a successor. The only sheet of that series which did obtain a successor is PN-C-04566-09:1974 (sulfates by the gravimetric method), superseded by PN-ISO 9280:2002. At the same time ISO 10530 has never been adopted into the collection of Polish Standards — a search of the PKN catalogue for "ISO 10530" returns no item at all. The practical conclusion: the reference document is ISO 10530:1992 itself, and the entry in the accreditation scope must reflect that (an ISO standard, not a PN), or else the laboratory describes the same course of action in its own test procedure with a reference to ISO 10530.

Status of ISO 10530:1992. The standard is current. First edition of 15 September 1992, prepared by ISO/TC 147 "Water quality", subcommittee SC 2 "Physical, chemical and biochemical methods"; Annex A (iodometric titration for standardising sulfide solutions) is an integral part of the standard. The DIN Media catalogue marks it with the tag [CURRENT] as ISO 10530:1992-09, 9 pages, original in English. It is neither withdrawn nor superseded.

What this description does not contain. We worked on a preview of the standard in the French version — it covers practically the whole content: scope, interferences, principle, reagents, apparatus, sampling and preparation of samples, procedure, calibration curve, calculation, part of the clause on precision and the test report. We did not, however, see Table 1 with the numerical precision characteristics or the full text of Annex A, and for that reason we give no figures on the repeatability or the reproducibility of this method. Nor do we give an official Polish title, because none exists — the Polish title in the heading is a working translation.

THE POLISH TITLE IN THIS ENTRY IS OUR WORKING TRANSLATION, not an official title: the standard has not been adopted into the collection of Polish Standards.

Method principle

The water sample is filtered in order to separate the suspended matter and the sparingly soluble sulfides, and the sulfide in the filtrate is preserved by adding an ascorbate solution. The filtrate is transferred into a reaction flask with a phthalate buffer of pH 4.0 and the sulfide is stripped from it with a stream of nitrogen, being carried into a scrubber with an aqueous solution of zinc acetate, where it is bound as zinc sulfide. In the scrubber, after the addition of a solution of N,N-dimethyl-1,4-phenylenediammonium chloride in sulfuric acid, leucomethylene blue is formed, which by the addition of iron(III) ions is oxidised to methylene blue. The absorbance of the dye formed is measured at a wavelength of 665 nm against water, and the concentration is read from the linear section of the calibration curve.

Applications

• Control of municipal wastewater and sewers for the release of hydrogen sulfide and the sulfate corrosion of concrete collectors
• Industrial wastewater from tanneries, paper mills (the sulfate process), coking plants, refineries and rubber-processing works
• Natural waters under anaerobic conditions — groundwater, bottom water of reservoirs, landfill leachate
• Assessment of the efficiency of aeration and of nitrate dosing in a sewer network as a means against the formation of sulfide
• Testing of sulfide (medicinal) waters and control of installations in which iron sulfide is deposited
• Diagnosis of the causes of blackening of sludge and of the precipitation of metal sulfides in treatment plants

Key parameters

Standard

Standard number

ISO 10530:1992

Title (PL)

Jakość wody — Oznaczanie siarczków rozpuszczonych — Metoda fotometryczna z błękitem metylenowym

Title (EN)

Water quality — Determination of dissolved sulfide — Photometric method using methylene blue

Step-by-step procedure

1. Deciding whether the method fits at all

   Check whether the water can be filtered in the manner described in clause 6 of the standard. If not — do not analyse it by this method; for such waters one determines the sulfide readily released at pH 4 according to ISO 13358. If it is not known whether the concentration will fall within the range 0.04–1.5 mg/l, the standard recommends filtering and preserving several portions of the sample so that smaller amounts can also be determined.

2. Filtration and preservation — waters easy to filter

   Into a 50 ml volumetric flask pipette 5 ml of the ascorbate buffer. Draw the water into a 50 ml three-ring piston syringe, fit a single-use 0.45 µm filter and filter the water into the volumetric flask up to the mark.

3. Filtration and preservation — waters difficult to filter

   Into a 50 ml volumetric flask introduce 5 ml of the ascorbate buffer. Flush the flask and the pressure filtration set with nitrogen for about 10 min. Fill the pressure vessel completely with the water under test and filter it into the volumetric flask up to the mark at a nitrogen pressure of up to 2 bar. Make the connection between the set and the flask so as to limit as far as possible the ingress of air. The filtration time must not exceed 5 min.

   ⏱ Time: ≤ 5 min of filtration

4. Time limit for the analysis

   Analyse the preserved filtrates as soon as possible, at the latest 24 h after the sample was taken. This is not an organisational recommendation but a condition of the validity of the result — dissolved sulfide is unstable.

   ⏱ Time: ≤ 24 h from sampling

5. Preparing the stripping apparatus

   Pour 25 ml of the phthalate buffer pH 4.0 into the reaction flask. Pour 20 ml of the zinc acetate solution into the absorption tube (scrubber). Assemble the apparatus and pass nitrogen through the solution at a rate of 40 l/h for 2 min in order to remove the air from the system.

   ⏱ Time: 2 min of flushing

6. Introducing the filtrate and stripping the sulfide

   Transfer the filtrate into the reaction flask through the dropping funnel, rinse it with a small amount of water, and then pass nitrogen through the solution at a rate of 40 l/h for 30 min. At pH 4 the sulfide is converted into hydrogen sulfide, carried along by the nitrogen and retained in the scrubber as zinc sulfide.

   ⏱ Time: 30 min

7. Developing the colour

   Take the absorption tube off the apparatus and through the side joint add 10 ml of the reagent for forming the coloured complex, and then 1 ml of the ammonium iron(III) sulfate solution. Fill the absorption tube completely with water, stopper it, shake and wait 10 min.

   ⏱ Time: 10 min

8. Measuring the absorbance

   Pour the solution into a 100 ml volumetric flask, rinse the absorption tube carefully with a small amount of water and add the washings to the solution in the flask. Make up with water to the mark and measure the absorbance at 665 nm against water. If the mass concentration exceeds 1.5 mg/l of sulfide, repeat the determination using a smaller portion of the preserved filtrate.

9. Blank

   Carry out a blank by exactly the same course of action. The response of the blank must not differ appreciably from the calculated value A₍s0₎ — the intercept of the calibration curve.

10. Calibration curve

    Into seven 100 ml volumetric flasks introduce 20 ml of the zinc acetate solution each. Into six of them pipette respectively 4, 6, 8, 10, 12 and 14 ml of the sulfide standard solution; the seventh serves as the blank. Into each flask add 10 ml of the colour reagent and 1 ml of the ammonium iron(III) sulfate solution, dilute with water to about 40 ml, stopper, shake and make up with water to the mark. After 10–20 min measure the absorbance at 665 nm against water in the reference cell. The mass concentrations of sulfide in these solutions are approximately 0 (blank), 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 mg/l, and their exact values are determined iodometrically according to Annex A. The curve is not always linear over the whole range — only its linear section may be used for the calculations. Every spectrometer and every optical path requires its own curve.

    ⏱ Time: 10–20 min of colour development

11. Calculating and rounding the result

    The concentration of dissolved sulfide is calculated as (A₍s₎ − A₍s0₎)·f / (h·V), where A₍s₎ is the absorbance of the sample, A₍s0₎ the calculated absorbance of the blank, h the slope of the calibration curve (sensitivity in l/mg), f = 100 ml (conversion factor), V = 45 ml (the volume of filtrate used for the analysis, that is the 50 ml of the flask less the 5 ml of ascorbate buffer). Every dilution must be taken into account in the calculation. Round the result to 0.01 mg and give it with at most two significant figures.

12. Supervision of the calibration

    Check the slope of the curve and its intercept with the ordinate axis at regular intervals in order to detect appreciable deviations; such checks are particularly advisable after starting a new batch of reagent.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The acidified sample and the reaction flask itself give off hydrogen sulfide — a strongly toxic gas which at higher concentrations paralyses the sense of smell, so that the warning smell of rotten eggs ceases to protect. Carry out the whole stripping stage in an efficient fume cupboard.
• The nitrogen outlet from the scrubber must end in the fume cupboard; over the 30 min of stripping the stream of 40 l/h carries the whole stripped amount of hydrogen sulfide.
• The colour reagent contains 200 ml of concentrated sulfuric acid per litre — prepare it by carefully adding the acid to the suspension in water and cool it before making up to the mark.
• N,N-dimethyl-1,4-phenylenediammonium chloride is sensitising and irritant — weigh it wearing gloves, in a fume cupboard, avoiding dust.
• Sodium sulfide is corrosive and in contact with acids releases hydrogen sulfide — do not acidify the stock solutions outside the apparatus and do not pour them into the drain.
• Pressure filtration is carried out under nitrogen up to 2 bar — use vessels intended for work under pressure and a shield; nitrogen in a closed room displaces oxygen.

Frequently asked questions