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<br />~~') D 4374
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<br />cyanide
<br />tom the
<br />ng cva-
<br />chieved
<br />fed out
<br />using a glass coil and 0.02 M sodium hydroxide solution
<br />(7.4).
<br />4.6 Colorimetric determination of the recovered cyanides
<br />is made by pyridine-barbituric acid reagent. The color is
<br />developed at pH 5.5 to 6.0 and is meazured at 578 nm (or
<br />use a 580 nm filter).
<br />4.7 The lower limit of detection (or these automated
<br />methods is <l µg/L. Actually decreasing the analytical
<br />concentration range aqd increasing the sensitivity scale
<br />allows for lower limits of detection if necessary.
<br />5. Significance and Use
<br />5.1 Cyanides are known to be toxic to man, but more so
<br />to fish and other aquatic life. The complexity of the
<br />chemistry of cyanides has led to the coexistence of seveml
<br />cyanide species in the environment. The presence of cya-
<br />nides in industrial, domestic, and surface water is cause for
<br />concern. Several regulations and standards require contin-
<br />uous monitoring of cyanides in different types of water and
<br />wastes. The automated test methods presented offer useful
<br />tools for such monitoring.
<br />6. loterferences and Treatment
<br />6.1 Several interferences are encountered with cyanide
<br />analysis. (See Test Methods D 2036.) The known interferants_
<br />with the automated system are turbid;ty and color contrib-
<br />uting substances sulfides. oxidizine materials, nitrate-nitrite,
<br />some metal cations, aldehydes, fat~t' acids, and some poten-
<br />cyanide Cormin maten'T any of these tote erences
<br />sou be treated, however care should be taken to reduce the
<br />time of sample handling and minimise exposure to UV light.
<br />6.2 Turbidity and Color Contributing Substances-These
<br />may interfere with color measurement. However, most of
<br />these substances are removed automatically through the thin
<br />film distillation step prior to color development.
<br />- 6.3 Sufdes-Sulfides may cause direct or indirect, or
<br />both, interferences with cyanide measurements.
<br />~ 6.3.1 Direct Sulfide Interjerence-Sulfide rmm~etrg
<br />with c snide in the tea color compounds. The
<br />egree of sulfide interference depends on the concentrations
<br />o su1 t o c ant a an c orarotne- so utton. At a
<br />chloramine-T concentration of 0.4 %, and all the other
<br />system conditions satisfied az specified, the~c snide auto-
<br />mated item can tolerate the presence of sulfides tin to
<br />about IS me/L withouLSil:^ifi am ;nirrfrrrnre
<br />6.3.2 /ndirect Sulfide Interjerence-Sulfide may react
<br />with cyanide and form thiocyanate. The reaction kinetics
<br />d~Pend on the concentration of sulfide and cyanide ions az
<br />well az the pH. Higher pH values accelerate the reaction.
<br />~~.3.3 Treatment jot Sulfides-Sulfide containing samples
<br />should be treated az follows. Treatroent by dilution is
<br />!Mtommended when samples are to be analyzed rapidly,
<br />whereas treatment with lead or cadmium carbonate should
<br />bee done before storage.
<br />~-$.3.3.1 Treatment by Dilution-Dilute the sample with
<br />dwtstilled water, within the detection limits of the test method,
<br />~tnil the lead acetate paper test becomes negative (the
<br />sensitivity of this test is about 5 mg/L sulfide). Thus the
<br />st+ fide concentration will be below the interfering level.
<br />~,ceed with [he analysis taking into account the dilution
<br />6.3.3.2 Treatment N~ith Lead or Cadmium Carbonate-
<br />This treatment is recommended to be made before preserva-
<br />tion, if possible. Add small amounts of powdered carbonate
<br />to the sample. Repeat until no further dark lead sulfide or
<br />yellow cadmium sulfide precipitates, and [he lead acetate
<br />paper test becomes negative. Filter [he sample from the
<br />residue. Do not use cadmium nitrate because nitrates intro-
<br />duce interferences. Also avoid large excess of carbonate and
<br />long contact time to minimize cyanide losses by adsorption
<br />on the precipitate. However significant cyanide losses could
<br />occur, up to 40 %.
<br />6.3.4 For more information refer [o Appendix X I, Sulfide
<br />Interference.
<br />6.4 Oxidizing Ma[eriats:
<br />6.4.1 Oxygen, ozone, chlorine, tend other oxidants may
<br />oxidize the free and some weak con plex cyanides to cyanate
<br />and result in lower cyanide values. t should be realized that
<br />treatment with reducing agents is to prevent further oxida-
<br />tion but can not recover what hay. been already oxidized.
<br />Several reducing agents, such as ascorbic acid, C6H6O6,
<br />sodium hydrogen sulfite, NaHSO3, stannous chloride, SnCI=,
<br />and hypophosphoms acid, H~PO3, we-e evaluated (8) and found
<br />not satisfactory. They either cause irterferences themselves or
<br />did not demonstrate emcient reduction. On the other hand
<br />oxalic acid (COON), and sodium arsenite (NaAsOz) are found
<br />to be effective.
<br />6.4.2 Oxalic acid, about 2 g/L, c:an reduce up to 50 mg
<br />chlorine per litre (8). The reaction is satisfactory, though
<br />somewhat slow and requires acidic ~~r neutral pH. Therefore
<br />treat with oxalic acid and close the sample container. Wait
<br />I S min then preserve with sodium hydroxide.
<br />6.4.3 Sodium arsenite was found to be very efficient in
<br />reduction (8). Only 0.1 g/L is required to reduce 50 mg
<br />chlorine per litre. The reaction is fast and the arsenite could
<br />be added before or after sodium hydroxide. However, in
<br />several cases treatment of field sam ales with arsenite caused
<br />interferences and resulted in unexpected high cyanide values
<br />(refer to 6.5 and Appendix X2, Nitrate-Nitrite interferences).
<br />Also arsenite is very toxic and its application may not be
<br />desirable.
<br />6.5 Nitrate-Nitrite Interference:
<br />6.S.1 Nitrate and nitrite do itot interfere with the
<br />dissociable cyanide determination, ~:ontrary to total cyanide
<br />measurements where significant interferences are encoun-
<br />tered. The UV irradiation conditions may cause nitrite to
<br />react with other sample components and be measured as
<br />cyanide. Nitrate may be reduced to nitrite under certain
<br />conditions and interferes as well.
<br />6.5.2 The interference is much higher with most field
<br />samples than with synthetic water standards. The interfer-
<br />ence is more pronounced in presence of arsenite. The
<br />interference is positive and significant even at a level of 1
<br />mg/L nitrate or nitrite. Therefore, arsenite must not be used
<br />for treatment of oxidants in field samples known or sus-
<br />pected [o contain nivate or nitrite, instead oxalic acid is
<br />preferred (6.4.2).
<br />6.5.3 sulfamic acid and urea re:tet with nitrous acid or
<br />nitrite (3, 4). However, sulfamic acid waz found to be more
<br />effective than urea in removal of tt a interference. Addition
<br />of 2 g sulfamic acid per litre of digestive acid mixture readily
<br />removes up to-100 mg/L of nitrate or nitrite.
<br />105
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