leased by oxidation were several orders of magnitude greater t h a n t h e measured amounts in solution. Controls in which no tracer was added gave t h e same mercury release to solution, indicating t h a t Hg(I1) added with t h e tracer had little effect on oxidation or t h e partition between solution and bound-Hg. Evidently, the large amount of boundHg made a 20-ppb addition insignificant.
Discussion These laboratory studies indicated t h a t significant rates of oxidation of cinnabar a n d release of mercury to solution can occur under conditions prevalent in acid mine drainage waters. Other studies in our laboratory have shown t h a t Hg(I1) is strongly adsorbed by HgS, a n d that, in t h e process, anions such as SO*’-, NO,j-, a n d C1- may also be removed from solution. Since mercury released by oxidation is largely held on t h e remaining cinnabar, t h e oxidation rate cannot be directly inferred from mercury released to solution. Larger concentrations of cinnabar increase not only t h e surface area available for oxidation b u t also t h a t available
for removal of Hg(I1) released by oxidation. T h e results illustrated in Figures 3 and 4 can be explained if t h e effect from increasing surface area was greater on Hg(II) removal t h a n on oxidation. Additional work is needed to identify t h e nature of t h e bound-Hg. Studies are continuing on this problem a n d on t h e kinetics of cinnabar oxidation over a range of pertinent variables broader t h a n those in acid drainage environments. L i t e r a t u r e Cited ( 1 ) Bailey, E. H., Clark. A. I,.. Smith. R. M., l T . S Geol. SurL’e) Prof’. Paper, 820, 401 (1973). (2) Krauskopf. K.,Econ. G P O / . . 46,498 (1951). ic‘i) Tunell, G., Mercury in “Handbook of Geochemistry.” LToi. 11/2, pp 80B-M, Springer-Verlag, New E’ork. N.Y., 1970. (4) Saukov, A. A., Aidinyan, N. Kh., Acad. N a u k . S.S.R., Inst. Geol. N a u k . , 39 Min.-Geokhim., 8,37 (1940) (Russ).
Keceii’ed /or rwieu Aug 29, 3974. Accepted Februarj, 13, 1975. This u,ork u‘as supportsd bi. ths Nationai Science Foundation through the Tracs Contaminants Program of’ the R A N N Dicision, Grants GI-32943 and 40614.
Interference of Sulfate Ion on SPADNS Colorimetric Determination of Fluoride in Wastewaters Richard F. Devine* and Gerald L. Partington Calgon Laboratories, Calgon Corp., P.O. Box 1346, Pittsburgh, Pa. 15230
Serious errors have been experienced in t h e analysis of wastewater samples for fluoride concentration by t h e S P A D N S colorimetric method. T h e source of positive interference in this method was due primarily to sulfate ion carry-over during t h e preliminary distillation step.
In accordance with t h e Federal Water Pollution Control Act Amendments of 1972, t h e October 16, 1973, F e d e r a l R e g i r i e r ( I ) specifies t h e approved test procedures for t h e analysis of water pollutants. T h e approved method for t h e determination of fluoride ion is t h e S P A D N S colorimetric procedure preceded by distillation from a sulfuric acid solilt ion. T h e S P A D N S method a n d t h e fluoride electrode method we had been using are both described in detail in Standard Methods (2) and P a r t 23 of ASTM Annual Book of Standards ( 3 ) . In t h e process of adopting t h e S P A D N S method for routine laboratory use, comparative d a t a were gathered on prepared standard fluoride solutions a n d actual wastewater samples using both methods. T h e d a t a showed t h a t after sample distillation, t h e S P A D N S method gave consistently higher results t h a n t h e electrode method. A study was initiated to discover t h e source and extent of the apparent interference. Since it had previously been reported t h a t a t t h e 1.0-mg/l. fluoride level, 200 mg/l. of sulfate ion will cause a positive error of 0.1 mg/l. in t h e S P A D N S results (2, 31,t h e study was concentrated on sulfate ion. T h i s a p peared to be a logical beginning because of t h e possibility of sulfate carry-over during distillation.
Methodology T h e colorimetric procedure is based upon t h e measurem e n t of t h e loss of color of a zirconium-dye lake owing to t h e reaction of fluoride ion with zirconium to form a color678
Environmental Science & Technology
less complex ion. T h e dye used is sodium 2-(p-sulfophenyla z o ) - 198 - d i h y d r o x y n a p h t h a l e n e - 3 , 6 - d i s u l f o n a t e (SPADNS). T h e range of t h e test is 0.1-1.4 mg/l. of fluoride. T h e method is subject to many interferences, most of which are supposed to be eliminated by t h e preliminary distillation stem T h e distillation procedure was first proposed by Bellack (1)in 1958. I t involves t h e distillation of 300 ml of sample from a water solution of sulfuric acid which has previously been heated to 180°C and cooled. Distillation is continued until t h e temperature reaches 180°C a n d approximately :100 ml of distillate have been collected. This procedure is supposed to assure complete recovery of u p to 3 mg/l. of fluoride ion and removal of most of t h e interfering constituents in the water sample. Sulfate carry-over is reported to be minimal as long as t h e temperature in t h e distillation flask does not exceed 180OC. T h e electrode method requires t h e use of a fluoride ion selective electrode, a calomel reference electrode, and a p H meter with a n expanded millivolt scale. A buffer solution is added to a portion of t h e raw sample or preferably to t h e distillate to adjust pH, to complex iron and aluminum which would interfere, a n d to provide a similar ionic strength background between samples and standards. T h e fluoride electrode develops a potential which is specific for fluoride ion and is proportional to t h e fluoride ion activity ( i f the sample. T h e range of this method is 0.1-1000 mg/l. fluoride. E s p P ri m e n t a 1
Excellent comparative d a t a were obtained when t h e two methods were used t o analyze undistilled prepared standard fluoride solutions in the range 0.0-1.4 mg/l. Marked differences were found, however, when actual wastewater iarnples were analyzed following distillation. T h e results ohtained by colorimetric analysis were in all cases higher
T a b l e I . Results of Analysis of Wastewater Samples Following Distillation SO: -, Electrode, SPADNS, Sample identification
mgjl.
rngjl. F -
mgjl. F -
Distilled water Distilled water Mining operation-well water Mining operation-well water Mining operation-well water Mining operation-well water Mining operation-well water Power plant-river water Power pla nt-a s h basin Power plant-circulating water Power plant-sewage Power plant-floor drains Power plant-screen wash Power plant-cooling tower water Mining operation-well water Mining operation-well water Steel mill-thermal Steel mill-main Steel mill-river Power plant-river Power plant-septic tank Power plant-discharge
400 1300 475 800 285
