Anal. Chem. 1086, 58, 1845-1848
1845
Determination of Diethyl Dithiophosphate in Flotation Liquors by Solvent Extraction and Ultraviolet Spectrometry Michael H. Jones* and James T. Woodcock CSZRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne, Victoria, 3207, Australia
Diethyl dlthbphosphate, (C2HIO),PSS- (DTP) (diethyl phosphorodlthloate), k one of a series of homologous compounds used as flotatlon collectors for sulflde mlnerals. Diethyl dlthkphosphate can be determined by UV spectrometry at 293 nm In a chloroform extract of the Pb(DTP), salt. At pH 4-5 virtually 100% of the Pb(DTP), Is extracted from the aqueous phase at 1:l phase ratk after addition of lead perchlorate to the sample. The absorbance of the chloroform extract Is directly proportionalto the DTP concentratlon up to at least 60 mg/L Pb(DTP), (40 mg/L DTP). Modifications of the dmpie extraction are provided to overcome Interference due to sulfate or cresyllc acid. The lower llmit of dbtectlon Is 0.4 mg/L DTP.
Diethyl dithiophosphate, (C2H50)2PSS-(DTP) (diethyl phosphorodithioate), as the acid or its sodium or potassium salt is one of a homologous series of dialkyl dithiophosphates used extensively as flotation collectors for sulfide minerals. The collectors include the Aerofloats made by the American Cyanamid Co. and the Phosokresols made by Farbwerke Hoechst A.G. The alkyl dithiophosphates are also used as additives to lubricants and as anticorrosion agents ( I ) . There is a need to determine small amounts of DTP in solution so that collector levels in flotation plant pulps can be measured and possibly controlled. Furthermore, because of the stability of DTP (2)there is a need to monitor flotation plant effluents to determine if DTP is being recycled or discharged to the environment. There is an extensive literature on the chemistry of dithiophosphates (3-8),and proposed analytical techniques include spectrometry of the copper complex (9),the bismuth complex (6),and the nickel complex (10). The first method (using copper) has been rejected due to lack of reproducibility (II), no doubt due to the instability of the CunDTP complex, which rapidly disproportionates to Cu'DTP and the dimer (DTP)2 (12). The bismuth complex, Bi(DTP)3, has a high molar absorptivity (17000 L mol-' cm-') but is unsuitable because it is light-sensitive and decomposes rapidly (6). Data on the molar absorptivity of the Ni(DTP)2 complex appear to be conflicting (10) even though the complex is more stable than either the bismuth or copper complex. Other techniques suggested involve polarography (13), pyrolysis/gas chromatography (14),and potentiometry with a sulfide ion selective electrode (15)or a silver electrode (16). Examination of the literature indicated that the lead complex (Pb(DTP),) (4,7,8)could be readily formed and, after extraction into an organic solvent, could form the basis of a simple W spectrometric method. Chloroform was considered to be a convenient extractant.
EXPERIMENTAL SECTION Equipment. Spectral scans were obtained with a Unicam SP8OOA recording spectrometer. Ammonium Diethyl Dithiophosphate. Ammonium diethyl dithiophosphate (Aldrich Chemical Co., Milwaukee WI) was purified by fmt dissolving it in water and filtering off the insoluble 00092700/86/0358-1845$01.50/0
oils and other residue. The filtrate then was extracted with ethyl acetate and the excess solvent evaporated under vacuum. The resulting oil, on exposure to air, formed white granular crystals. Anal. Calcd for (CzH50)2PSz.NH4:C, 23.64; H, 6.94; N, 6.89; P, 15.24; S, 31.55. Found: C, 23.98; H, 6.77; N, 6.90; P, 15.5; S, 31.8. Lead Diethyl Dithiophosphate. The lead diethyl dithiophosphate (Pb(DTP)2)was prepared by a metathetical reaction using aqueous solutions of lead perchlorate (analytical reagent grade) and ammonium diethyl dithiophosphate in stoichiometric amounts. The resultant precipitate was fitered off, washed, and recrystallized twice from acetone. Fine white needles were obtained; melting point, 74 "C (lit. value, 74 "C (17)). Anal. Calcd for ((CzH60)2PSz)zPb: C, 16.64; H, 3.49; P, 10.72; S, 22.20; Pb, 35.87. Found: C, 16.69; H, 3.44; P, 10.8; S, 22.0; Pb, 36.0. Chloroform. Ajax (analytical reagent grade) chloroform was used without further purification. It is desirable to check each batch for freedom from free chlorine (18)because dithiophosphate is readily oxidized by chlorine to the dimer. Procedure. Stock solutions of Pb(DTP)2were prepared by dissolving 0.100 g of the salt in 100 mL of chloroform. Appropriate aliquota were diluted with chloroform to give concentrationsin the range 0-40 mg/L DTP (0-60 mg/L Pb(DTP),). Two types of calibration curves were obtained in this work. One was the concentration of Pb(DTP)zin chloroform against absorbance at 293 nm and the other was the concentration of NHIDTP in aqueous solution against the absorbance at 293 nm of a chloroform extract after addition of lead perchlorate to the aqueous solution. Extractions were done at 1:l aqueous sample/organic by shaking for 2 min at room temperature. The pH of the aqueous phase was adjusted to pH 4.5-5.0 before extraction by adding 0.2 M sodium acetate/acetic acid buffer. The Pb(DTP)zcomplex was formed by adding, typically, 5 mL of 2% (w/v) Pbto a 25-mL aliquot of the dithiophosphate solution. (C10&2-3Hz0 The UV absorption spectrum of the extract, using the extract from a similarly treated blank as a reference, was determined between 250 and 400 nm.
RECOMMENDED METHODS Method I. For Solutions Containing up to 40 mg/L DTP and up to 400 mg/L Cuprocyanide (as CU(CN~~-) or 300 mg/L Sulfite (as SOa2-).To a 25.0-mL aliquot, add 5 mL of 0.2 M sodium acetate/acetic acid buffer to give pH 4.5-5.0; then add 5.0 mL of 2% (w/v) Pb(C104)2.3H20,and mix. Add 25.0 mL of chloroform and shake for 2 min. Allow the phases to disengage, and filter the chloroform extract through a Whatman 1PS paper (or equivalent). Measure the absorbance of the extract a t 293 nm using the extract from a similarly treated blank as a reference. Determine the original DTP concentration from a calibration graph or from eq 1 for 2-cm cells where C, is the concentrationof DTP in milligrams
per liter in the original aqueous solution and AZg3is the absorbance at 293 nm in 2-cm cells. Method 11. For Solutions Containing up to 40 mg/L DTP and up to 2700 mg/L SO?-. T o a 25.0-mL aliquot, add 5 mL of 0.2 M sodium acetate/acetic acid buffer and 10 mL of 5% (w/v) Ba(C104)2and mix. Filter off any precipitate, and to the filtrate add 5.0 mL of 2% (w/v) Pb(C10$2.3H20, then proceed as in method I. 0 1986 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
1846
l’L
I ’ ” ’ ” ’ I
& O ‘ *
I n I
0 x
-
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U
5 0.4 -
n 0
::0.2 0.
0 250
275
300
325 Wavelength l n m l
LOO
350
Fbure 1. uv absaptbn spectnm h 2cmoeHof 40 me/L leaddkthyl dithlophosphate (25.7 mg/L DTP) In chloroform.
i
jI 0.61
,” 0.L
s 0.2 0
-1
0
1
2 3 L 5 p H o f oqueousphose
6
7
8
9
Fcgure 2. Effect of pH on extraction of lead dlethyl dithiophosphate with chloroform from aqueous solutlon.
Method 111. For Solutions Containing up to 40 mg/L DTP and up to 100 mg/L Cresylic Acid. To a 25.0-mL aliquot, add 1 M NaOH or HC104 to pH 8. Add 25.0 mL of chloroform and shake for 2 min. Allow the phases to disengage, discard the chloroform extract, and proceed as in method
I. RESULTS AND DISCUSSION The UV absorption spectrum of 40 mg/L Pb(DTPI2 in chloroform in a 2-cm cell is shown in Figure 1. A plot of absorbance at 293 nm against Pb(DTP)2 concentration in chloroform was linear up to at least 60 mg/L Pb(DTP)2(38.5 mg/L DTP). Linear regression analysis of 45 pairs of values from five separate calibration runs gave eq 2 (at 95% confidence limits) A293
= (0.0395 f 0.0003)C0,, - (0.0089f 0.0060)
(2)
where A,, is the absorbance of a chloroform solution of Pb(DTP)2at 293 nm in 2-cm cells and C , is the concentration of diethyl dithiophosphate in milligrams per liter. The molar absorptivity at 293 nm of Pb(DTP)2in chloroform was calculated to be 7340 f 90 L m o P cm-* (Bode and Arnswald (7) give 6295 = 7700 in CClJ. Extraction of Lead Diethyl Dithiophosphate. A 2-min shake time was found to consistently extract about 98% Pb(DTP)2over a range of diethyl dithiophosphate concentrations. The effect of pH on the extraction of Pb(DTP)2is shown in Figure 2. High recoveries were achieved over a wide pH range, confirming the results of Bode and Arnswald (7) who
--
= I
I
I
I
I
1 0 I
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4
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Table I. Effect of Cuprocyanide and pH on Extraction of Pb(DTP)2 DTP recovered,
DTP," mg/L
cu(CN)4S-,b mg/L
extraction pH
%
25.9 25.9 25.9
80 400 400
0.6e 0.6c 4.7d
23 19 100
Table 11. Effect of Various Oxidants and pH on Recovery of Pb(DTP)2from Solutions Containing Sulfite sulfite,b mg/L
25.9 25.9 25.9
1600 1600 1600
25.9 25.9 25.9 25.9
400 1600 400 1000
reagent added, g/25 mL 0.1 g NaAsOp 0.1 g Mn02 0.1 g FeNH4(S04)2. 12H20 nil nil nil nil
extraction pH
% DTP recovered
0.6 0.6 0.6
>200 19' 190
0.6 0.6 4.7 4.7
155 >200
100 100
"Expressed as mg/L diethyl dithiophosphate; added as 28.5 mg/L NH4DTP. Expressed as mg/L Na2SO3-7H20. Not Pb(DTPL: see text.
Sultite (or aqueous sulfur dioxide) is also a common addition reagent in flotation plants that u8e dithiophwphate collectors. On acidification, any residual sulfite gives SO2, which is partially extracted by chloroform (19)(thiosulfate also decomposes to SO2 but much more slowly (20)).The amount of SO2 liberated depends on the sulfite concentration and pH (20). The extracted SO2 has a broad absorption maximum at about 288 nm, which is close to that of Pb(DTP)2so that it can be difficult to determine if the absorbance is due to either compound. Because sulfite can be readily oxidized it was considered that preoxidation and extraction from strong acid solution might be a satisfactory alternative to extraction from weakly acid solutions. Table I1 gives details of the various oxidants that were examined and compares the DTP recoveries achieved with those obtained by using the standard method. With manganese dioxide, the DTP was oxidized to the dimer (eq 4) and no Pb(DTPI2was detected in the extract.
+
+
Table 111. Effect of Sulfate on Extraction of Pb(DTP)2 DTP," mg/L
SO>-,mg/L
Ba(C10412, g/25 mL
% DTP recovered
25.9 25.9
2700 2700
0.0
0.5
91 99
"Expressed as mg/L diethyl dithiophosphate; added as 28.5 me/L NH,DTP.
OExpressed as mg/L diethyl dithiophosphate; added as 28.5 mg/L NH4DTP. bAdded a~ NaSCu(CN)& 'pH adjusted with HC104. dpH adjusted with 0.2 M sodium acetate/acetic acid buffer.
DTP" W/IJ
1847
-+
2(C2H50)2PS2- M n 0 2 4 H + (C2H50)2PS2S2P(C2H50)2Mn2++ 2H20 (4) With arsenite or ferric ammonium sulfate, no (or insufficient) oxidation of sulfite occurred and gross interference resulted. With extraction at pH 4.7, however, quantitative extractions of Pb(DTP)2 were obtained from sulfite solutions whose concentrations were in excess of those normally expected in flotation plant liquors. A pH of 4-5 was therefore adopted for the recommended method. Sulfate was considered to be a possible interferent because high concentrationscan occur where plant liquors are recycled. The precipitation of lead sulfate could result in insufficient lead being available to form Pb(DTPl2. Barium perchlorate was consideredto be a suitable precipitant to remove the bulk of any sulfate present in the sample. Table 111gives the result obtained in the extraction of Pb(DTP)2from solutions containing 2700 mg/L sulfate.
Table IV. Effect of Cresylic Acid on Determination of DTP
DTP," mg/L 25.9 25.9 25.9 25.9 25.9
cresylic acid TD,bmg/L 100 100 100
100 100
pH of preliminary extraction C
4.7 6.8 8.0 12.0
% DTP recovered
135 95 95 98 123
"Expressed as mg/L diethyl dithiophosphate; added as 28.5 mg/L NH4DTP. *Supplied by Union Carbide Australia Ltd. No preliminary extraction.
Higher concentrationsof sulfate may require higher barium perchlorate additions to obtain quantitative recoveries. Two or more test runs may be needed for samples of unknown sulfate concentration. Cresylic acid is a frother sometimes used in sulfide flotation and is also a component of many of the commercial diaryldithiophosphate collectors (21). It is partly extracted by chloroform from acidified solutions with an absorption maximum around 280 nm (19))and at high concentrations interferes with the determination as shown in Table IV (no preliminary extraction). A preliminary extraction of the cresylic acid with chloroform from slightly alkaline solution (pH 8.0) was found to eliminate the interference, as shown in Table IV. A preliminary extraction from acid solution (pH 4.7) removed the cresylic acid but also appeared to remove some DTP. A preliminary extraction at pH 12 removed some only of the cresylic acid and interference still occurred. In an unknown situation, it would be necessary to determine the presence of interferences such as cresylic acid by examination of the UV spectrum of the original aqueous solution (20). Registry NO. DTP, 29806-6; Pb(DTP),, 1068-23-1;Pb(C104)2, 13637-76-8.
LITERATURE CITED Reid, E. E. Organic Chemistry of Bivalent Sulfur; Chemical Publishing Co.: New York, 1962; Vol. I , pp 304-306. Cote, G. L.; Bauer, D. Anal. Chem. 1984,56, 2153-2157. Wasson, J. R.; Woltermann. G. M.; Stoklosa, H. J. Topics in Current Chemistry; Springer-Verlag: Berlin, 1973; Vol. 35, pp 65-129. Handley, T. H.; Dean, J. A. Anal. Chem. 1962,3 4 , 1312-1315. Handley, T. H. Talenta lB85, 12, 893-901. Bode, H.; Arnswald, W. 2.Anal. Chem. 1962, 785, 99-110. Bode, H.; Arnswald, W. 2.Anal. Chem. 1962, 785, 179-201. Busev, A. I.; Ivanyutin, M. I. T r . Kom. Anal. Khim. Akad. Nauk SSSR Inst. Qeokhim. Anal. Khim. 1960, 7 1 , 172-191; Chem. Abstr. 1961,55. 24381b. Norris. M. V.; Vaii, W. A.; Averell, P. R. J . Agric. FoodChem. 1954, 2 , 570-573. Ermolina, G. I.; Lebedev, V. D. Tsvetn. Met. (Moscow) 1977,50(7), 87-88. Ware, J. H. J . Assoc. Off. Agric. Chem. 1962,4 5 , 529-530. Busev. A. I.;Ivanyutin. M. I. J . Anal. Chem. (USSR) 1957, 7 7 , 559-564. Makens, R. F.; Vaughan, H. H.; Cheiberg, R. R. Anal. Chem. 1955, 27, 1062-1084. Legate, C. E.; Burnham, H. D. Anal. Chem. lB60, 32, 1042-1045. Arsent'ev, V. A.; Borisov, B. M.; Ievlev, Yu. V.; Posypkin, L. D. USSR Patent SU 900 859; Chem. Absh. 1982,97, 96094k (CSIRO Translation No. 13252). Du Rietz, C. Sven. Kem. Tklskr. 1957,69, 310-327. Mastin, T. W.; Norman, 0. R.; Weiimuenster, E. A. J . Am. Chem. Soc.1945, 6 7 , 1682-1664.
1848
Anal. Chem. 1086, 58, 1848-1852
(18) AnalaR Standards for hbwatory Chemicals, 6th ed.; Analar Standards, Ltd.: London, 1967; p 153. (19) Jones, M. H.; Woodcock, J. T. Anal. Chem. 1975, 4 7 , 11-16. (20) Jones, M. H.; Woodcock. J. T. Ultraviolet Spectrometry of Flotation Reagents with Special Reference to the Determination of Xanthate in Flotation Liquors: The Institution of Mining and Metallurgy: London, 1973
(21)
Mingione, P.
A. In Ragants in the Minerals Industry; Jones, M. J., Oblatt. R., Eds.; The Instltution of Mlnlng and Metallurgy: London, 1984; pp 19-24.
for review February 18,1986. Accepted March 26,
1986.
Determination of Picogram Quantities of Methyltins in Sediment Cynthia C. Gilmour,' Jon H. Tuttle,*2 and J a y C. Means Center for Environmental & Estuarine Studies and Department of Chemistry, Chesapeake Biological Laboratory, University of Maryland, Solomons, Maryland 20688-0038, and College Park, Maryland 20742
An extremely sensitlve purge and trap method is descrlbed for the determlnatlon of methyltins In complex matrlces. Organotlns were determined dlrectly from sediments and culture medlum as the volatlle methylstannanes. Hydrlde derivatives were prepared with NaBH, In a closed, flowthrough system conslstlng of a purge vessel, gas chromatograph, and mass spectrometer. Borate buffer added to samples generated H, from NaBH,, resulting In high purge efflclencles for mono-, dl-, and trhnethyttln. Selected lon mode monitoring with the masg spectrometer gave detectlon lknits for methyltins of 3-5 pg as Sn. The concentratlon detection llmits for a 5 3 sedlment sample were < w / g wfth a standard devlatlon of 6-18 "6, depending upon the methyltin specles and sample type. Sensitlvlty achieved was 2 orders of magnitude lower than previously reported for methyltins In sedlment. The method reported is both selective and specific, ellmlnatlng most Interferenceswhlle permitting podtlve ldentlflcatlon of lndlvldual methykin specles.
Methyltin species are ubiquitous in natural waters, although their concentration is usually low (less than 1 ng/L) in waters relatively unimpacted by anthropogenic activity (1,2). Monoand dimethyltin are the dominant species (1-3)) suggesting that methyltins, like methylmercury species, arise via stepwise methylation of the inorganic metal (4). Not only are sediment slurries capable of methylating added inorganic tin (5), but concentrtions of methyltin species increase with estuarine surface-to-volume ratios (1). Thus, tin methylation in aquatic environments likely occurs in sediments. Measurements of sediment methyltin concentrations show monomethyltin to be the dominant species in anoxic sediments while trimethyltin is found in highest concentrations in oxic sediments (6). This suggests that tin methylation probably occurs in anaerobic sediments, while degradation of higher molecular weight organotins such as tributyltin, an antifouling agent, occurs in oxygenated environments. In recent studies of inorganic tin methylation, we have confirmed that biomethylation occurs preferentially in anaerobic estuarine sediments (7). Methyltins were produced to a maximum level of about 2 ng/g (dry weight) of sediment in 21 days. Present address: Harvard University School of Public Health, Interdisciplinary Programs in Health, 665 Huntington Ave., Boston, MA 02115.
*Address correspondence to author a t Chesapeake Biological Laboratory, University of Maryland, Solomons, MD 20688-0038.
Methyltins may be extracted from complex matrices and analyzed by conventional gas-liquid chromatographic (GC) techniques (8). However, the procedure is lengthy, involving multiple steps where speciation may be altered and vessel adsorption effects may be large. Detection limits achievable with a flame ionization detector are 10-100 ng (9). Butylation of methyltin species before solvent extraction and use of atomic adsorption spectrophotometry shortens the extraction procedure and reduces detection limits to about 0.1 ng (IO). Analysis of organotins is often based on hydride generation, as the methylstannanes produced are both stable and volatile with boiling points ranging from 0 to 59 OC ( I , 2 , 6 , I I ) . Purge and trap (F/T) procedures followed by boiling point separations and detection by spectrophotometric methods yield detection limits in water of between 0.01 (2) and 1 ng (3). Detection of SnH emission by flame emission gives the greatest sensitivity (2). Chromatographic methods have also been applied with hydridization. Jackson et al. (12) used a commercial P/T apparatus fitted to a packed GC column and flame photometric detector to achieve a 0.1-ng detection. Our studies of microbial tin methylation required low part-per-trillion, or less than 10 pg, detection limits for methyltins in complex matrices such a~ bacterial culture medium and natural sediments. A rapid and direct method of analysis was preferable in order to preserve speciation and minimize transfer loss of tins on surfaces. We combined a modified hydride generation technique, used directly with sediment samples, with cryogenic trapping and selected ion mass spectrometry (SIM). The use of the P/T technique with SIM removes the majority of interferences while allowing positive identification and quantitation of methyltin species in sediment with detection limits of about 1 pg/g (dry weight) of sediment. Borate buffer added to samples increased the purging action of the hydridization reagent, NaBH4,by in situ production of H2 and allowed nearly complete recovery of methyltins added to 5 g wet weight of sulfidic sediment. This technique allowed measurement of part-per-trillion concentrations of methyltin species in a relatively pristine portion of Chesapeake Bay.
EXPERIMENTAL SECTION Apparatus. Methyltins were determined as their hydride derivatives with a Hewlett-Packard 5985B gas chromatograph (GC)-quadropolemass spectrometer (MS) equipped with a liquid nitrogen, cryogenic temperature programming accessory and an attached hydride generator. The hydride generator consisted of a 15 cm3glass readionlpurging vessel with side arm, screwed into a Teflon ring seal mounted in a stainless-steel headpiece. The headpiece was fitted with gas inlet and outlet lines. The hydride
0003-2700/86/0358-1848$01.50/00 1986 American Chemical Society
