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Anal. Chem. 1983, 55, 1823-1826
rectly from the CRA 90 26 Vac supply, if caution is used not to overload the power rsupply. The operation of the “modified” CRA-90 furnace power supply is relatively normal with the exception of the setting of the oxygen ashing time and temperature. Oxygen ashing is selected by closing SW2. This enables the various logic and timer chips, and lights lLED1. The O2 i3sh time is set with ash time potentiometer (P2). The total ash time (0, ash + inert gas ash) is set with the front panel ASH control, which has been doubled by the addition of the extra timing capacitor. Thus a reading of 30 011 the scale will eqlual a 60 s ash. The length of the inert gas flush before atomization is the total ash time minus the O2 ash time. This shlould be a minimum of 20-30 s to allow complete flushing of the gas lines and furnace of any residual 02. The inert gas ash cycle temperature is set by depressing the ASH set button and adjusting the temperature potentiometer to the desired 3etting. The O2 agh cycle temperature is set by turning O2 ash set switch (SW1) to the on (SET) position. Press the ASH set button andl adjust the temperature with the O2 ash cycle temperature potentiometer (P2) to the desired setting. SW1 also disable13the operate button and turns on the O2 solenoid so that the O2 flow rates can be adjusted. SW1 must be opened (OPERATE) before the power supply can be activated.
OXYGEN ASHXNG CIRCUIT OPERATION At the start of dry cycle, RLDl (timer PWB) closes and pin 10 goes high (+15 V). The signal is input to pin 1of IC3. If the O2 ash enable switch is closed, pin 13 is pulled high and the CMOS analog switch is closed. This routes the dry signal to the input (pins 2 and 9) of IC4. This forces pin 10 of IC4 low, turning off the inert gas solenoid (Vl), and the inverted output turns the O2sollenoid (V2) on. If SW2 is open, the CMOS analog switch if3 held open and the inert gas solenoid remains on throughout the cycle. At the start of the ash cycle, RLEl (timer PWB) closes and pin 10 goes high. The signal is capacitively coupled to the base of the switching transistor (Tl). Thle transistor is briefly turned on, and pin 2 of IC2 is pulled low. This triggers the start of the O2ash timing cycle. The output of IC2 is input into IC1, the CMOS analog switch, which switches the ash cycle temperature control to P2. This controls the ash tem-
perature during the O2 ash. At the end of IC2 timing cycle, the output goes low, pin 13 of IC1 opens the connection between P2 and the ash temperature control. However, the inverted output is input to pin 12, which connects the ash temperature control to the front panel mounted ASH potentiometer. This now controls the ash temperature for the remainder of the ash cycle, which is utilized as an inert gas flush. If SW2 is off, the reset line of IC2 is pulled low by R3, and triggering i s prevented. The sheath gas control system consists of two triacs (TR1 and TR2) and two normally closed, 26 Vac solenoid gas valves (V1 and V2). When SW2 is off the inputs to the logic control (IC4) are held low, disabling the O2 triac (TR2), and the inert gas solenoid is held on through out the timing cycle. For O2 ashing, the logic is as follows. If the dry or O2 ash cycle is on, the O2solenoid is on. If any other cycle is on, the inert gas solenoid is activated. The outputs of IC4 are input into a divider network of resistors (R5-R8) to lower the voltage and current to the triac gate specifications. The logic is designed so that at no time can both gas solenoids be on the same time. It is recommended when using oxygen ashing that the atomizer temperature not exceed 1000 “ C when oxygen is present within the furnace to prevent any detrimental effect on the furnace lifetime. Circuit provision allows for the changeover from oxygen to an inert gas ash prior to atomization. The cycle is provided to ensure that there is no oxygen on the atomizer under high temperature conditions. It is also recommended that the inert sheath gas ash cycle temperature be held at a relatively low value to prevent the desorption of oxygen which ‘has been attached to the sample and surface during the oxygen ash cycle. Typical operation is such that a 900 “C oxygen ash for 20-30 s is followed by a 200-300 “C nitrogen ash for 20-30 s. Registry No. 02,1182-44-7.
LITERATURE CITED (1) (2) (3) (4)
Beaty, R. D ; Cooksey, M. M. At. Abs. News/. 1978, 17, 53-58 Beaty, R. D ; Barnett, W. A t . Spectrosc. 1980, 1 , 72-77. Salmon, S. G.;Holcombe, J. A. Anal. Chem. 1978, 50, 1714-1716. Eaton, D. K : Hoicombe, J. A. Anal. Chem. 1983, 55, 946-950.
RECEIVED for review May 5, 1983. Accepted June 3, 1983. This work is supported in part by a grant from the National Science Foundation, CHEW-07632.
Extraction and Spectrophotometric Determination of Molybdenum with Thiocyanate and Amides Khageshwar Singh €’atel,*Hiralal Khatri, and Rajendra Kumar Mishru’ Department of Chemistry, K. Gout. Arts & Science College, Raigarh 496-001, Madhya Pradesh, India Amidine dimer (1) was recently reported for the spectrophotometric determination of Mo in complex materials. Similarly, the commonly available amides (HAL) such as N-phenylacetamide l(PAA), N-(methylpheny1)acetamide (MPAA), N-(dimethylpheny1)acetamicle (DMPAA), N-(diethylpheny1)acetamide (DEPAA), N-phenylpropionamide (PPA), N-phenylbutramide (PBA), and benzamide (BA) are described as very coiivenient extraction reagents for the spectrophotometric dletermination of IMo(V). The organic solution of HAL derived from aliphatiic acid and aromatic Present address: Department of Chemieitry, Ravishankar University, Raipur 492 010, Edadhya Pradesh, India.
amine or aromatic acid and ammonia is capable of extracting Mo(V) from SCN- solutions. Mo(V1) was reduced into Mo(V) with mild reducing agent like ascorbic acid and allowed to react with S C N in strongly acidic media and then the species formed was extracted with benzene solution of amide dimers (H2A2L2)as a mixed-ligand complex. HAL may dimerize through intermolecular hydrogen bonding by association of enolic and keto tautomers (2) in the fashion similar to carboxylic acid (3) to act as a N,O-dentate chelating agent in the formation of six-membered Mo(V) complex as does the amidine dimer (1). The special advantage of the method is that the HAL has a good chemical stability with a very simple method of preparation from readily available chemicals.
0003-2700/83/0355-1823$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
f
WAVELENGTH, nm
Absorption spectra of PAA and Mo(V)-SCN--H,A,L, cornplexes in benzene: [SCN-], 0.5 M; [HCI], 4 M; [ascorbic acid], 0.1 M; (O), Mo(V)-SCN-~,~-(DM)PAA;(0),Mo(V)-SCN--o-(M)PAA; (+), Mo(V)-SCN--PAA; (A),PAA after its equilibration of 0.6% (w/v) benzene solution with 1 %, (w/v) 4 M HCI solution in equal ratio. Flgure 1.
Moreover, it eliminates most of the drawbacks of established methods (4-16)such as interference of many metal ions, variation of color intensity of the complex with respect to amounts of reagents, absorption of the reagent at ,A,, or prolonged extraction of the metal.
EXPERIMENTAL SECTION Apparatus. A Carl-Zeiss Specord recording UV-VIS spectrophotometer and an ECIL UV-VIS spectrophotometer Model GS-865 equipped with 1-cm quartz and silica cells were used for recording and measuring the absorbance values, respectively. Reagents. A stock solution of Mo(V1) was prepared by dissolving weighed ammonium molybdate in double distilled water, standardizing gravimetrically( I 7) and diluting to working solution (1mL = 20 pg of Mo). A 20% (w/v) freshly prepared solution of ammonium thiocyanate and ascorbic acid was used. HAL is soluble in acidic water as well as organic solvent; therefore, fresh solutions of both in 10 M HCl/H2S04 (1% (w/v) PPA, PBA, DMPAA, or DEPAA; 2% (w/v) BA or PAA) and in benzene (0.4% (w/v) PPA, PBA, DMPAA, or DEPAA; 0.6% (w/v) PAA; 1.0% (w/v) BA) were used at a time. All the chemicals and reagents employed were of analytical grade. All the solutions used were presaturated with benzene. Procedure. An aliquot of solution containing 40 pg of Mo(V1) was taken in a 100-mL separatory funnel with subsequent addition of 2-, 3-, and 7-mL solutions of ascorbic acid, ammonium thiocyanate, and HAL in HC1/H2S04,respectively, and made up the total volume to 15 mL with water. The aqueous phase was equilibrated with 15 mL of benzene solution of relevant amide for 2 min. The benzene layer was collected over anhydrous sodium sulfate (2 g) in a 50-mL beaker. The absorbance of the complex against a suitable reference. was measured at A,, RESULTS AND DISCUSSION Absorption Spectra. Figures 1 and 2 show the absorption spectra of BA, PAA, and some Mo(V) complexes in benzene, obtained by the aforesaid procedure. All the complexes showed a sharp A,, around 475 nm in benzene and the position of ,A, was intact with respect to the nature of HAL while the value of molar absorptivity was evidently affected. The reagent has a negligible absorption at .,A, Choice of Solvent and Reducing Agent. The Mo(V)SCN- -PAA was extractable into solvents like 1-pentanol (PN), acetophenone (AP), cyclohexanone (CHN), ethyl acetate (EA), diethyl ether (DEE), carbon tetrachloride (CTC), chloroform (CF), benzene (B), toluene (TI,and xylene (X), showing the value of molar absorptivity in the range (1.08-1.65) x lo4 L mol-l cm-' between A,, 465 and 475 nm. The position of ,A, showed a slight hypsochromic shift with respect to decreasing basicity of solvent: B, T = 475 nm; AP, CHN, CTC, CF = 470 nm; PN, EA, DEE = 465 nm. The
1
WAVE LENGTH, nm
Flgure 2. Absorption spectra of EA and Mo(V)-SCN--H,A,L, cornMo(V)-SCN--2,3(DM)PAA; (0);Mo(V)plexes in benzene: ,).( SCN--BA; (*), Mo(V)-SCN--p-(M)PAA; (A),BA after equilibration of 1 %, (w/v) benzene solution with 1%, (wlv) 4 M HCI solution in equal ratio.
value of molar absorptivity had no trend with respect to nature of solvent: B = 1.65 X lo4;AC, CF = 1.60 X lo4; T = 1.55 x 104; CTC = 1-36x 104; CHN = 1.29 x 104; PN, EA = 1.20 X lo4;DEE = 1.08 X lo4L mol-' cm-l. However, the bulkiness of the aryl group of the aromatic hydrocarbon greatly affected the ease of extracted complex: it was stable for a t least 10 h in B, least stable in T, and unstable in X a t room temperature. Of these, B was chosen for the later experimental work owing to high extraction efficiency of the complex while others were not recommendable owing to either low absorbance (DEE, PN, EA, CHN), unstable color reaction (EA, T, X), prolonged separation of two phases (AP), or poor extractability (CTC, CF). Tin(I1) chloride and ascorbic acid were tried for the reduction of Mo(VI) to Mo(V). Of these, ascorbic acid was found to be adequate for the extraction of the metal while tin(I1) chloride was proved to be unsuitable owing to relatively low absorbance of the complex and decreasing color intensity with its increasing amounts. A 100% extraction of the metal was also achieved in the absence of the reducing agent but a longer time was required (Z30 min). Effect of Acidity, Reagents, and Other Variables. The effect of various acids like CH,COOH, HN03, HC1, and H2S04 in the extraction of the metal was studied. CH,COOH and HNOB proved to be unsuitable owing to low absorbance of the complex in benzene or partial extraction of the metal. Therefore, HC1 and H2S04were employed to get maximum absorbance of the easily extractable complex and the optimum acidity range was found to be between 1.0 and 6.6 M HC1/ 1.0 and 6.0 M H2S04as shown in Table I. Beyond 6 M H2S04, the complete extraction of the metal was inadequate owing to miscibility of benzene into aqueous phase. A ~ 0 . M 1 solution of PAA in benzene or water + benzene (its total concentration in both 4 M HC1 and benzene being 0.10 M) was necessary for the complete extraction of the metal and addition in excess up to 0.30 M caused no adverse effect on the absorbance of the complex. The minimum necessary amount of other HAL were also studied: 00.15 M BA in water or benzene and 0.05 M PPA, PBA, DEPAA, or DMPAA in benzene was found to get maximum and constant absorbance of the complex. BA solution either in water or benzene and other HAL solutions in benzene or benzene + water were usable for the complete extraction of the metal. Owing to increased solubility of HAL in higher acidic aqueous solution, an acidity value of 4 M HCl was preferred for the experimental work. The effect of SCN- on the extraction of Mo(V) was studied at 4 M HC1 and a t least 0.3-1.0 M NH4SCN was adequate for full color development. The order of mixing of
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
Table I. Spectral Data of Mo(V)-SCN--,H,A,L, Complexes in Benzene optimum acidity R'CONHR" range, M, HC1 R' R" (HZSO,) 1.5-6.0 (1.2-6.0) 3.0-6.5 (1.8-6.0) 2.5-6.5 (1.7-6.0) 1.2-6.5 (1.0-6.0) 1.2-6.5 (1.0-6.0) 1.2-6.5 (1.0-6.0) 1.0-6.6 (1.0-6.0) 1.0-6.6 (1.O-6 .O) 1.0-6.4 (1.0-6.0) 1.O-6.4 (1.0-6.0) 2.5-6.0 (1.9-6 .O) 2.5-6.0 (1-9-6 .O) 2.5-6 .O (1.9-6.0)
C6H5
H C, H ,C! H C6H5
mol-' cm-'
Sandell's sensitivity, pg cni-, Mo
16 500
0.005 81
1 7 300
0.005 54
17 800
0.005 38
1 7 800
0.005 38
17 800
0.005 38
1 8 000
0.005 33
17 500
0.005 48
18 000
0.005 33
1 5 500
0.006 18
1 3 300
0.007 21
1 5 000
0.006 39
1 5 000
0.006 39
14 000
0.006 85
e, L
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C6H5
Table 11. Determination of M o in Ore, Alloy Steel, and Synthetic Matrices with PAA and SCN- at 4 M HC1 % Mo
sample
certified valuee
found
re1 std dev, %
2.901 2.865 0.56 b 64a 4.11 4.078 0.62 0.595 b 21913 0.60 0.81 b 22512 0.34 0.90 0.342 c l 0.85 0.844 0.88 c2 0.20 0.206 0.96 30 pg of M o + 20 mg of A13++ 1 0 mg of Cr3++ d l 0.98 30.2 pg of Mo 1 0 mg of Fe3+t 5 me of Zr4+ d2 35 pg of M o + 10 mg of Cr3++ 10 mg of U6+t 35.0 pg of Mo 0.90 5 mg of W6++ 20 mg of Pa:d3 40 pg of Mo + 10 mg of Fe3+t 2 mg of Ti4++ 39.8 pg of M o 0.83 5 mg of Zr4++ 10 mg of PO:d4 45 pg of M o t 10 mg of Mnz- + 5 mg of Znz++ 45.1 pg of Mo 0.70 2 mg of C!u2++ 1 0 mg of Fe3+ d 5 50 ,ug of Mo + 10 mg of Fe3++ 4 mg of V5++ 49.7 pg of M o 0.66 2 mg of Nb5+t 10 mg of C20,2a Ore, Indian Bureau of Mines, Nagptir. Alloy steel, British Chemical Standard. Alloy steel, Bhilai Steel Plant, India. Synthetic matrix, e' Average of six determinations, a
reagents was not critical. Variation in temperature from 10 to 40 OC and volume of aqueous phase 'from 10 to 30 mL did not alter the nature of the complex. No remarkable change in the extraction of the metal was noticed if salting out agents like NaCl, KC1, or (NH4)2S04were added up to 2 M. Amide as an Analytical Reagent. All HAL mentioned above reacted with Mo(V)-SCN- in a similar manner and followed Beer's law. The optimum coricentration ranges on the basis of Beer's law and Ringbom plot's (18) lie between 0.3 and 5.2 and 0.6 and 4.8 ppm of Mo, respectively. The detection limit of the metal with PAA is 0.35 pg of Mo at a dilution ratio of 1:1.4 >< lo6. The precision of the method was evaluated by taking 10 measurements, each containing 40 pg of Mo/15 mL, and the relative standard deviation of the system was found to be 2~0.72%.The molar absorptivity and Sandell's sensitivity of the complexes lie in the range of (1.33-1.80) X lo4 L m(o1-l cm-l and (5.33-7.21) X pg cm-2
Mo at, X 475 nm, respectively. Of the various HAL tested, only BA, PAA, MPAA, DMPAA, DEPAA, PPA, and PBA rapidly and completely extracted Mo(V)-SCN- into benzene, used for the selective and sensitive spectrophotometric determination of the metal over wide acidity range. Acetamide was unable to extract the desired species while C6H5CONHCsH5partially extracted and its extractability can be enhanced by breaking conjugation on either side >C=N- and a 100% extraction of the metal was observed with C6H5CONHCH2C6H5 and C6H5CH2CO~HC6HS. In PAA series, the introduction of -I substituents like C1, Br, em. in the N-phenyl ring retarded the extraction of the metal while the reverse effect was observed with +I substituents like CH3, CzH5,etc. The lengthening of the C atom of PAA suppressed the value of molar absorptivity with increased extractability while the bulkiness of the N-phenyl group enhanced both values. The rate of extraction of the metal was dependent on the nature
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Anal. Chem. 1983, 55, 1826-1829
/ I
C
n
I
If'
,511 [REAGENT]
Flgure 3. Determination of ratio of variables in Mo(V)-SCN--PAA complex by the curve-fitting method: [Mol, 2.79 X M; [KCI], 0.1 M; [HCI], 4 M; [ascorbic acid], 0.1 M; (e),log D vs. log [P2A2A2] a t 0.5 M SCN-; (0),log D vs. log [SCN-] at 0.12 M PAA.
of HAL and a prolonged extraction ( 2 5 min) was required when C6H&O~HCH2C6H5 and CsHsCH2CONHCsHjwere used as reagents. However with other HAL, a fast extraction (-2 min) was observed and a longer time (=30 min) had no effect in the degree of extraction of the metal. The rate of extraction of the metal was observed in increasing order: Cp,HbCONHz < CH3CONHCeHS < CH3CH2CONHCeH5 CH3CHZCHzCONHC6H5 = CH&ONHCp,H4CH3 < CHBCONHCsH,(CH3)2. Composition of the Complex. The ratio of SCN- and HAL to Mo(V) in the complex was determined by the curve-fitting method ( I 9 ) ,using log D (the distribution ratio of Mo) vs. log [HzAzLz)]/[SCN-]as in amidine complex ( I ) , and slopes are found to be 1.1and 2.2 very close to integer 1 and 2, respectively, Figure 3. Therefore, the formation of MoO-2SCN.HAzLz complex is expected in benzene and an overall reaction can be written as follows:
-
Moo3++ 2SCN- + (H2A2LZlo== (Mo02SCNHAZL2),,+ Ht where subscript o denotes the organic phase. Effect of Diverse Ions. The effect of diverse ions in the determination of 1.6 ppm Mo was studied separately at 4 M HCl with PAA and SCN- as described earlier. Nb5+interfered in the determination of Mo and masked with sodium oxalate. C1-, Br-, NO3-, S042-and alkali metals did not interfere up to 2 M. The tolerated amounts of other ions (inppm) causing
an error
