Improvements in the Manganese Dioxide Collection of Trace Lead and Bismuth in Nickel St. John H. Blakeley, Alan Manson, and Vladimir J. Zatka The International Nickel Company of Canada, Ltd., J. Roy Gordon Research laboratory, Sheridan Park, Mississauga, Ontario, Canada, L5K 729
A detailed study of the scavenging properties of manganese dioxide for coprecipitating microgram quantities of lead and bismuth in nickel metal prior to atomic absorption analysis was reported by Burke ( I ) . When the proposed method was tested for ASTM in this laboratory, it was found that lead recoveries were frequently low (in the range 50-70%) and not reproducible. The collection of bismuth was, however, satisfactory. An investigation revealed that formation of manganese dioxide in a boiling solution, as used in the method ( I ) , results in a precipitate which has its collecting power for lead substantially reduced by the high ionic strength of the solution due to the large excess of nickel ions. I t has been found that, in the presence of a high concentration of soluble salts, only a slow formation rate of the manganese dioxide precipitate provides a strongly adsorbing form. In addition, raising the initial pH of the solution from the suggested p H 2-3 to p H 4-4.5 further improves the lead recovery u p to 94-99% in a single precipitation step. However, for the highest accuracy, a second precipitation is recommended for complete collection of lead. In the newly proposed procedure, a simpler and faster dissolution of the manganese dioxide precipitate is used. The working range of the method is 5-120 pg each of lead and bismuth. The precision, expressed as relative standard deviation, is 0.03 pg lead and 0.05 pg bismuth. EXPERIMENTAL Apparatus. All atomic absorption measurements were made with a Perkin-Elmer Model 403 atomic absorption spectrophotometer equipped with a single slot 10-cm titanium burner head and a plastic nebulizer insert. Resonance lines selected were 217.0 nm for lead (slit 1.0 m m ) and 223.1 n m for bismuth (slit 0.3 mm). A slightly fuel-rich air-acetylene flame was used (flow rates: acetylene 6 l./min a t 8 psi, air 19 l./min a t 30 psi). Reagents. Standard stock solutions containing 1.000 g/l. of lead and bismuth, respectively, in 8M nitric acid were prepared from high purity metals. A ten-fold dilution gave 100 ppm in 0.8M nitric acid. Working calibration solutions prepared from these solutions were 3M in hydrochloric acid and 0.05M in manganese(I1) chloride (to match sample solutions) and covered the ranges 0 to 15 ppm lead and 0 to 10 ppm bismuth. Other reagents required are 0.25M manganese(I1) sulfate and 0.08M potassium permanganate. A hydrogen peroxide-hydrochloric acid mixture (2.5 ml 30% H202/50 ml 6M HC1) must be prepared just before use. Procedure. Accurately weigh a sample of nickel which contains between 5 and 120 pg of lead and bismuth. Transfer the sample to a 400- or 600-ml beaker and add 25-50 ml of 8M nitric acid depending on the size of the sample. Warm to complete the dissolution. Adjust the volume to 200-300 ml with water and neutralize the excess acid with ammonia to a p H of 4.0-4.5. Add 5 ml of 0.25M manganese(I1) sulfate and 2.5 ml of 0.08M potassium permanganate. Stir, and heat the solution slowly to boiling. To prevent bumping, use a small piece of filter paper held on the bottom of the beaker with a stirring rod. As soon as the solution starts boiling, reduce the heat and maintain the covered beaker a t 80-90 "C for 1 hour. Filter the separated manganese (1) K. E. Burke, Ana/. Chem.. 42, 1536 (1970)
dioxide onto a Whatman No. 30, 12.5-cm paper. Wash the precipitate five times with 0.008M nitric acid. Dissolve the manganese dioxide precipitate on the filter paper by the dropwise addition of a freshly prepared mixture of hydrogen peroxide and hydrochloric acid, collecting the filtrate in the original precipitation beaker. A maximum of 8 ml is needed for each precipitate. Rinse the filter with 5 ml of 10M hydrochloric acid and then 2 or 3 times with water. Carefully dissolve all manganese dioxide particles remaining on the walls of the beaker and on the stirring rod. Evaporate on a hot plate without boiling until the volume is reduced to about 5 ml. Transfer the solution quantitatively to a 10-ml volumetric flask with water. Simultaneously carry a reagent blank through the whole procedure using the same reagents, but omitting the sample. Prewarm the burner of the atomic absorption instrument, aspirate the sample solutions, and record the absorbance at the selected wavelength. Follow with aspiration of the reagent blank solution and of selected working calibration standard solutions with a concentration range bracketing that of the samples. Correct the absorbance readings of the sample solutions for any reagent blank values and convert the results to pg/ml of lead or bismuth using the appropriate calibration graph.
RESULTS AND DISCUSSION Mode of Manganese Dioxide Precipitation, In the method it appears that, unlike with bismuth, the most decisive factor for the extent of the lead collection is the rate of the manganese dioxide precipitation. At a high rate, such as in a boiling solution, an insufficiently active, finely divided granular precipitate is formed. However, if the permanganate is added to a cold solution which is then heated to boiling, a flocculant form of the manganese dioxide results which has an excellent collecting power for lead. Digestion a t 80-90 "C assists in the transformation of the initial precipitate into the flocculant form; a high ionic strength of the solution slows it down. Therefore, the original procedure was modified accordingly. Even so, in high nickel solutions a t pH 2-3, two consecutive collections were necessary to obtain a quantitative recovery of lead (cf. last three analyses in Table I). A single precipitation was sufficient to get a complete recovery of bismuth irrespective of conditions used. Table I shows the difference in the recoveries resulting from precipitation in a boiling and in a cold solution. Influence of pH. From theoretical considerations of the role of the manganese dioxide precipitate, which acts as a trace ion adsorbent, it can be concluded that the extent of adsorption will depend not only upon the charge of the adsorbed particle, but also on the stability of the compound formed by the ion contained in the collector with the counter ion adsorbed from the solution [Paneth-Fajans-Hahn rule ( 2 ) ] .Consequently, the coprecipitation of lead which occurs in weakly acid unbuffered medium should be p H sensitive. During the precipitation step, free acid is liberated in the solution as a result of oxidation of manganese(I1) ions: (2) H. A. Laitinen. "Chemical Analysis," McGraw-Hill Co., New York, N.Y., 1960, p 167.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1941
Table I. Recovery of Lead and Bismuth-Comparison of Precipitation Methodsa YOLead recovery % Bismuth Added pgPb
pg Bi
gNi
100 100 100 100 100 100
...
... ...
50 50 60 80 100
recovery, 1st pptn,
~
1st pptn, 2nd pptn. hot cold
~
0.4 0.4 0.4 0.4
55
50
44 36 33 33 30
53 48 53 53 56
Total
hot
105 97 84 86 86 86
...
% Lead recovery
100 98 97 101 97
recovery, 1st pptn,
1st pptn, 2nd pptn, fig Bi
g Ni
cold
100
...
,..
100
50 50
...
98 98 98 100 87 7e
100 100 100 25 25 50 a
60 60 10 10 20
0.4 0.4 0.4
5.0 5.0 ..
80 100
,
'
cold
Total
cold
,..
98 98 98 100 87 92 96 101
100 102 98 102 100 90 95
... ,..
.. ... ,
16 16 1
...
Volume 200 ml; 2.5 ml 0.08M KMn04 for each precipitation; pH 2-3.
Table II. Change of pH during the Precipitation Processa pH after neutralization
pH after buffer addition
pH after precipitation
Lead recovery
2.90 2.75 2.30 2.40 2.40 2.60 2.85 3.45 2.40 2.55 2.90
No buffer No buffer No buffer No buffer No buffer
2.15 2.20 2.00 2.00 2.10 2.30 2.40 2.50 2.25 2.30 2.45
74 75 68 62 65 71 72 73 66 66 88
2.60 2.70 2.75 2.45 2.60 2.70
%
+ 2H20 = 5Mn02 + 4H+
Final
1st pptn
10 10
200 200 200 200 200 200 300 400
30 30 30 30 30 30 30 30
3.10 3.60 4.05 4.60 5.00 4.03 4.05 3.95
2.35 2.42 2.60 4.33 4.90 2.77 3.90 3.37
82 85 95 102 99 95 97 96
5 5 5 5 5 5
200 200 200 200 200 200
60 80 100 120 150 200
4.45 4.40 4.55 4.32 4.30 4.40
4.20 3.90 4.30 3.05 3.05 4.05
97 96 98 94 89 90
2nd pptn
Total
. . . . . .
. . . . . . . . . . . . 1
103 99 100 104 102
0
5 7 6
100 101 101 99 97 97
3
5 3
5 8 7
Used: 2.5 ml 0.08M KMn04.
Table IV. Reproducibility and Accuracy Obtained on Simulated Samplesa Addedrg
Found pg, mean
No.of determinations
Std dev
Recovery range,
Pb 50.0 Bi 20.0
49.6 19.9
19 25
0.80 0.44
96-1 02 95-1 05
Pb 10.0 Bi 5.0
10.1 . 4.9
9 11
0.24 0.22
97-1 05 90-1 02
%
Found, ppm Designation X-914
(1)
However, precipitation in buffered solutions (pH -2.6) brought about only insignificant improvements in lead recoveries even when a doubled volume of permanganate was used (Table 11). The reproducibility still remained poor. Only by raising the pH to 4-5 and by performing the permanganate oxidation in the cold solution was the extent of lead collection increased to as high as 94-9970 in a single precipitation step (Table 111). Large amounts of sample (10 g nickel or possibly more) can be easily handled even in twice the usual volume, without affecting the high recovery of lead. With 2.5 ml of 0.08M potassium permanganate solution, up to 120 pg of P b can be collected with a recovery of better than 94%. In actual analysis, the optimum initial pH range is 44.5. It is not advisable to go higher than p H 5 since the solution becomes buffered and there is danger of coprecipitating large amounts of nickel with the manganese dioxide precipitate. All of the iron and some aluminum present in the sample are collected in the precipitate. However, with flame conditions as recommended, even a 100fold excess of iron and aluminum does not affect the
* ANALYTICAL C H E M I S T R Y , VOL.
Initial
Table V. Analysis of Nickel Materials
Q
1942
5 5 5 5 5 5
Lead recovery, % PH
In each set, amounts varying from 0 to 4 g Ni added as nitrate.
Published procedure ( 1 ) . 100 pg Pb/200 ml voiume; 5 ml 0.08M KMn04; 3 ml 1M chloroacetate buffer (pH 2.7).
3Mn2+ + 2Mn04-
g Ni (present as Volume, pg Pb nitrate) ml added
% Bismuth
Added pg Pb
Table 111. Recovery of Lead in the Modified Procedurea
NBS-671' NBS-6720 NBS-673' 63871 63872b 63874b
Material
Pb
Bi
Standard NiO (0.024% Pb) Nickel Oxide No. 1 Nickel Oxide No. 2 Nickel Oxide No. 3 ASTM Test Nickel Sample ASTM Test Nickel Sample ASTM Test Nickel Sample
240 16.4 38.4 2.5 1.6 8.0 29.3
32.5
0.7 0.4
0.5 1.7 10.2 27.8
The NBS standards are not certified for lead and bismuth content. Analyses by other techniques are given by Burke ( I ) . Other results available from ASTM sub-committee E03.05.08.
atomic absorption signals of lead and bismuth, provided manganese(I1) chloride is present in the solution. Reproducibility and Accuracy. The modified procedure was tested on simulated samples. These contained, in a volume of 200 ml, varying amounts of nickel nitrate corresponding to 0-4 g nickel, and were spiked with either 50.0 pg P b and 20.0 pg Bi, or with 10.0 pg P b and 5.0 pg Bi. The analytical results obtained are summarized in Table IV. The per cent recovery range covers the span between the lowest and the highest result after two precipitations for lead and one precipitation for bismuth. Results from the analysis of various nickel metal and nickel oxide samples are presented in Table V. Detection Limit. The lowest concentration of an element which can safely be determined will depend upon the sensitivity of the resonance line selected as well as upon the stability of the instrument reading a t zero ab-
45, NO. 11, SEPTEMBER 1973
sorption level. A very close agreement was found between the standard deviations of the measured blank readings a t the respective lead and bismuth wavelengths and the standard deviations calculated from the corresponGing fluctuational concentration limits, defined by RamlrezMuiioz ( 3 ) as the peak-to-peak noise at zero absorption level, and set equal to 4 n. Taking a 95% probability, one
can expect the spread of results for 2 CT on both sides of the true value. Due to instrumental fluctuations close to zero concentration, the uncertainty range which determines the lower concentration limit has been found to be 1 pg lead, and 2 pg bismuth, respectively.
Rami&-Muiioz, "Atomic Absorption Spectroscopy," Elsevier Publishing Co..Amsterdam-New York, 1968, p 227.
Received for review October 2, 1972. Accepted March 2, 1973.
(3) J.
Determination of Thiocyanate as a Rhodamine B Complex Ariel H. Guerrero and Antonio M. Roig facultad d e Ciencias Exactas y Naturales, Ciudad Universitaria, Nhiez, Pabellon 2, Buenos Aires, Argentina
The determination of small quantities of thiocyanate is commonly done through the reaction with iron(II1) in acid media ( I ) . This method has several drawbacks, the main ones being moderate stability and poor sensitivity (5 to 10 pg/ml). In another method used for biological fluids ( Z ) , the sample is treated with bromine in order to convert thiocyanate into cyanide bromide, which after elimination of excess reagent with As3+, reacts with benzidine and pyridine solution. Cyanide interferes, but sensitivity is good (1 crg/ml). We have previously proposed (3) the reaction between thiocyanate and rhodamine B in benzene for the determination of small quantities of this ion ( 4 ) . In this method, after extracting an acid solution of thiocyanate with benzene and adding colorless rhodamine B solution dissolved in benzene to the isolated organic phase, a purple color is obtained, similar to the reagent aqueous solution. In this paper, we wish to report that the method may be considerably improved if thiocyanic acid is extracted in the presence of sulfite ion, salting out with sodium sulfate. The reaction proceeds efficiently if water is eliminated from the organic phase. Stability and sensitivity are satisfactory. EXPERIMENTAL Apparatus. A Bausch & Lomb Spectronic 20 was used for all absorbance measurements. Reagents. AR grade reagents were prepared as follows: standard thiocyanate solution (25 pg/ml as SCN-); rhodamine B solution (0.01M in benzene. Drug is purified according to ( 4 ) by dissolving in absolute ethanol and precipitating with 10 volumes of ethyl ether.); sodium sulfate (saturated solution); sodium sulfite (saturated solution); and sulfuric acid (30% (w/w), saturated with sodium sulfate). Procedure. Place 0.05 ml of sample in a 5-ml centrifuge tube. A small drop (ca. 0.02 ml) of sodium sulfite, 1 ml of benzene, and two drops of the sulfuric acid solution are added successively. Cover, shake for 1 min, and centrifuge for 2 min a t 2500 rpm. Transfer the organic phase carefully with a dropper, without touching the aqueous solution, to a dry tube (5 ml) and add 0.5 P. Dansen, H. E. Bass. 8. Dandou, and E. W. Jones, Arch. Eur. Health, 14, 865 (1967). W. N. Aldridge, Analyst (London), 70, 474 (1945). A. M . Roig and A. H. Guerrero, "Determination of thiocyanate and iodide by means of rhodamine," XX IUPAC Congress, Moscow, 1965. A. M .
Roig. Ph.D. Thesis, Facultad de Ciencias Exactas y Naturales, Univ. de Buenos Aires, 1963.
ml of 0.01M rhodamine B solution in benzene. A purple color is a positive signal, the sensitivity limit of identification being 0.5 pg and of concentration, lO-5M. If the sample is obtained from an alkaline treatment, it should be neutralized with 30% sulfuric acid. Under these conditions, there are no interferences from common ions; b u t if they are present, SeCN-, TeCN-, and W2would give a positive reaction ( 5 ) . On the other hand, it is well known that SbCls- and SbI4- react with rhodamine B under conditions similar to those necessary for thiocyanate to react and we have found that BiI4 may be specifically extracted and gives a positive signal (6). Iodide ion forms a complex in water solution which, extracted with benzene, is also purple ( 3 ) . The quantitative technique coincides with the above description, b u t the volumes are changed. Sample, 1.0 ml, is placed in a 25-ml centrifuge tube and 1 drop of sulfite solution, 10.0 ml of benzene, and 2.0 ml of the sulfuric acid solution saturated with sodium sulfate, are added successively. Cover, shake vigorously for 1 min, and centrifuge a t 2500 rpm for 2 min. Transfer, with the same precautions as mentioned above, 5.0 ml of the benzenic extract to a dry tube, to which 5.0 ml of the reagent solution are added, and after mixing, covering, and waiting 1 min, absorbancy is read a t 560 nm and 20 "C.
RESULTS AND DISCUSSION The absorption spectrum of the complex was read from a 20 pg/ml SCN- solution. It is very similar to the one obtained with the other ions which react, and to that of pure rhodamine in water (peak a t 560 nm). Beer's law is followed between 5 pg and 20 Fg, the limit for the instrument being 25 pg. The influence of temperature on rhodamine reactions is known and to choose an optimum (20 "C), trials were run with the results shown in Table I. The concentration of the reagent in benzene is not critical between 0.01M and O.O02M, but by working with a concentration of 0.01M better results are obtained. F o r instance, the absorbance of a 20 pg/ml SCY- solution gives the range shown in Table 11. Precision was studied while working under the conditions chosen for the technique proposed in this paper (Table ID). Extraction of HSCN is favored by salting out with Nad304 in the 30% HzS04 solution, with an estimated efficiency of around 95% (Table IV). When applied to saliva and urine with known added ( 5 ) A. H. Guerrero, A. M . Roig, and C. R u i z , Sesiones Quimicas Argentinas,,San Luis 1970. (6) R . Sivori and A. H. Guerrero, A n . Asoc. Quim. Argent.. 55, 157 (1967),
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
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