Spectrophotometric Method for Mercury RAY U. BRUMBLAY University of Wisconsin, Extension Division, Milwaukee,
OR determining verj- small amounts of mercury, in the 1 ange Fof 0.012 to 0.12 mg. by colorimetiic methods, Cambar ( 1 ) eniployed the reverse Nessler reaction. Karaoglanov ( 2 ) found the L-olhard method satisfactory for larger quantities of mercury, but reported that it is unsatisfactory for small quantities. The method described here seems to bridge the gap between these two methods and can be useful throughout a wide range of quantities of mercury when a rapid and flexible method is desired for routine work. APPARATUS AND REAGENTS
Experimental work was done with a Beckman Model B spectrophotometer and a Beckman Model G p H meter with glass electrode. Spectrophotometric measurements were made at 460 mg, because this is the wave length of maximum absorption for solutions used in this study containing ferric thiocyanate. The solution of potassium thiocyanate was made up and standardized against silver nitrate, which in turn was standardized against primary standard sodium chloride. The mercuric solution was repared by dissolving 1 gram of purified mercury in nitric acix, oxidizing the mercury t o the plus two state with bromine, and then boiling off the excess bromine and all traces of bromide with nitric acid. The solution thus prepared n-as diluted to 1liter to give a stock solution with 1 mg. of mercury in each milliliter. Standardization was accomplished by electrolysis, using a gold electrode as cathode. Solutions of ferric ion were nitrate containing no tiace of halogen or sulfate, as both these ions tend to reduce the intensity of ferric thiocyanate color and halogens reduce the mercuric ion activity in solutions. The ferric iron solutions were standardized against dichromate, nhich in turn had been standaidiLed using
Wis.
electrolytic iron. The solutions standardized showed no trace of ferrous iron with ferricyanide. PROCEDURE
The solutions checked in the epectiophotometer were made u p in 100-nil. graduated flasks. The feriic ion was pipetted in first, the mercuric ion next; some water \\as added, then the thiocyanate, followed by the quantity of nitric acid previously founil to adjust the particular solution t o a pH of 2. Water \vas then added t o the mark, the p H Tvas checked, 1- and 4-cm. cells were filled, and readings were taken. If thiocyanate ion is added t o strong ferric ion or t o a solution too strong with nitric acid, the thiocyanate is partially destrored and the intensity of ferric thiocyanate color developed will be accordingly reduced.
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0
MILLIGRAMS
Figure 2.
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15
20
3
OF M E R C L R Y
Standardization Curves
Increasing molar ferric ion concentration with constant thiocyanate concentration increases sensitivity to mercuric ion. Cells used were 40 mm. except for curve labeled 10 m m . Precision is increased but range is decreased by increasing ferric ion concentration
n o
5
a 0
2.0 5
G O
1.0 4.0
10
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1.5
2.0 8.0 20
6.0 15
The first step in the determination of mercury by this method, assuming the mercuric ions to be properly separated from 811 other ions except nitrate, ir: finding the most satisfactory colicentration of thiocyanate for the quantities of mercury expected to be determined. This can be accomplished by a simple stoichiometric calculation followed by a few preliminary tests. The data given in Figure 1 cover a fairly wide variety of quantities of mercury and indicate other possibilities. Three standardization curves with different concentrations of thiocyanate and a fLved concentration of ferric ion are shonn ill Figure 1. Curve A shows a small but fairly uniform change in transmittance as mercuric ion is added, since both the ferric and thiocyanate ions were so dilute as to give only a slight color even with no mercury present. Curves B and C show greater changes in transmittance because of the wider range in thiocyanate 1011
10#0 25
M I L L I G R A M S OF M E R C U R I C ION
Figure 1.
Standardization Curves
Increasing thiocyanate concentration increases quantity of mercury that can be determined. Precision increases with approach t o equivalence point. 40-mm. cells. Ferric ion concentration 2 X 10-4 M
905
ANALYTICAL CHEMISTRY
906 conccntration and consequent color intensity. However, these curves represent larger quantities of mercuric ion as well. The concentration of thiocyanate to be used in analysis solutions should be such that the amount of thiocyanate ill be slightly greater than equivalent to the largest quantity of mercury to be determined. The exact amount of thiocyanate needed will depend somewhat on the ferric ion concentration, so that a small adjustment may be required attt,r t l i v tt,iric ion concentration has been chosen. 44
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+ Z
g 35
\
tration and by using cells of greater thickness, the interference of ferric ions becomes important. It was found that at 460 mp in a 4-cni. cell, concentrations of ferric ion of 10-3 M or greater interfere wit,h the transmittance of light even a t a p H of 2. This makes it advisable to use ferric nitrate of the same strength and p H in the comparison cell as in the solution being analyzed whenever the ferric ion concentration used in an analysis approaches or is greater than 10-3 M . A further difficulty arises with use of high concentrations of ferric ion and cells of large cross section. Small variations of pH greatly change the transmittance of 460 nip light through solutions containing ferric ions, {vhether thiocJ-anate ions are present or not. Figure 3 shows this to anlourit to several per cent for IO-? 31 ferric ions in 4-cm. cells with thiocyanate ions present. Figure 4 shows a similar effec*t in the absence of thiocyanate. ;is it is very difficult to control the p H within 0.1 pH unit by adjustment with nitric acid, the ferric ion concentration is best kept below 10-3 M where slight variations in pH are of little consequence, when using cells up to 4 em. thick. Cells of less thickness are affected only by larger concentrations of ferric ion, as would be expected.
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DlSCCSSIOZl
This method is applicable to smaller quantities of mercury than the standard Volhard and it is lapid, as are most photometric methods. The limitation in not being applicable to large quantities of mercury because of precipitation of mercuric thiocyanate in the mi\;ing flasks is one w r m u disadvantage. Csing larger
30 I
1.
, 1.80
1
1
190
200
210
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2.20
PH Figure 3.
Change in Transmittance with Change in plI
0.01 41 ferric nitrate solution nith thiocjanate ion 40-mm. cells. Referconcentration of 4.4 X 10-4. ence cell contained 0.01 M ferric nitrate at pH 2.25
After the concentration of thiocyanate required for a series of analyses ii? fairly well established, the ferric ion concentratiori should be adjusted to give a stantlard curve of sufficient range of quantity of mercuric ion. The curve with ferric ion concentration of 2 X lo-' M ferric ion in, Figure 2 is essentially the same curve a? .I i n Figure 1. I t is obvious this curve covers the range of from 0 to 2.1 mg. of mercury with about cqu:il change in transmittance per unit of mercury. Ho~vever,t h e curve is too fiat for any great precision. By doubling the ferric. ioii concent,ration, the next curve in Figure 2 is obt,ained, which giws better precision but narroxvs the range to between 1 and 2 1119. of mercury. The other curve in Figure 2, with the ferric ion concentration a t lo-* M , has a range of only 1.8 to 2 mg. of mercury, but gives very large changes in transmittance for small changes in quantity of mercury. JVith this curve and within this range the author was able to get results that were reproducilile bvithin 1 % of the yuantity of mercury present. Curves with a short range have the advantage that small errors in reading are possible for relatively large amounts of mercury, thus giving good results even in the 80 to loo'% trnsmittance range, in spite of the fact that instrumental errors become large near 100%. Each analyst will have to decide upon the ferric and thiocyanate concentrations most suitable to the precision, range, and speed required. Figures 1 and 2 show trends in curve form with variations in ferric and thiocyanate ion concentrations and act as a guide for assisting the analyst in his choice of concentrations. FERRIC ION IXTERFERENCE
In attempting tQ increase the sensitivity of ferric thiocyanate to small changes in mercuric ion by increasing the ferric ion concen-
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1.90
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2.00
2.10
2 20
PH
Figure 4.
Change in Transmittance with Change in pH
0.01 M ferric nitrate solutions i n 40-mm. cells. Reference cell contained ferric nitrate at pH of 2.25. Compare with Figure 3, where thiocyanate is also present
volumetric flasks for making up solutions can help to some degree in solving this problem, as can aliquoting and choice of sample size, but aliquoting reduces precision proportionally and large flasks become awkward and slow to handle. The reverse Xessler reaction produces a suspension of solid and is not suitable for use in the ordinary photometer because of light scattering. The lower limit of amount of mercury determined by this method can be extended by using high concentrations of ferric
V O L U M E 24, NO. 5, M A Y 1 9 5 2
907
ion, cells of greater thickness, and smaller volumetric flasks for making up the test solutions. The minuteness of mixing flasks is limited by the volume of the cells used, and increasing the cell length causes interference by ferric ion to become a problem in the same way as increasing the ferric ion concentration. Another precaution must be cnonsidered. The author found that some solutions kept in the dark gave readings that changed less than 1%transmittancy in 24 hours. Other solutions changed as much as 6% in 24 hours, but none of the solutions used in this study changed in transmittancny as much as 1% in 30 minutes. It therefore seems safe to sa) that ferric thiocyanate in solution3 of p H 2 is stable enough to be used to determine mercury if readings are taken well within half a11 hour after the solutions are mixed.
SUMMARY O F METHOD
The concentration of thiocyanate to be used in the mixing flask is first chosen so as to be slightly greater than equivalent to the largest quantity of mercury to be determined (Figure 1). Then the concentration of ferric ion is adjusted to give a range of quantities of mercury broad enough to include all samples expcctcti, or to give acceptable precision (Figure 2). A standard curve is r u n and analyses are made in the usual nay. LITER 4TURE CITED (1) Cambar, R., Bull. trar. sop. phurm. Bordeaux, 78, 112-26, 132-64 (1940). (2) Karaoglanov, Z., 2. anal. Chem., 125, 406-16 (1943). RECEIVED for review M a y 5, 1951.
.4ocepted December 12. 1951.
Microdetermination of Sulfur Using the Grote Combustion Apparatus Combustion of Sulfanilamide 0. E. SUNDBERG AND G. L. ROYER American Cyanamid Co., Calco Chemical Division, Bound Brook, N . J . previous publication ( 5 1 method was described the I Smicrodeterminat,ion of sulfur in organic compounds by the A
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Grote combustion procedure. Since then, the method has been successfully applied in this laboratory in the determination (Jf sulfur in a wide variety of sulfur compounds. Recently, Steyermark, Bass, and Littman (4)reported low and erratic sulfur values for sulfanilamide by the Grote combustion procedure, and indicated that other workers had experienced similar d i f f i d t y . No further analytical data on the determination of sulfur i n SUIfanilamide have been published up to this time, but srvrral improved titration methods for sulfur have been described t)y Ogg, Willits, and Cooper ( 1 ) and hy Walter (6). Although Steyermark et al. (4)report low sulfur values (approxiniately 1 to 30/, Ion.) for sulfanilamide, no difficult!- \vas observed by the authors in the combustion of other organic sulfur compounds, including sulfanilic acid. The object of this publication is to present recent data ahouing that sulfur in sulfanilamide can be determined successfully by combustion in the Grote apparatus, and to indicate the wide temperature range under which acceptable sulfur values were obtained. The data presented herewith were obtained by studying the behavior of sulfanilamide in the Grote combustion apparatus under various conditions. Sperial emphasis was placed on the question of combustion temperatures. EXPERIMENTAL
-4series of sulfur determinations was made on two saniplrs of sulfanilamide according to the published procedure (6). The samples ranged in weight from 6 to 22 mg. Table I shows the
Table I.
Values for Sulfur Obtained in a Sample of U.S.P. Sulfanilamide (Theory 18.627,) Furnace Temperatures, Auxiliary. C.
Sulfur Found,
-Mg.
11.27 12.18 5.695 I O . 07 21.96 15.57 18.55 15.85 17.58 16,71
350 350 350 350 400 400 400 400 400 400
18.63 18.56 18.46 18.72 18.40 18.43 18.40 18.51 18.50 18.42
Weight of Sample,
%
Table 11. Values for Sulfur Obtained in a Sample of Sulfanilamide Labeled Reference Standard' (Theory 18.62%)
wt. of
Sample, hlg.
Furnace Temperatures, C. Movable Stationary
Sulfur Found, %
Soteb
720 730 18.58 a 720 730 18.54 a 730 720 18.70 a 730 720 18.50 a 720 730 It 18.64 720 730 18.51 a b 720 730 18.72 720 730 18,72 C c 720 730 18.35 620 730 18.53 a 18.50 540 730 a ti20 18.46 540 e 620 18.43 a 540 620 540 18.34 a 620 540 18.39 a 620 540 18.49 a S.50 540 18.40 a 550 540 18.42 a 550 540 18.34 b d 440 460 16.80 Sample received from -4. Steyermark. Satisfactory sulfur value: were obtained for this sample by the micro-Carius method (4). 10 10 11 19 5 11 10 10 10 9 11 11 11 13 15 12 11 10 9 10
b
a. b. e.
d.
87 03 83 42 848 17 79 79 80 950 18 07 31 20 08 66 30 93 790 04
Quartz Grote tube, platinum catalyst. Quartz Grote tube, no platinum catalyst. Vycor Grote tube, platinum catalyst. Considerable undecoinposed material.
sulfur values obtained undpr normal operating conditions with the stationary furnace at 730" C'. and the movable furnace a t 720" C., except that the end auxiliary furnace was operated a t 350" and 400" C. A quartz tube containing a platinum catalyst was used. Table I1 s h o w the sulfur values obtained by various modifications of the originall>- published procedure with the auxiliary furnace held at 500" C. DI scussron-
The results in Tables I and I1 indicate that no difficulty is encountered in determining sulfur in sulfanilamide by the Grote procedure, even under v-ide1.v varying conditions of temperature and procedure. Correct sulfur values (within 3~0.3% of theor?) were obtained by combustion in a temperature range from 540" to 730" C. Equal success resulted when differsnt Grote combustion tubes were used, including a Vycor silica tube. A low value, accompanied by considerable undeconiposed material, was obtained by combustion a t 460" C. The platinum catalyst, although helpful, is not an essential requirement in the determina-
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