Determination of Mercury in Organic Compounds A Micro and Semimicromethod BURNETT C. SOUTHWORTH, JOHN H. HODECKER, and KENNETH D. FLEISCHER Sterling- Winthrop Research Institute, Rensselaer,
,A
simple and rapid method for the determination of mercury in organic compounds is based on the Schoniger method of combustion. After the sample has been burned in an oxygenfilled flask, the mercury is absorbed in concentrated nitric acid. The pH of the solution i s adjusted to 7.5, and the mercury is titrated amperometrically with (ethylenedinitri1o)tetraacetic acid. The 95% confidence interval for both the micro and semimicrodetermination
is 1%. must frequently be determined in organic compounds, particularly in the field of diuretics. A rapid method for the destruction of organic matter and the mineralization of several elements (4]6), consists of the combustion of the sample in an oxygen-filled flask. As indicated b y Schoniger and several other investigators, the process is both rapid and accurate. A reagent in the flask then serves to fix the desired element and convert it, if necessary, to a form suitable for determination. The classifical micromethods for the determination of mercury use either a combustion in the usual combustion tube or digestion with mixed acids. The mercury is amalgamated with gold by electrodeposition or by absorption of the vapor. These methods are both long and tedious. The method of Spacu and Suciu ( 7 ) is n o v in common use but, as noted by Parry ( S ) , the precipitate is too soluble for microdeterminations. This method is somewhat long but is capable of giving excellent results. When a n organomercurial is burned in an oxygen-filled flask, the mercury is converted to Hg, H&++, and Hg++. Concentrated nitric acid oxidizes and fises both of the lower forms as Hg++. It is then a simple matter to adjust the p H of the solution to 7.5 and titrate the mercury viith (ethylenedinitri1o)tetraacetic acid. CRCURY
EXPERIMENTAL
The details of the combustion have been published (4, 5 ) . Weigh the sample on a piece of filter paper (Whatman KO,54) 1 inch square. Fold the 1 152
ANALYTICAL CHEMISTRY
N. Y.
paper to prevent loss of the compound, attach a fuse of filter paper, and affix the whole to a platinum wire sealed in a ground-glass stopper. Charge a 300-ml. Erlenmeyer flask with 3 nil. of concentrated nitric acid and fill with oxygen. (The nitric acid should be fresh and colorless.) Ignite the fuse and insert into the flask. Allow the flask to stand for about 30 minutes to complete the conversion of the mercury to Hg++. Wash the contents of the flask into a 100-ml. beaker. Adjust the p H of the solution to 7 . 5 , using 20 to 50%, then 1 to 20/, sodium hydroxide. As the p H approaches 2 , add a volume of a 20% sodium acetate solution n hich will yield a 0.2M final solution. Prevent high local concentrations of alkali and see that the p H of 7 . 5 is not overshot, as a precipitate appears a t a p H of about 9.5. Titrate the mercury amperometrically with either 0.01 or 0.001N (ethylenedinitri1o)tetraacetic acid a t zero applied potential us. a saturated calomel electrode. The platinum electrode had an area
of 0.0245 sq. em. and was rotated a t 891 r.p.m. Current measurements ivere made with a General Electric galvanometer] Catalog No. 98929100, with a suitable shunt. Typical titrations are illustrated in Figure 1. The only commonly encountered interference comes from chloride ion, which stabilizes the mercury as mercurous chloride. It is then necessary to reflux the sample in the nitric acid to oxidize the mercury to the divalent form. A two-necked flask \!-as used instead of an Erlenmeyer to permit the ready addition of a condenser. The condenser must be of an efficient type to prevent loss of mercury. There are few volatile organic mercury compounds. Presumably these could be treated b y the procedure outlined by Schoniger for liquid samples. DISCUSSION
Several attempts were made to use a simple indicator titration. Schwarzenbach (6) and Flaschka (1) have described an indirect method for the titration of mercury. A solution of zinc or magnesium complexone mas added to the mercury before the solution was buffered. This n-as necessary to prevent the precipitation of mer-
\
I 0
4
v m I. E D $A
2
Figure 1. Titration of 1.00 and 10.00 ml. of 0.00939M mercuric nitrate with 0.001 04 or 0.01 037M (ethylenedinitri1o)tetraacetic acid
curie oxide and (because both authors use an ammonia buffer) to prevent the formation of an insoluble complex with mercury. Difficulties were encountered in this method. The indicator, Eriochrome black T, was destroyed by the oxides of nitrogen present from the nitric acid. If the solutions were boiled to remove these oxides, the mercury n-as volatilized. Because a simple and straightforward amperometric titration vias available, it was decided not to pursue the search for an indicator further. Mercury is reduced a t a rotating platinum electrode us. a saturated calomel electrode a t zero applied potential. The reduction wave of Hg++ 2e = H g is observed before the end point is reached; afterwards this wave drops sharply to the small constant value observed beyond the end point. The exact point is determined by extrapolation. It is necessary to reach a compromise between the optimum p H for the formation of the complex and the pH a t which mercury precipitates. From electromotive force data given by Latimer and Hildebrand (b), the solubility product
+
Table 1.
Titration of Mercury with (Ethylenedinitri1o)tetraacetic Acid
5.2 0-
Sample [3-Hydroxymercuri-( 2-methoxuypropy1)carbamyl]phenoxyacetic acid
Calcd. 43.06
Anhydro-N-( ~-methoxy-~-hydroxymercuripropyl)-2-pyridone-5-carboxycyclic acid 48.9
Hg
Founda 42.5 49.4
Sample Size, Mg.
10-40 2-4
KO. % Hg Found of Detns. 4v. Min.’ Max. 42.5 43.0 42.4 11 42.6 43.0 42.2 5 6
1
49.1 49.1 54.3 47.6
54.63 47.44
54.5
...
10-40 2-3 15-30 15
37.86 [4.3.0l-nonane-7,Q-dione 3-Acetoxymercuri-4-methoxy-8-methyl-1,6,8-triazabicyclo [4.3.0Inonane7,Q-dione 43.81 3-Hydroxymercuri-4-methoxy-1,6,8-triazabic~clo [4.3.0Inonane-7,Cl-dione 49.93 a Found by method of Spacu and Suciu (7). Xeohydrin, registered trade mark, Lakeside Laboratories.
37.8
20
1
37.6
43.6 49.6
35 25
1 1
43.5
3-Chloromercuri-2-methoxyprop ylureab
3-Carbethoxymethylmercaptomercuri-2-methoxypropylurea 3-Acetoxymercuri-8-carbethoxymethyl-4methoxy-l,6,~triazabicyclo-
for the reaction Hg++
+ 20H-
HgO
+ H20
equals 1.6 x The mercury present should start to precipitate a t a p H 9.3. This is in excellent agreement n i t h t h e observed value of 9.5. The composition of this precipitate, observed as a faint white turbidity, has not been determined. It might be the basic nitrate or carbonate because the simple oxide should be yellow or orange. Possibly the precipitate is a mixture of these compounds. Nevertheless the upper limit for the p H is 9, and for practical purposes 8 is as high as useful. A series of titrations was performed a t various p H values. At a p H of 6.5 and below a n appreciable amount of mercury was uncomplexed. However, a t a p H of 7 and above, the uncomplexed mercuric ions remaining in solution were estimated to be less than 5 X lo-@J1. A pH of 7.5 was therefore adapted for routine work. If 10 nig. of mercury are titrated in 100 ml. of
solution the recovery will be 99.9%, while for 1 mg. of mercury under the same conditions, the recovery will be 99%. The results of several determinations have been tabulated. Using the results from 0- [ (3-hydroxymercuri-2-methoxypropyl)carbamyl]phenoxyacetic a c i d , the 95% confidence interval is 15 parts per 1000 for both the micro and semimicrodeterminations. CONCLUSION
The Schoniger method may be applied to the determination of mercury in organic compounds. The accuracy compares favorably with other methods (3, 8 ) . The titration of aliquots of a mercury solution yielded results accurate to within 0.01 ml. of titrant. The relatively wide variations of the results for organically bound mercury may be due to the difficulty encountered when one tries to obtain a pure mercurial. These variations may be due to a poor
2 2
50.0
49.4 49.4 54.4
48.7 48.7 54.2
... ...
...
...
...
sampling technique. It is difficult to obtain a representative sample in 20 mg. and almost impossible to obtain a representative 2-mg. sample. T h e compounds reported are those received for routine analysis and were not specially purified. LITERATURE CITED
(1) Flaschka, H., Mikrochemie ver. V i k r o -
chim.Acta 39,38 (1952). (2) Latimer, W. M., Hildebrand, J. €I., “Reference Book of Inorganic Chemistry,” revised ed., p. -474, Macmillan, Xew York, 1940. (3) Parry, E. P., ANAL. Cmar. 29, 546 (1957). (4) Schoniger, IT.,Mikrochim. Acta 1955, 123. ( 5 ) Ibid., 1956, 869. (6) Schvarzenbach, G., “Die Komplexometrische Titration,” F. Enke, Stuttgart, 1955. (7) Spacu, G., Suciu, G., Bul. soc. sliinle Cluj 4, 403 (1929). (8) Ralton, H. F., Smith, H. -4s.4~. CHEhf. 28, 406 (1956). RECEIVED for review September 21, 1957. Accepted January 1, 1958.
Cobalt Determination in Soils and Rocks with 2-Nitroso-1-naphthol LEWIS J. CLARK’ Ferfilizer and Agricultural lime Section, Soil and Water Conservation Research Division, Beltsville, Md.
U. S.
b A sensitive colorimetric method for the determination of submicrogram traces of cobalt in soils and rocks depends upon the development of an extractable red complex between cobalt and 2-nitroso-1 -naphthol. Isoamyl acetate extracts are color-stable. The procedure is direct and effective a t room temperature; no preliminary separation of interfering metals is required. From 0 to 10 y of cobalt can b e determined accurately in the pres-
submicrogram quantities of cobalt. T h e determination of these minute traces requires highly sensitive and precise analytical techniques. Available procedures presented the disadvantages of color instability or complicated length. This investigation \vas undertaken t o develop a colorimetric method having
ence of copper, iron, manganese, nickel, palladium, and tin. Spectrophotometric measurement a t 530 rnp permits the determination of 0.1 y of cobalt in 10 ml. of solution.
C
occurs in soils and rocks in amounts usually less than 50 y per gram with average concentrations less than 10 y per gram. Sandy soils, limestones, phosphate rocks, and other inorganic materials may contain only OBALT
Deparfmenf of Agriculture,
Present address, Metallurgy Division, U. S. Naval Research Laboratory, Washington, D. C. VOL. 30, NO. 6, JUNE 1958
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