Mercuric Acetate as an Analytical Reagent in Nonaqueous Titrimetry KIRON KUMAR KUNDU and MlHlR NATH DAS Physical Chemistry Department, ladavpur University, Calcutta 32, lndia
The reaction of mercuric acetate with halides has been utilized for some specific analytical procedures involving acid-base titration in glycolic solvent media. These include analyses of quaternary halides, mixtures of sodium and mercuric acetates, and binary mixtures of mineral acids containing hydrochloric acid as a common constituent. In the absence of neutral halides, perchloric or nitric acid has been estimated in mixtures with relatively large quantities of hydrochloric acid. The rapid and quantitative a.ddition of mercuric acetate to styrene in methanol has been utilized for determination of binary mixtures of this acetate with other metallic acetates. Mercaptans (thiols) have been estimated with mercuric acetate and also with phenylmercuric acetate by nonaqueous titration.
M
has found an ingenious application in the determination of halogen acid salts of organic bases as well as of several inorganic anions by titration n i t h perchloricacid in glacial acetic acid medium. The method, devised by Pifer and Wollish ( 7 4 , is based on the fact that if mercuric acetate is added to a metallic or amine salt-e.g., chloride-the anion is tied up with the mercury as undissociated mercuric chloride, and the acetate formed can be titrated with perchloric acid as a base. The excess nirrcuric acetate present does not interfere. TomiEek and Zukriegelova (//), on the other hand, electrometrically and visually titrated lithium chloride and bromide in glacial acetic acid directly with mercuric acetate, and vice versa. Kolling (3) has recently established the stoichiometry of the reaction between mrrLuric acetate and halides in glacial acetic acid medium by potentiometric mrasurements. I n glycolic solvent media, mercuric acetate cannot be titrated with perchloric acid, but with hydrochloric acid an exceptionally sharp inflection is obtained in propylene glycol-chloroform medium (2, 6), and this was utilized by Das (1) for the indirect estimation of some unsaturated compounds which take u p mercuric acetate quantitatively in methanol solution. As in glacial 1358
ERCURIC ACETATE
ANALYTICAL CHEMISTRY
acetic acid medium, halogen acid salts can be determined satisfactorily by titration with perchloric acid in glycolic media in the presence of mercuric acetate. Conversely, mercuric acetate can be indirectly titrated with perchloric acid in the presence of an added halide such as sodium or potassium chloride. These reactions of mercuric acetate in glycolic media have been utilized for several analytical procedures and the results are reported here. EXPERIMENTAL
All solvents were of laboratory reagent grade, and were tested for neutrality before use. Small amounts of bases were sometimes present in the solvents, and in such cases, the bases were exactly neutralized with 0,OlAperchloric acid, using thymol blue as indicator. Mercuric acetate was prepared by dissolving pure mercuric oxide in pure glacial acetic acid, and cautiously evaporating the solution. The samples of the quaternary halides were of unspecified purity, except lauryl trimethyl ammonium bromide (dodecyl trimethyl ammonium bromide) which was a recrystallized sample. The samples of the metallic acetates were of reagent grade and the purity was checked by titration with standard hydrochloric acid in ethylene glycol2-propanol. Hydrochloric acid (mostly 0.lN) in 1-butanol was standardized against mercuric oxide after conversion to acetate ( I ) . This has been found to be by far the best method for standardizing hydrochloric acid solutions in glycolic or alcoholic solvents. The same procedure has also been used for 0.01N hydrochloric acid solutions (1 ml. of 0.1-Y hydrochloric acid = 0.01083 gram of mercuric oxide). Perchloric acid (0.lN) in 2-propanol or butanol was standardized against sodium carbonate after conversion to acetate, using ethylene glycol-%propanol as the titration medium and thymol blue as indicator. Sodium acetate solution, 0.01.1- in ethylene glycol-2-propanol (1 to 3), was standardized with 0.01.V hydrochloric acid, using thymol blue indicator. Potentiometric titrations n ere carried out x i t h a Cambridge p H meter, using a glass electrode with a saturated calomel electrode. I t was convenient to use the p H scale rather than the e.m.f. scale, the p H values, as recorded
1
HC104(0,1 N
0'
2
Figure 1 . 1. 2.
4
6
)
d
a
,
d
10
1;
Typical titration curves
Mercuric acetate plus potassium chloride (excess) VI. perchloric acid Lauryl trimethyl ammonium bromide plus mercuric acetate (excess) vs. perchloric acid
on the p H meter, being used for plotting the titration curves. These values may not have any fundamental significance, but this is immaterial for titration purposes. RESULTS
Titration of Cationic Soaps. Some long-chain quaternary halides have been titrated in glycolic solvent medium with perchloric acid, in the presence of added mercuric acetate. The titration is based on t h e same principle as for similar halide titrations in glacial acetic acid medium. Glycolic solvent mixtures possess several advantages as titration media (5, 6 ) , one being their far greater dissolving power as compared with solvents such as glacial acetic acid. Cationic soaps are freely soluble in glycols and their mixtures with alcohols, which serve as almost ideal media for their titration. The titration can be extended to a nunibrr of various halogen acid salts which show poor solubility in glacial acetic acid medium. Moreover, titration in glycolic media is reasonably free from interferences and can be done in the presence of appreciable quantities of PFater. The glass electrode also functions very smoothly in glycolic media and presents no trouble.
A typical titration curve is presented in Figure 1, which also includes a curve for the titration of mercuric acetate in the presence of potassium chloride added in excess, for comparison. Some analytical results are given in Table I. The titration may be followed with an indicator such as bromophenol blue, but thymol blue is unsuitable. Better results are obtained by potentiometric titration. Diphenylcarbohydrazide was tried as an indicator for the direct titration of the halides against a standard solution of mercuric acetate in glycolic solvents, but no sharp end point could be obtained. Diphenylcarbohydrazide, however, is a very good indicator for titrating free hydrochloric acid with mercuric acetate or vice versa. The data reported in Table I were obtained by potentiometric titration with 0.1N perchloric acid in 2-propanol. Titration of Binary Mixtures of Acetates Containing Mercuric Acetate. For a mixture of mercuric acetate with sodium acetate, the following method has been successful. The mixture is taken up in ethylene glycol2-propanol (1 to 1) or propylene glycol-chloroform (1 t o 1) solvent mixture and potentiometrically titrated with perchloric acid in 2-propanol. An inflection is obtained when all the sodium acetate has been neutral- ized. At this stage, sodium chloride solution in ethylene glycol is added to the solution, whereby mercuric chloride and sodium acetate are formed, resulting in an increase in pH. On continuing the titration with perchloric acid, a second inflection is obtained giving the amount of mercuric acetate initially present in the mixture. The titration curve is shown in Figure 2. Attempts were made to extend the method to mixtures of mercuric acetate with a bivalent metal acetate-e.g., zinc, cadmium, and lead. But the first inflection for these mixtures is very poor and useless for analytical work. Such mixtures can, however, be analyzed by the following procedure. A known amount of the mixture of the acetates is dissolved in propylene glycol and diluted to a definite volume with methanol, the volume of the alcohol added being 2 to 3 times that of the glycol. An aliquot of this solution is taken up in propylene glycol-chloroform medium and titrated with standard hydrochloric acid in butanol, using thymol blue as indicator. This gives the total acetates. Another aliquot of the glycol-methanol solution of the acetates is treated with a few drops of styrene, Mercuric acetate readily and quantitatively reacts with the added styrene to form an addition compound as follows: CsHE CH CHz Hg (0Ac)n CH3OH + CsHS. C-CH2 HOAc (1)
+
I
1
+
0 Hg CHS(0Ac)
+
Table 1.
Analysis of Cationic Soaps
Sample Lauryl trimethyl ammonium bromide Lauryl pyridinium chloride Lauryl butyl dimethyl ammonium bromide
Cetyl trimethyl ammonium chloride Cetyl butyl dimethyl ammonium bromide Cetyl dimethyl ethyl ammonium bromide
Taken, Gram 0.2566 0.3195 0.2218 0.2652 0.2210 0.2908 0.3042 0.2784 0.3354 0.2685 0.3041
Found,
Obtained, Gram 0.2578 0.3193 0.2184 0.2600 0.2279 0.3026 0.3055 0.2639 0.3200 0.2640 0.2980
%
100.4 100.0 98.5 98.1 103.1 104.0 100.4 94.8 95.4 98.4 98.0
the original sample. The difference between the two titers directly gives the amount of mercuric acetate in moles, and hence the amount of the other constituent also can be calculated. Results of analyses of typical acetate mixtures are presented in Table 11. I n place of styrene, any other suitable unsaturated compound, which reacts readily and quantitatively with mercuric acetate under the conditions stated, ought to serve the purpose equally well. The results reported here were, however, obtained by using styrene only.
-
0
9
d -
HCiOd (O.lN)
Figure 2. Titration of sodium acetate plus mercuric acetate
The addition compound formed will consume only 1 equivalent of acid, when titrated with hydrochloric acid, whereas mercuric acetate consumes 2 equivalents as shown belorr. Hg (0ilc)z
+ 2HC1
+
HgCl?
+ 2HOA.c (2)
CsHE,C---CH?
I
4,
OCH3
+ HCl
+
(OaC) =
CH?
I
CsHsC
I
+
OCHi HOAC (3)
HgCl
If the mixture is nom titrated with hydrochloric acid, the titer will, therefore, be less than that required for
Table II.
Titration of Binary Mixtures of Mineral Acids Containing Hydrochloric Acid. PROCEDURE A. The following procedure can be used for titration of binary mixtures of hydrochloric with a strong monobasic acid such as perchloric, nitric, or p-toluenesulfonic acid. The acid mixture is taken up in glycolic solvent medium and titrated with standard sodium acetate in glycol-2-propanol, which gives the total acid. After complete neutralization, mercuric acetate is added. The sodium chloride formed during the above titration reacts with this reagent, forming a n equivalent amount of sodium acetate, which is now titrated with perchloric acid.
X typical potentiometric titration curve for a hydrochloric-perchloric acid mixture is shown in Figure 3. Curve A represents the neutralization of the total acid by sodium acetate used as the titrant. At point B denoted in the
Analysis of Mixtures of Mercuric Acetate (A) with Other Acetates
Second Constituent (B) Sodium acetate Zinc acetate Cadmiumacetate Lead acetate
Milliequivalent rl Taken Found 0 1163 0 1155 0 0686 0 0686 0 1132 0 1132 0 0693 0 0695 0 1147 0 1147 0 0686 0 0686 0 1163 0 1155 0 0686 0 0686
hfillequivalent B Taken Found 0 1132 0 1147 0 0973 0 0987 0 1193 0 llT8 0 0i69 0 0762 0 1354 0 1331 0 0742 0 0749 0 1323 0 1331 0 0560 0 0560
(B)
Found, % A 99 100 100 100 100
B 3 0 0
3
0
100 0 99 3 100 0
VOL. 31, NO. 8, AUGUST 1959
101 3 101 4
101 3 99 1 101 7 100 9 100 6 100 0
1359
Figure 3. Titration of hydrochloric acid plus perchloric acid
z \ c
3 CH3
coo NI
6
(0 IN) ml
-
9
0
2 HCiO4
4 (0.N)
6
ml
0
-+
Nitric
Perchloric and Nitric Acid (A) in Hydrochloric Acid (8) 4/B hfilliequivnlent -4 Molar Ratio A Found, Taken Found (A p p o x . ) 3 !l 1:s 98.1 0,0912 0.0895 08 . 5 1:8 0.0838 0 0842 1:l5 98.0 0,0845 0 0862 1:20 97.6 0 0702 0 0685 1:6 98.0 0.0615 0.0603 1:9 99 5 0.0588 0.0585 1:18 08 7 0.0481 0.0475 0.0512 0.0495 1:18 96 6
figure, mercuric acetate is added. Curve C represents the neutralization of sodium acetate formed by perchloric acid used as the titrant. I n calculating the amount of hydrochloric acid from the titration curve C, allowance must be made for the perchloric acid consumed by excess sodium acetate titrant added during the first titration. If z is the amount of sodium acetate required for neutralization of the total acids, y the excess sodium acetate added after the first inflection point, and z the amount of perchloric acid required for neutralization of the acetate in the second titration, all the quantities being espressed in milliequivalents, z - y = milliequivalents of hydrochloric acid, and z - ( z - y) = milliequivalents of perchloric acid. PROCEDURE B. An alternative method can also be used for the titration of a mixture of hydrochloric with perchloric or nitric acid. A measured amount of mercuric acetate is added to the mixture in glycolic solvents, and the solution is titrated with sodium acetate, which directly gives the amount of perchloric or nitric acid. -4ftrr the inflection point, the solution is titrated with hydrochloric acid. The potentiometric titration curves are shown in Figure 4. Curve A represents the neutralization of perchloric acid \pith sodium acetate used as the titrant. At point B , titration with hydrochloric acid is started, and the curve C represents the neutralization of excess sodium and mercuric acetate present.
1360
ANALYTICAL CHEMISTRY
4
0
ml-
2
4 HQ
(0,IN)ml.-
Figure 4. Titration of hydrochloric acid plus perchloric acid plus added mercuric acetate
Table 111.
Acid Perchloric
2
CH3CCQNa (0,IN)
If 21: is the amount of mercuric acetate added to the mixture, z the amount of sodium acetate required for neutralization of perchloric acid, y the excess sodium acetate added after the inflection point, and z the quantity of hydrochloric acid required for neutralization in the second titration, all quantities being expressed in milliequivalents, z = milliequivalents of perchloric acid, and w y - z = milliequivalmts of hydrochloric acid. I n both the above procedures, neutral halides will interfere and must be absent, which imposes a serious limitation on either method. Perchlorates, nitrates, or sulfates, however, will not interfere. Procedure A does not offer any special advantage in practical analysis, but Procedure B, which directly gives the perchloric or nitric acid present in the mixture, may be applicable in the determination of small amounts of these acids in relatively large quantities of hydrochloric arid. The method has been tested for such estimation, and analytical data are presented in Table 111. The second phase of the titration, which would give the hydrochloric acid content in the mixture, was omitted in the quantitative work, because the object was to determine only the perchloric or nitric acid in the misture. The rcwlts indicate that the mcthod is capable of yielding a reasonable degree of accuracy and has the advantage of being simple and straightforward. Too much free mercuric acetate, if present, seriously impairs the sharpness of in-
+
flection. Determination of total acids or separate estimation of hydrochloric acid as chloride, even if approsimatc, would indicate t'lie amount of mercuric acetate t o be added. For the work reported here, mercuric acetate was added to the acid mixture in the form of a 0.05M solution in ethylene glycolbutanol (I to I ) , and the titration was done wit,li 0.01S sodium acetate. Estimation of Mercaptans. Mercuric acrtate readily reacts with mercaptans, giving either a dimercaptide or a n acbetosymeicuri mercaptide and the react'ion limy be utilized for the estimation of mercaptans by nonaqueous titration. The sample is treated with a known amount of mercuric acetate in methanol, and the escess acetate is titrated with standard hydrochloric acid in butanol, using thymol blue or diphenylcarbohydrazidrl as indicator. The method has h e n testrd 2nd is satisfactory, for a pure sample of n-octyl mercaptan, the purity of which was checked by iodometric as well as ampwometric titration. Unsaturated compounds, thiophene, phenols, and inany other materials which tend t o react readily with niercuric acetate ill obviously interfere. The scope of the method can be enlarged by using n suitable organomercuric acet'ate-e.g, phenylmercuric acetate-in place of mercuric acetate. Phenylmercuric acptate! like mercuric acetate, can bc directly t i t r a t , d as a base in glycolic or alcoholic media with hydrochloric acid using thymol blue or diphenylcarbohydrazide as indicator (4, 10). Direct tidration Ivith phenylmercuric acetate in met,hanol was tried, but the reaction nit11 mercaptans, towards the end point, is not fast enough, and it is more convmic>nt to back-titrate, using a slight exc(m of the reagent. Fairly good results have been obtained (*2%) Kith n-octyl and benzyl mercaptans which were used as 0.01M solutions in benzene. I'nsaturated compounds and
phenols do not interfere. Thc estimation can also be carried out in the presence of amine bases if diphenylcarbohydrazide is used as the indicator. Thiophene seriously interferes. ACKNOWLEDGMENT
The authors thaidi tlir Council of Scientific and Industrial Research, India, for financial assistance.
LITERATURE CITED
(1) Das, 11. N., -4.1-AL.CHEM.26, 1086
(1954). (2) Das, M.S . ,J . Indian. Ckent. SOC.31, 39 (1954). (3) Killing, 0. W., J . Ani. Chem. SOC.79, 2711 (1957). (4) Kundu, K. K., Das, M. Tu’., Sci. and Culture (Calcutta) 23, 660 (1958). (5)?Palit, 8. R., IND. ENG.CHEM.,. ~ N A L . hD. 18,246 (1946). (6) Palit, S. R., Das, hf. N., Sornayajulu, G. It., “Non-r\qrieous Titration,” Indian
ASRJC. Cultivation of Sci., Calcutta, 1954. (7) Pifer, C. N7., \Vollish, E. G., - 1 ~ 4 ~ . CHEM.24,300 (1952). (8) Zbzd., p. 519. (9) Pifer, C. W-., Wollish, E. G., J . -1m Phnrm. Assoc., Sei. Ed. 42,509 (1953). (10) Sporek, K . F., Analyst 81, 474 (19.56). (11) Tomitek, O., Zukriegelova, >I.,Chent. listy 46, 263 (1952). It13 EIYED for review December 12, 1058. hccepted April 16, 1959.
Estimation of Arsenic in Biological Tissue by Activation Analysis HAMILTON SMITH Department o f Forensic Medicine, Universify of Glasgow, Glasgow, Scotlund
b Activation analysis is quick and accurate for the estimution of arsenic in very small samples of biological materials. After nitric-sulfuric acid digestion of the activated sample, a Gutzeit separation is combined with an estimation using a Geiger tube which accepts liquid samples. Blanks are not required. A single hair weighing 0.5 mg. can b e analyzed and two people can analyze up to 100 samples of hair in 2 days. The arsenic content of hair from over 1000 living subjects has been estimated by this technique.
T
I!HhE problems iii tlic quantitative clrtrrmination of arsenic in biologiea1 tissue are: the destruction of tissue whik keeping the arsenic in the rcaction nirdium, the analysis, and the final detrrmination of arsenic. Of the methods tricrl, a Geiger counter detection of the r:itlioisotopes of arsenic was the most sensitive. -1complete analysis consists of preparation of the specimen and irradiation in the atomic pile, digestion of the s:iinple in nitric-sulfuric acid mixture, Gutzeit separation of the active isotope : i i i d added carrier arsenic, and detection of the activity by a Geiger counter.
REAGENTS AND APPARATUS
J\-lliw possible AnalaR (British Druy IIoiises) reagents were used. IXgestion mixture of concrntratctl iiitric and sulfuric acids (5 to 3). 16- to 22-mesh zinc pellets. Sodium iodide, 15% solution. Stannous chloride, 4001, solution i i i 500/, hydrochloric acid. Mercuric chloride, 1.6% solution. 0.001N iodine solution in 4001, sodium iodide solution. A solution containing 10 y of arsenic per ml. The apparatus was a modification of
that of Thomas and Collier ( 1 2 ) . All joints n-ere ground glass sealed with water. DIGESTION
OF
SAMPLES
I k a u s e organic material reduces the accuracy of the final separation of arsenic by 10 to 40% ( 3 ) , the sample must undrrgo a digestive process to drstroy or eliminate all organic matcirial. This takes place after activation of the samplr in the pile and can he perfornirtl bj- wet ashing ( 9 ) ) dry :ishing (5-6,12), or ashing in an oxygen bomb ( 2 ) . Recauqc the second and third methods iiivolrc either a length\ procrdure or the uso oi high tcmpt~inturrsantl do not result in a significantly higher arsenic recol-ery ratio. the \$et digestion method n n s IISPd. WET DIGESTION METHOD
?‘lie quick, siniplt., and accurate method of Milton (9) was tried first on the organic samples. A sample is boiled with dilute caustic soda solution t o fix any free arsenic, antl then wetashed using a mixture oi concentrated sulfuric and nitric acids. The results on a standard sample were reproduciblv nithin 4o/c. Because the results were ithin the same region of accuracy 1%hen thc digestions were repeated without thv alkali treatment, this latter digestion was further investigated. The standard sample was a piece of filter paper to which radioactive arsenic had been added, which g a l e approximately the same digestion conditions as in actual practice. The following series of experiments was made: Standard samples were placed in borosilicate glass test tubes measuring (iX 6/* inches and of about 25-ml. (hapacity. Eight milliliters of a mixture
of concentrattd sulfuric and nitric acids in the proportions of 3 to 5 were added t.0 each. The tubes were placed on a digestion stand and heated to boiling, using fine Carborunduni as bubblcr. Tlie boiling was continued until tlic paper was digested and the nitric acid was rvaporated. The digestion was completed in under 1 hour. The vessels were cooled, the cont.ents diluted to a st,aridard volume, and the activity as estimated by counting in an M - 6 Geiger t’uhe accept,ing liquid s:tlllpl(~s. Four additional samples w r e made up and digested in the same manner, with t h r addition of 10 y of inactiye arsenic as a carrier. The results wried from 90 to 08% recovery, iiidrpriident of the addition of carrier arseiiic. Some splasliirig out of the tubr probably ~ o c t ~ u i i tfor ~ ~ the d loss. Subsrqucntly flasks of n, greater capacity were used. ontl set of digestions was carried out in 50-ml. Kjeldahl digestion fla.sks. The flasks were too large and the digrstion took 3 hours because the x i d refluxed in the necks of the flasks. The results show a large loss probably due to the lengthy digestion and bhe high tefnperature required to r i m w e t,he nitric acid from the flasks. A third set of digestions was carried out in conical-bottomed flasks; with 6inch necks, and of 25-ml. capacit.y. This digestion took less than 1 hour and all samples showed 100% recowry within the statistical counting error (1%). This method was used in all furt.her investigations. Summary of Digestion Method. The sample after irradiation with 3 ml. of concentrated sulfuric acid and 5 ml. of concentrated nitric acid is heated in a conical-bottomed flask (as above) until all t h e nitric acid is removed. The reaction mixture is cooled and then diluted. T h e reaction should be complete within 3/4 hour, after which inactive arsenic is added as a carrier. VOL. 31, NO. 8, AUGUST 1959
1361
