Sensitized Paper for Estimation of Mercury Vapor FRED STITT AND YOSHIO TOMIM.4TSU Western Regional Research Laboratory, Albany, Calij". A sensitized paper was required for use with a portable instrument for estimation of ethylene in air. A method has been developed for preparing sensitized paper, which is both uniform and reproducible ( & 5 % ) in its response to mercury vapor. Filter paper is impregnated with red selenium by soaking in potassium selehocyanate solution, draining, and exposing to a hydrogen chloride atmosphere. The quantity of reactive material per unit area of paper is easily controlled by adjusting the concentration of selenocyanate. The properties of this paper have been compared with those of selenium sulfide papers
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course of developing a portable instrument for measuring small amounts of ethylene in air by use of hot mercuric oxide as a n oxidizing agent ( 6 ) , sensitized paper suitable for detecting and estimating mercury vapor was studied. The use of selenium sulfide for coating paper for this purpose was first described by Sordlander ( 4 ) , who prepared a selenium sulfide powder by a carefully controlled precipitation and drying procedure, followed by a mechanical application of the polvder to the surface of the paper. Paper prepared by this method is used in a commercially available mercury vapor detector, in which the quantity of mercury present as vapor is estimated from the degree of darkening which occurs when the reactive surface of the coated paper is exposed t,o the air in a specified manner. Beckman, PIIcCullough, and Crane describe an improved procedure for preparing and using paper containing selenium sulfide ( 1 , 2 ) . Filter paper is soaked in selenious acid, exposed to hydrogen sulfide, allowed to dry, and then subjected to a baking process. The length of blackening which occurs when a standard volume of sample is sIo~vIypassed over a strip of the hot sensitized paper is a measure of the mercury vapor present in the sample. The authors wished to eliminate the need for separately calibrating each sheet of paper prepared by the met,hod of Beckman, McCullough, and Crane. Omission of the baking step in their procedure produced a uniform, stable, reactive paper u-ith properties that were readily reproducible but rather sensitive to the temperature a t which the paper was used. Sat,isfactory paper with properties relatively insensitive t o temperature has been prepared by a method xhich impregnates the paper with only selenium as the reactive material. The properties of sensitized paper prepared by these methods are compared and the relative advantages of each are discussed.
prepared by three different procedures, when used in the form of a strip over which a fixed volume of sample is slowly passed. A temperature of 65" C. or above was required for maximum reactivity of the mercury vapor with the reactive material on each of the sensitized papers. The length of blackening of selenium paper is directly proportional to the mercury vapor concentration and is insensitive to paper temperature between 65' and 200" C. Both selenium and selenium sulfide papers retained their original calibration after a year of storage at room temperature in the dark.
used without further processing. Paper prepared in this manner is referred to below as unbaked selenium sulfide paper. Within a few days of preparation sheets of unbaked selenium sulfide paper develop an irregular spotty appearance, the spots being of darker hue than the remainder of the paper. After several more days the area of these spots has spread, so that the entire sheet once more appears homogeneous, but of darker color than the original. The colors involved vary from light yellow to deep orange, depending on the concentration of selenious acid used in making the paper. This change in color probably reflects a change in phase of the deposit on the paper, but the evact nature of this change is not understood. Indeed, the nature of the reaction product usually called selenium sulfide is not understood, for available data on the selenium-sulfur phase diagram show no indication of a compound of formula SeS? ( 3 ) .
100 140 180 BAKING TEMPERATURECOC) Figure 1. Effect of Baking Condi-
PREPARATION O F SELEZIIUM SULFIDE PAPERS
The steps in the preparation of selenium sulfide papers as described by Beckman, IIcCullough, and Crane ( 1 , 2 ) were examined. It was verified that the reaction of selenious acid with hydrogen sulfide is quantitative. The time of soaking the paper in the selenious acid may be reduced t o 1 to 2 minutes. After soaking, the papers should be drained approsimatelv 15 minutes, but should not be allowed t,o become dry. Use of a iarge covered glass jar for the drainage operation eliminates atmospheric relative humidity as a factor; drainage periods from 10 t o 60 minutes have yielded satisfactorily uniform sheets under these conditions. If the paper is permitted t,o air-dry a t this stage, it can be humidified a t a later date and the preparation completed, but this is not recommended because of the difficulty of uniformly moist,ening the dried paper. The time of exposure to hydrogen sulfide is not critical so long as reaction is complete; at least twice as long as is required for no further visible change in color is recommended. After these papers have dried, they may be cut into strips and
tions on Response of Baked Selenium Sulfide Paper (R--l, 0.10 ,VI) A . Baking t i m e 30 m i n u t e s B . Baking t i m e 10 m i n u t e s C. No baking
Beckman, McCullough, and Crane ( 1 , 2 ) bake the selenium sulfide paper prepared as above for 30 minutes a t 140' C. This baking process is rather critical, because the uniformity and calibration properties of the baked paper depend on how this step is done. The net effect of the process is t o remove much of the sulfur and some of the selenium. The effect of varying the time and temperature of baking sheets of selenium sulfide paper on the length of blackening produced by a standard sample of mercury vapor passed over a 0.125-inch wide strip of the baked paper a t 150" C. is shown in Figure 1. The horizontal line, C,
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V O L U M E 2 3 , NO. 8, A U G U S T 1 9 5 1
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represents the response when the baking step was omitted. The papers were baked in a forced-draft oven, each sheet being held between two larger sheets of untreated filter paper during the baking process. Increase of temperature or baking time is seen to result in increased sublimation, as indicated by the longer length of strip required to react viith the mercury vapor. When sheets were baked between glass plates or in sealed tubes, sublimation was prevent,ed and these papers showed the same calibration properties as unbaked paper.
PROPERTIES O F SENSITIZED PAPERS
PREPARATION OF SELENIUM PAPERS
The difficulty of preparing baked selenium sulfide papers with reproducible calibration properties, and the sensitivity of response of unbaked selenium sulfide papers t o the temperature a t which they are used (see below), indicated the need for a method of preparing paper with calibration properties which are easily reproduced and are relatively insensitive t o temparature. Because the temperature sensitivity of response of unbaked selenium sulfide papers above 65' C. was shonn t o be due t o sublimation of active material from the paper, preparation of a paper containing selenium but no sulfur as the active agent was undertaken. I
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1 I I 140 180 220 100 PAPER TEMPERATURE ('G) Figure 2. Effect of Temperature on Response of I
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Sensitized Paper A, B.
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able loss in reactivity of the deposited selenium. If the paper is allowed to air-dry after soaking in the selenocyanate solution, it still reacts immediately when placed in the hydrogen chloride atmosphere, but, the selenium deposited under these conditions is not reactive to mercury vapor even a t 200" C. The hydrogen chloride treatment should be carried out in a well ventilated hood. The outer margin of each sheet of sensitized paper is discarded, only the 15 X 13 cm. central rectangle being cut into strips 0.125 inch wide and 15 cm. long.
Baked and unbaked selenium sulfide paper (W-1, 0.10 M ) , sample volume 72 CC. Selenium paper (SS-610, 0.03 M ) , sample volume 38 CC.
Such paper was prepared in several ~vays. The method finally adopted consists of soaking filter paper in a solution of potassium selcnocyanate, allowing the paper t o drain, and exposing it to hydrochloric acid vapors. The selenocyanate is decomposed by the acid, forming hydrogen cyanide and leaving potassium chloride and red selenium deposited on the paper. The steps of the procedure are the same as in the preparation of unbaked scilenium sulfide paper, except for the difference in the reactions involved. Potassium selcnocyanat,e solution can be easily prepared by vigorously shaking potassium cyanide solution with excess pure selenium powder in a glass-stoppered flask. Formation of selenocyanatc is nearly quantitative. The cyanide content of both the cyanide and selenocyanate solutions is convenient,ly dekrmined volumetrically with standard silver nitrate in the presence of iodide. Selenocyanate solutions are stable when stored in glassstoppered containers. They may be gravimetrically analyzed by precipitation of the selenium in acid solution (6). The authors have either used 21.5-cni. circles or have cut 22 em. X 16.5 em. rectangles from large sheets of filter paper. The papers are soaked in selenocyanate solution for 2 minutes or more and are hung in a closed container t o drain for 15 minutes. They are then introduced int,o a n at,mosphere of hydrogen chloride for 15 t o 30 seconds, removed, and allowed t o dry before being cut into strips. The hydrogen chloride atmosphere is obtained by bubbling tank anhydrous gas through concentrated hydrochloric acid, preferably with t,he aid of a sintered-glass type of gas washing bottle. It is important that the atmosphere be prepared before introducing the paper. Reaction is imnicdiate and full development of color should be complete in a matter of seconds. Prolonged exposure (20 minutes) t o the acid at,niosphere results not only in a weaker, somewhat brittle paper, but also in detect-
The characteristics of the various sensitized papers have been studied by passing standard samples containing mercury vapor over the paper in the form of strips 0.125 inch wide inserted in glass tubing of slightly greater inside diameter. Except where ot'herwise noted, the mercury vapor sample has always been obtained by passage of standard ethylene-oxygen samples over mercuric oxide a t 285" C. Usually the instrument , its equivalent, was described in the accompanying paper (j)or used. The volume of sample was usually 38 cc. and the flow rate about 20 cc. per minute. Choice of Paper and Flow Rate. Most of the authors' work has been done irith Whatman S o . 1 or Schleicher and Schuell (-1merican) S o . 610 filter paper. The lat,ter is a very thin paper a n d usually showed a smaller difference between t,he lengths of blackening of the trvo sides of the strip. This difference seldom exceeds 1 mm. for the thinner paper. I n the following discussion the sensitized paper used is identified by the symbol W-1 or SS-610 and the molar concentration of selenious acid or potassium selenocyanate used in preparing the paper. The shape and sharpness of the boundary between the darkened and unreacted portions of a detecting strip are affected by ~ sample over t,he paper. However, the length the rate of f l o of of blackening of unbaked selenium sulfide paper (15'-1, 0.05 M ) a t 125" C. was found to be unaffected when the flow rate was varied from 5 to 45 cc. per minute. -1flow rate of 10 t o 20 cc. per minute is recommended. Effect of Paper Temperature. The length of blackening produced by a standard sample of mercury vapor on unbaked selenium sulfide paper is sensitive to the temperature a t which the paper is used. On the other hand, the responses of baked selenium sulfide paper and selenium paper are comparatively insensitive t o the temperature of usage if that temperature is not too loxv. Curves A and B of Figure 2 were obtained with selenium sulfide papers (W-1, 0.100 M) which differed only in that the former was baked and the latter was not. The baking process (30 minutes a t 150" C.) removed much of the sulfur, so that a t lower paper temperatures a longer length of baked paper was required to react n-ith the standard sample of mercury. However, as the paper oven temperature n-as increased, sublimation from the unbaked paper increased, so that the difference in response of the baked and unbaked papers was comparatively small above 180" C. That sublimation of material from the strip occurs is evident from visible condensation of sulfur on the cool portion of the strip. When pure oxygen is passed over hot unbaked selenium sulfide paper in a long glass tube, sublimate forms where the gas rcachcs the cool portion of the tube. At higher paper temperatures the sublimate consists of a reddish deposit of selenium followed by a y e l l o ~ deposit of sulfur. Curve C of Figure 2 was obtained with selenium paper (SS-610,0.03 M). The concentration of mercury in the standard sample used for curves -4 and B corresponded t o saturation a t about 115' C., n-hereas a more dilute standard sample vias used for curve C. The response of three types of sensitized paper over a wider temperature range was studied by passing a 3-liter sample of oxygen saturated with mercury vapor a t 26 9' C. over 0.125-inchwide strips of the paper in a glass tube surrounded by a small cylindrical brass block oven a t a flow rate of about 45 cc. per niinut'e. The results are shown in Figure 3. Paper A was coated selenium sulfide paper supplied by the General Electric Co. Two strips of the paper with the uncoated sides back to back were used for ,each measurement,. Paper B was unbaked Eelenium sulfide paper (W-1, 0.033 AI), and C was selenium paper (SS-610, 0.080 M). All measurements were made a t least in duplicate. All three types of paper show a minimum length of blackening a t about 65' C. The 1on:;er the lenzth of blackening, the less
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ANALYTICAL CHEMISTRY
intense was the blackening, as is expected for deposit of a fixed amount of black mercuric sulfide or selenide over a varying area of paper. At temperatures above 65°C. the effect of sublimation is again marked with the selenium sulfide papers; it is even appreciable for the selenium paper a t 175' with this large sample volume. At the flow rate used in these experiments, the rate of reaction below 65" was apparently too slow for complete removal of mercury vapor by the first unreacted sulfur or selenium which the sample encountered on the sensitized paper. I n accordance with this explanation, it was found a t these lower temperatures that the reacted portion of the strip was not uniformly darkened, the length of blackening was not well defined, and the length of blackening was sensitive to the flow rate.
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ethylene concentration are blanks obtained with pure oxygen samples and result from dissociation pressure of the hot mercuric oxide. The mercury vapor concentration resulting from the oddation of the ethylene is 5.4 times the ethylcne concentration (6). For the selenium papers, the length of blackening (corrected for blank value) was directly proportional to the ethylene content of the sample, the proportionality constant being inversely proportional to the concentration of selenocyanate used in preparing the paper. These results may be conveniently summarized by the equation ac =
K
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where a = length of blackening in millimeters per part per million of mercury per cubic centimeter of sample measured at 25" C.; c = molar concentration selenocyanate solution used; for 0.125-inch-wide SS-610 selenium paper and K = 1.26 X strips. As the response of selenium paper is not sensitive t o temperature in the range 65" to 200" for small volume samples, Equation 1 may be used for estimating the proper concentration of selenocyanate t o use in preparing selenium paper for use in this temperature range. The corresponding value of K for W-1 Relenium paper strips is 1.0 X lo-'.
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I I I 80 I20 I60 PAPER TEMPERATURE ('C) Figure 3. Effect of Temperature on Response of Sensitized Paper A . Coated selenium sulfide paper B . Unbaked selenium sulfide paper (W-1,0.033 M ) C. Selenium paper (SS-610,0.080 M )
The authors do not understand why a longer blackening was found a t 125" than a t 145" C. for the coated paper. (An average value of 49.0 mm. with a mean deviation of 2.4 mm. was found in five experiments a t 125" performed a t three different times, whereas the average value of 40.4 mm. with a mean deviation of 3.5 mm. was obtained from four experiments a t 145".) Calibration Properties. The quantity of reactive material per unit area of sensitized paper may be adjusted by choice of paper thickness and concentration of selenious acid or selenocyanate solution used in its preparation. When the sensitized paper is used in the form of strips as described above, the width of the strip and the volume of sample used may also be adjusted to yield appropriate lengths of blackening for estimating mercury vapor a t various concentrations. Of these various factors, the solution concentration is the easiest to vary over a wide rangc with satisfactory results. The lower limit to the solution concentration that may be used in preparing the paper is set by the degree of blackening required to produce perceptible contravt between the reacted and unrcacted portions of a strip. Thiq limit is approximately 0.002 31 selenious acid or potassium selenocyanate. The length of blackening of selenium papers (SS-610, 0.010 ,If, 0.030 M , 0.090 M ) and of unbaked selenium sulfide papers (SS-610, 0.010 M, 0.030 M , 0.090 M ) was determined as a function of mercury vapor concentration with the portable instrument for estimating ethylene in air (6). Ethylene-oxygen samples of known composition were made by preparing a standard mixture by use of a gas pipet and gasometer ( 2 ) and then diluting portions of this mixture with oxygen by use of a gas buret and a mixing bulb of known volume. In Figure 4 the observed lengths of blackening for some of these papers are plotted against the ethylene concentration of the sample. The values a t zero
IO Figure 4.
20 30 40 50 60 70 ETHYLENE CONCENTRATION (p.p.m.)
Calibration Properties of Sensitized Papers
Temp. 1 2 5 O C. Sample volume 38 cc. A D . Selenium paper (SS-610 0.010 M , 0.030 M ) B: C. Unbaked selenium sulfide paper (SS-61t3,0.010 M,0.030 M ) For curve C, ethylene concentration scale should be multiplied by three
The calibration curves for the unbaked selenium sulfide papers are not linear, as illustrated by curves B and C of Figure 4. I n general, they consist of an initial portion of eteep slope followed by a linear portlon of lesser slope. The higher the concentration of selenious acid used in preparing the paper, the shorter is the initial region of higher slope. The nonlinearity is due to sublimation of sulfur from the first portion of the strip until the sample becomes saturated with sulfur vapor, thus leaving less active material to combine with mercury vapor in the first portion of the strip. For paper containing very small amounts of selenium sulfide, a major portion of the strip may he influenced by sublimation, as in curve B of Figure 4. On the other hand, for paper containing relatively large amounts of selenium sulfide, the initial region of high slope becomes very short and the calibration curve is linear over the concentration range for which it is useful. Because of the influence of the flow rate and volume of sample on sublimation effects, it is difficult to prepare unbaked selenium sulfide paper with reproducible calibration properties for use under conditions where the calibration curve is not linear in the mercury vapor Concentration range of interest.
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Uniformity and Reproducibility. The variations in response of strips cut from a single sheet of sensitized paper and of strips cut from different sheets of sensitized paper prepared by the same procedure a t different times were studied by use of the instrument and standard samples. The main factors determining these variations are differences in paper thickness within a sheet and among different sheets, reproducibility of the experimental procedure in leaving a uniform deposit of active material, and variations of the widths of the paper strips The widths of the paper strips employed showed a mean deviation from the average of about 1%. The lengths of blackening of strips cut from the same sheet of paper showed a mean deviation from the average of 2 to 3% when only the bottom ends or the top ends of the strips were used. The difference in response of bottom and top ends of strips from the same sheet %’as usually about 5%. The mean tieviati in from the average response of strips cut from different sheets proc(lssid in the same way \vas 3 to 5%. These figures \\ere the same for selenium paper (0.03 -21, SS-SlO), unbaked seleniuni sulfide paper (0.05 31, W-1), and baked when the baked papers u W P selenium sulhde papci (0.05 .\I, W-1) I-laked in a vari:iljle forced-draft oven in successive batches i t ithout changing conditiolis in the oven. .MI these figures xvere obtained with lengths of blackening of a t least 20 mm. Storage Properties. Selenium papers and both baked and unbaked selenium sulfide papers have been kept over a year a t room temperature in the dark without change in their response to a standard sample of mercury vapor. In diffuse daylight both selenium and I)aktrl selenium sulfide papers tend to become a violet color ov(’1 a peri d ( i f months. Although this discoloration reduces thv contrast bf%u een the unreacted and blackcried portion a1tc.t IISCX,it does iict seem to affect the response c.h*rrhcpt actwistics. I t is recommended that the sensitized strip? in the d a r k in rlc sed containers. 1 x 5
DISCUSSIOY 4 N D COVCLUSIOYS
Of tiic, four types of paper considered, only the selenium paper ,111 ilir f Iloi\ing properties: low sensitivity of reiponse
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to temperature above 65” C., linear calibration, high degree of uniformity, high reproducibility in preparation, and satisfactory storage properties. Baked selenium sulfide paper is essentially similar to selenium paper, as most of the sulfur is removed in the baking process, but it lacks high reproducibility in preparation because of the sensitivity of its properties to the details of the baking process. Unbaked selenium sulfide paper lacks low sensitivity of response to temperature, whereas the coated paper seems also to have a lover degree of uniformity. Selenium paper is accordingly ,recommended for use under conditions of sample volume and temperature where appreciable volatilizatioii of sulfur may be anticipated, such as estimation of mercury vapor formed u hen hot mercuric oxide is used as an oxidizing agent ( 2 , 5 ) . Either selenium paper or unbaked selenium sulfide paper zhould serve satisfactorily under conditions where volatilization of sulfur is negligible, as in the estimation of mercury vapor in atmospheres, although the resonance absorption photoelectric instrument of Woodson ( 7 ) is superior for this purpose. LITERATURE CITED
Beckman, A. O., McCullough, J. D., andICrane, R. A., AN.4L. CHEM.,20,674 (1948). (2) Beckman, A. O., McCullough, J. D., and Crane, R. A., Office of Publication Board, U. S. Dept. Commerce, R e p t . PB5921, 1945. (3) International Critical Tables, Vol. IV, p. 24, Xew York, hIcGrawHill Book Co., 1928. (4) Sordlander, B. W., Ind. Eng. Chem., 19,518 (1927). (5) Stitt, F.. Tjensvold, A. H.. and Tomimatsu, Y., . 4 ~ . 4 CHEII., ~. 23, 1138 (1981). (6) Treadwell and Hall, “Analytical Chemistry,” Vol. 11, 7th ed.. Sew Tork, John Wiley & Sons, 1930. ( i )Koodson, T. T.. Rec. Sci. Instruments, 10,308 (1939).
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RECEIVEDFebruary 21, 1919. Presented before the Division of Analytical SOCIETY, San Chemistry a t t h e 115th Meeting of t h e . ~ M E R I C A V CHEMICAL Francisco, Calif. T h e mention of products does not imply t h a t they are endorsed or recorninended b y the D e ~ ~ i t r t i n e noft .\griculture over others (if a similar nature not mentioned.
Determination of Trace Amounts of Selenium in Water JACK L. LAMBERT’, PAUL ARTHUR,
AYD T H O J I i S E. MOORE Oklahoma Agricultural und Mechaniral College, Stillwater, Oklu.
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E L E X I U ~ I because , of its toxicity and cumulativr effect in the body, presents a health problem whrn it occurs in food or water in even minute amounts. It is t h e onlj element known to be taken up b> plants from the soil in sufficient quantities to be toxic to animals (IO). In natural waters, selenium may have its source in selenium-containing soils or rocks or in water draining from areas containing seleniferous vegetation. In such L-iaterP, it may occur as the selenite or selenate ion or as organically combined selenium in concentrations usually of the order of 1 p,p.m. or less. A concentration of 0.5 p.p.m. of selenium in water is considered potentially dangerous (11). This research n as undrrtaken to develop a procedure f6r quantitatively deterniining selenium in whatevtxr form it may occur in nntural waters without first concentrating it. REAGENTS
All reagents used were of the C . P . or analytical reagent grades, except the carbon dioxide, tvhich was of commercial grade. Blank determinations, made to test the purity of the reagents, proved negative. Present address, Department of Chemistry, Kansas State College, M a n h a t t a n , Kan.
1. Sulfuric acid, 12 11’ (334 ml. of concentrated sulfuric acid per liter of solution). 2. Bromine, saturated aqueous solution. 3. Carbon tetrachloride. 4. Carbon dioxide, commercial cylinder. 5 . Potassium bromide, saturated aqueous solution. 6. Sodium hypochlorite, 1%. Zonite, a readily available, carefully prepared, and stabilized 1% solution of sodium hypochlo_rite is recommended. i. Sodium nitrite, saturated aqueous solution prepared fresh daily. 8. Urea, saturated aqueous solution prepared fresh daily. 9. Formic acid, 90%. 10. Phosphoric acid, 85%. 11. Tartaric acid, saturat,ed aqueous solution prepared fresh daily. U.S.P. grade n-as substituted satisfactorily for the more highly purified grades i n many determinations. 12. Cadmium iodide, 5.00yo (5.00 grams in 100 ml. of solution). The cadmium iodide was dissolved in somewhat less than the necessary volume of water and the solution was boiled gently for 1 5 minutes to remove iodine, after which it was diluted to the correct volume and stored in a brown glass bottle. 13. The cadmium iodide-linear starch reagent ( 1 ) was prepared from twice recrystallized linear A fraction potato starch as described by Schoch and conorkcrs (8, 9). The crude A fraction was recrystallized from a hot aqueous solution saturakd with 1butanol, centrifuged, and finally dehydrated with successive portions of 1-butanol. The starch fraction so prepared dissolved easily and completely in boiling water.
