INDUSTRIAL AND ENGINEERIKG CHEMISTRY
226
Practice on other coverslips before attempting to use this method on the beveled slip is advised. The refractive index of the glass being known, the scale for the three beveled slips was calculated by substituting in Equations 1 and 2, a separate calculation of scale distance being made for each 0.01 increment in the value of N I from 1.330 to 1.850.
TABLE I Liquid
Refractive Index Microrefractometer Pulfrich refractometer
Deviation
% Water Amyl alcohol Benzene Pyridine Aniline
1.33279
0.1-
1.491 1.504
1 49297
1.576
1,57854
0.2+ 0.1+ 0.20.2-
1.332 1.398
1.40123
1,60651
The instrument when completed was found to require volumes of liquids of the order of 0.01 ml. It gave values of t h e refractive index agreeing with those obtained by the use
VOL. 10, NO. 4
of a Pulfrich refractometer to within 0.3 per cent. This is illustrated by the data in Table I obtained by setting up the two instruments side by side and making corresponding determinations practically simultaneously. Variations of room temperature within a few degrees do not cause deviation of the values obtained beyond the limit of experimental error given above. However, if values a t exact or elevated temperatures are required, they can easily be obtained, as suggested by Jelley ( I ) , by fitting a hollow circular disk into the frame behind the microscope slide and circulating water of the desired temperature through this disk. The total cost of the instrument was $1.00 for materials and $3.00 for the services of a mechanic.
Literature Cited (1) Jelley, E. E., J . Roy. Microscop. Soc., 54, 23445 (1934).
RECEIVED January
12, 1938.
Microdetermination of Arsenic ALFRED E. HOW D e p a r t m e n t of Ophthalmology, The Oscar Johnson Institute of the Washington University Medical School, St. Louis, Mo.
A modified Gutzeit procedure capable of determining as little as 0.1 microgram of arsenic w i t h a probable error of 5 per cent and sensitive to 0.01 microgram of arsenic is described.
A
PROPOSED investigation of conditions conducive to the entrance of arsenic into the eyes of individuals undergoing treatment with organic arsenicals necessitated a n analytical procedure capable of determining 0.1 microgram of arsenic with less than a 5 per cent error.
process was studied. In one of these ( 6 ) the probable error ranged from 50 to 8 per cent; in the other (55) it was uniform at 10 per cent for single determinations. These errors have been greatly reduced by usin the apparatus and procedure developed in this laboratory fo%owing a study of the various conditions controlling the reaction.
A search of the literature revealed seven micromethods for arsenic. Of these, the Bettendorf (19,39, 66) test was discarded almost immediately; for, although it is sensitive, little or no successful work has been done toward its ap lication as a quantitative measure. Three others, the DenigPs 11,& 61),Reinsch (13, 56), and titration (1, 5, 9, 16, 64, 43, 49, 51, 63) methods seemed to lack the requisite sensitivity; although one variation, the sensitive and accurate conductometric titration of Jander and Harms (37), possesses the single disadvantage of requiring the separation of arsenic from the digestion residues. The Marsh-Berselius method (67, 31, 46, 47, 52,, 60), although very sensitive, is cumbersome and in common with the origmal Gutzeit spots possesses the disadvantage that quantities intermediate to those used in the standard determinations cannot be determined with a measurable degree of accuracy. The most accurate use of the nephelometer (3, 41, 42)involves a fairly elaborate setup, and the procedure is long and does not appear to be as sensitive as the hlarsh or Gutzeit methods. The electroGutzeit method (65, 45,57) has been used very successfully, but in preliminary experiments at this laboratory the zinc-acid process was somewhat more sensitive. Fliickiger (66) used the zinc-acid Gutzeit procedure as long as 48 years ago to detect 0.1 microgram of As203, though he placed the workable limit of the process at 17. Since then at least four authors (18,38, 50,58) have differentiated between 0.17 and 0.27 by means of this procedure. Heretofore, the advantage of this sensitivity was offset by the relative inaccuracy of the method. Where figures are available, the approximate average error in the lower concentration ranges of the nephelometric, DenigBs, and titration procedures (with the exception of the very accurate titrations of Gang1 and Sanchez, 88) is about 5 per cent, that of the Gutzeit method about 8 per cent. The author could find only two papers in which the accuracy of this
Kearly all authors used paper disks or strips in the Gutzeit procedure. The former not only possess the disadvantage of inaccuracy, but also, as shown by the work of Cribb (179, allow a measurable portion of the arsine to pass through apparently unchanged. The paper strips, prepared in any of the usual ways, must necessarily possess some unevenness of deposition due to unpreventable drainage. Also it seems difficult to avoid errors up to 5 per cent in width when cutting the smallest size (1 mm.) strips. As a first effort to avoid these difficulties, the usual tube and paper strip were replaced b y a capillary tube with a thin film of mercuric chloride deposited by evaporation of a n alcoholic solution on the inner wall. When using these tubes i t was necessary to pass the gases from the generator over a drying agent such as calcium chloride, or through a microcondenser, since condensed moisture inside the mercuric chloride tube would streak or wash away the deposit. A series of such tubes, matched for depth and uniformity of deposit, gave very consistent results when exposed to arsine and “developed” by hydrogen iodide. However, the difficulty of reproducing a standard deposit was so great that the method was considered impractical. Deposits from solutions of mercuric chloride in acetone, in ether, and in water were not as good as those obtained from the alcoholic solution. Results from the use of mercuric bromide and silver nitrate
&,
Experimental
APRIL 15, 1938
ANALYTICAL EDITION
227
gave much more intense, definite, and permanent developsolutions were decidedly inferior to those obtained from ment. One of them, osmic acid, is too expensive and mercuric chloride deposits. A lesser hindrance to the use noxious to be satisfactory, although the developed color was of these tubes was the indefinite termination of the stain. as good as that produced through the use of ammoniacal This is also observed on paper strips, and may be due to the silver nitrate, the reagent finally selected as most satisfactory large volume-surface ratio. for this procedure. The silver stain does not fade; several This ratio may be reduced to a minimum by substituting hours' exposure to daylight darkrns the whole string, but cotton for paper as practiced by Hmerbein (55), b u t much this can be avoided by TI-aehingthe string in dilute ammonium better results are attained by having the mercuric chloride hydroxide solution and rinsing in distilled water. However, deposited on a string which fits fairly tightly in a capillary i t is not necessary to preserve the stain, since the averages of tube. T h e termination of the stain is then consistently the measured length. of the standard stains are graphed and definite, but a string which hangs freely or not quite snugly the measurements of stains produced by unknown quantities stains with a n indefinite end point. This may have been a of arsenic are applied to this curve. The earliest use of the cause of Thompson's (59) lack of success with thread as regraph that the author could find was Collins' (15) adoption of ported by Sanger and Black (&?), although Cahill ( I S ) used i t in 1918. It has since played an indispensable part in the a KO. 60 thread impregnated with mercuric bromide to deprocedures of a t least five other workers (A, SO,36, 40, 5 5 ) . ~ 10 to 15 per cent error. termine arsenic down to 0 . 5 n-ith The physical treatment of the string during the reaction During the work on mercuric chloiide-coated tubes, it was was found to be more important than the subsequent color obserired that an apparent sublimation of the finely divided development. Changes in the length and character of the deposit occurred a t temperatures as low as 80" C. This stain accompanying marked variations of room temperature suggested that the mercuric chloride might be distributed led Heidenhain (54),Crossley (18), and the author to control very evenly over impregnated and cut strings by heating the temperature of the absorption tube. Heidenhain imthem in sealed tubes. Experiments at various temperatures and periods in evacuated and nonevacuated glass tubes sealed mersed the whole apparatus in a water bath at 30" C. Crossley made a condenser inner tube of the absorption tube and a t both ends indicated that a temperature of 100" C. for one passed t a p water through the jacket, with the reaction flask hour in a nonevacuated tube was most successful. It was submerged in a 20" water bath. The author employed two necessary to pack the strings tightly in the tube, and advanwater baths: one to control the termerature of the reaction tageous to keep the tube revolving during the heating process. When small auantities (less than vessel and scrubber tubes, the other to twelve) of strings were so treated, there control the temperature of the absorption tube, as shown in Figure 1. With was a noticeable increase in uniformity this apparatus it was possible to study of results. This was not observed, however, when large quantities were the relationship between temperature and character of stain in some detail. heated; and since i t was necessary to have a t least 100 uniform strings for a Twenty-two temperature combinations series of experiments, the heat treatwere tried, ranging from 40":50° C. to ment was discarded. An effect similar 15":lO" C. for reaction and absorption, to that obtained by heating a small respectively. The optimum range for quantity of strings was secured by the reaction and for the absorption closely packing about 150 strings in a temperature was from 15" to 30" C. tightly stoppered tube and storing in However, the number of working comdarkness a t room temperature for 1 to binations is limited by the fact that 4 weeks. The stains were then more best stains are obtained when the abuniform and somewhat longer. sorption temperature is about 5" lower than the reaction temperature. When Strings impregnated from alcoholic this condition is observed, the length solutions were superior to those soaked and character of the stains vary alin aqueous, ethereal, or acetone solumost imperceptibly from 20":15"C. to tions. Mercuric chloride proved much 30":25O C. The author is using the more satisfactory than the mercuric latter combination because i t is more bromide which has become so popular suitable for all-year use in St. Louis. with the user9 of paper strips. Strings Exact temperature control is not very were prepared from a number of alcoholic solutions of different concentraimportant, and a variation of 1" or even 2" makes no perreptible differtions. The most satisfactory solution ence in the stain; but the end point bestrengths were found to differ considercomes rather indefinite when the two ably from those in previous use. The lowest concentration (0.25 per cent) that temperatures are equal, and almost imgave a discernible stain was selected for possible to determine with any accuracy use where less than 4 r of arsenic would when the absorption temperature is t-H much higher than the reaction tembe involved. String soaked in a 5 per perature. The success of the temperacent solution was used where 4 to lO0y of arsenic were to be determined. ture ratio set forth here might be ascribed to condensation of water vapor. A number of subdances were tried The desirability of a high, uniform as developers. The iodides of lithium, moisture content for the gases has been potassium, and cadmium all developed frequently mentioned (la.22,Z4,2Q,SS, a light brownish to yellow color, with I 50, 53, 68). The usual method of considerable variation of intensity, and ensuring an adequate concentration of subsequent rapid fading. Two easily water vapor has been to pass the gases reduced compounds which have not through a cotton plug saturated with FIGURE1 previously been applied to this use
I
a,
IKDLSTKIAL A I D ENGINEERING CHEhIISTHl
228
water. For this purpose, however, the water trap described under Apparatus has proved definitely superior to the cotton or gauze plug. Lead acetate is a very satisfactory medium for removal of hydrogen sulfide from the generator gases. von Fellenberg’s (24) use of bandage material suggebted the gauze plug which is used in the lower scrubber tube. The 50-ml. Erlenmeyer flagks first used as reaction vessels were cliqcarded in favor of the more efficient form illustrated in Figure 1. The position of the metal in the inner tube makes it certain that the rihing gases will set up a circulation of the fluid as indicated by the arions. This flow s e n es two purposes: The fluid IS effectively cooled to the temperature of the water bath, assuring the maintenance of a constant temperature; and all the solution is constantly pashing over the active surface of the metal a t a uniform rate, ensuring a n even diminution of arsenic content and evolution of ai-ine.
FICCRE2
r or,.
10, YO. 4
adoptedforsubsequent work. The stains obtained were as consistent as before but longer, and the procedure was simplified. One lot of very pure zinc reacted very slowly with sulfuric acid. The tin alloy made from this zinc required 2 hours to develop a stain previously obtained in 30 minutes. The time could be shortened by using higher concentrations of acid, but the stains were not as uniform. The addition of 0.1 gram of Mohr’s salt to the reaction mixture when this alloy was used helped t’he reaction to “completion” in 45 minutes. T h e weighing of hIohr’s salt for each experiment was rather timeconsuming, and a n effort was made to eliminat,e the need for this salt. T o this end artificial inipurities in the form of platinum, lead, nickel, and iron were added to separate portions of the zinc-tin alloy in various amounts, each less than 1 per cent. Platinum was satisfactory in a concentration of 0.01 per cent. The lead alloy gave a very intense and definite coloration, but the reaction rras still slow and the stain not very long. On the other hand, the reaction with 0.1 per cent nickel \vas so vigorous that the strings were blown out of the tubes. Iron produced about the same result as nickel, and was used in diminishing concentrations for subsequent preparations. In the range of 0.001 to 0.005 per cent it rather remarkable variation in the rate of reaction x-as noted. Sticks of alloy containing 0,001, 0.002, 0.003, 0.004, and 0.005 per cent of added iron all originally of the same size c o d d , after reacting with 40 ml. of 3.5 h’ sulfuric acid at 30‘ C. for 45 minutes, he visually separated and arranged in order according t o the iron content (Figure 2 ) . In sticks originally veighing 6.5 grams, thcrc ivaj on the average a lose of 0.75 gram for each 0.001 per cent of iron over the first 0.001 pcr cent. Thc might lost and the amount of arsine produced by the alloy with 0,001 per cent of added iron varied little from the effect secnretl by the pure zinc-tin alloy. This was also trrie of Paniplcs containing 0.001 per ccnt of nickel, and 0.001 pcir ccnt of p1:itinum. On the basis of thc) appearance of stains ol)taincti through the u s of these alloys, the best concentration of iron seemcd to lie betwecn 0.002 and 0.003 per cclnt. An alloy TF-ith 0.0028 pcr cent of iron TT-~. compared w t h one riing 0.01 per cent of platinum. The former as more ent a n d n-as selected for further work, although the stains ed from its IIT 11-ere somexhat indefinite. I t had been noticed earlier that lead appeared to haTe a stabilizing influence on tho rcactioii t)etn-een the metal and acid. With this in mind, added to the selectcd iron alloy. The 0.01 per c m t of lrad stains obtained x(’rc dark and definite, n-it11 no loss oi reprodueibilitj-.
Tlie size of the reaction T-es-els and digestion flasks determined the coiicentration of d f u i i c acid in the reaction fluid at’about 3.5 S. At this concentration, ortlinary c, P.zinc dissolve: rapidly, but there is a fairly long lag before the reaction becomes visually perceptible. To aroid this period, varioii? iuethotls of sensitizing tlie zinc haye been uqed ( 2 , 14: 18,22, 44. 46, 47,53, 5 5 ) . Iinmereioii in 0.5 per cent copper ,sulfate aolution, in 1 to 3 liytlrochloric acid containing 0.5 per rent of stannous chloride, and in 1 to 3 liytlrochloric acid \\.as tried in this laboratory. Tlie last was most, buccessiul. Exact control of t,he length of tinie tlie zinc is allon-et1 to re:ict’ n-ith tlie acid is not iniportarit, a.: the iaine results were obtained from pieces of zinc n-hich liatl been in the acid for 5-, lo-, antl 15-minute periods. It Tvas found iinnece+iary to keep the A secoiitl lot of very pure zinc vas found t o h a m a slightly sensitized sticks under water. Those n-ashetl a n J dried by greater iron content than tlie first. A comparison of perspont,aneous emporation antl Iiandlecl by galvanized forceps centage of iron-veiglit lost ciiriTes of alloys made of zinc from n-ere just as good or better a t any period after preparation the tn-o lots (Figure 3) indicates that lot I1 contained apthan pieces which had been kept under water. A zinc-copper proxiniately 0.0008 per cent more iron than lot I. In concouple would eliminate the necessity for this preliminary formity with conclusions draim from the 71-orli on lot I, the treatment, but results obtained TI-ere not’ as satisfactory. ailditioii of 0.0020 per cent of iron to lot 11was most’ satisfacVery little arsine is eyol\-ecl in the absence of st,annous and ferrous ions ( 2 , 32). A series of experiments showed that wide variation in the amount’ of 0.005 stannous ions iyas allon-able, but that more than 10 drops of a 40 per cent solution of stannous chloride in concentrated hydrochloric acid were 0.0 0 4undesirable. The amount adopted was 5 drops n to 40 nil. of 3.5 S sulfuric acid. Similarly it LJ waq demonstrated that the best concent’ration n 0.0 0 3..0 of ferrous ions lay b e h e e n 0.05 and 0.20 gram 4 of Molir’s salt in each 40 ml. of the acid. Onea3 tenth gram was chosen as the standard. L. T. Throne, in the discussion of a paper 0.0 0 2by Goode and Perkin (&9), d a t e d that lie found an alloy of zinc with 2 per cent cadmium to he very 3ensitive and reliable. The author 0.001tried alloys of 0.5 and 2 per cent cadmium, 0.1 to 10 per cent tin, 0.5 to 5 per cent’ lead, ant1 0.5 per cent niagne~iiim. \Then these iiiet,al 1 2 3 mixtiires were used. the addition of stannous G R A M S WEIGHT L O S T or ferrous salt’s to the reaction fluid was unFICCRE3 necessary. An alloy of 0.5 per cent, tin was ~
:
APRIL 15, 1938
ASALYTICAL EDITION
tory. I n general, it s e e m that tlie best results are obtained when the iron content is such that under the given conditions of dissolution tlie m-eight loss of approximately 6.5-gram pieces lies between 2.0 and 2.5 grams. Use of this generalization iq illustrated in Figure 3. A small sample of a third lot of pure zinc was uqed to form an alloy containing 0.0020 per cent of added iron. Several sticks of this alloy were allowed to react nitli acid under the standard conditione noted ab07 e. The average lo- of weight was plotted again4 the percentage of atldetl iron as point 1.
FIGCRE 4
A line having the same tun-ature and slope as lilies I anti I1 was then drawn through this point. On tlie basis of this graph it m s predicted that an arerage weight loss of 2.1 grams could be secured by the addition of 0.0029 per cent of iron to alloy made n-ith tlie zinc of lot 111. How well this prophecy was fulfilled is demonstrated by point 2 , which shows the average loss of weight of several sticks of lot' 111alloy to d i i c h 0.0029 per cent of iron had been added. This use of the graph and the generalization macle aboi-e shortens tlie time and labor required to determine tlie percentage of iron needed by any particular lot of this pure zinc. Although the effect of small variations in the iron content of the alloy is pronounced, the quantity of iron in the acid solution seems to be relatively unimportant'. One-tenth gram of hiohr's salt added to each tube of a series made no difference in tlie length of stain. It was found t'hat a difference in casting temperature of as much as 100" C. caused a slight but definite difference in the length of tlie stain produced through the use of the alloy. Heretofore reference to this effect has been neglected by workers in this field, although Bolley (11) long ago recorded a more marked effect of extreme casting temperatures on t'he acid dissolution of pure zinc. Because of this variation, castings are made in this laboratory when the metal is a t a temperature of 480" to 500"C. I t is probably needless t o add a word of caution concerning the danger of contaminating the alloy. The porcelain or quartz vessels used as melting pots should be thoroughly cleaned by boiling in the digestion mixture of acids. Tlie mold and vessels should be seasoned and tested by casting several preliminary small quantitie. before a large amount of alloy is made. The digestion procedure Tvas selected by comparing resulk obtained from digestions of blood samples containing known amounts of neoarsphenamine. On the basis of rapidity or completeness of reaction, or both. the mixture of perchloric, nitric, and sulfuric acids described under Reagents was superior to nitric-sulfuric, yerchloric-.ulfuric, perchloric-nitric, hydrogen peroxide-sulfuric, and fuming nitric-sulfuric mixtures and combinations. The process can proceed more rapidly !hen some deyice is used to prevent bumping. Glass beads proved slightly hettpr for this purpose than crushed glass fused to the bottom of the flask. Prolonged boiling of the sulfulic acid solution after elimination of the last traces of perchloric acid is unnecessary. Two or three minutes of boiling folloning the disappearance of tlie intense yellon- coloration is sufficient. Residual traces of nitrogen need not be removed from the digestion fluid when the determination is made as described. The use of oxalic acid macle no detectable difference in the
m9 --
character or length of the stains obtained through the use of any of the digestion mixtures mentioned. Barnes and Murray (6) found, contrary to t'he usual belief, that charring during digestion does riot affect the final result. The author's experience, a t present limited to recovery of known amounts of arsenic from 1-nil. samples of blood, is in accord with their finding. Determinations were quantitative regardless of tlie amount cJf carbon fornied. Tlie autlior also coiicurs n-ith Klein (40)in finding t,liat the effect of the pre.;eiicc of inoderate amounts of chloride in the digestion is negligible. Follonkg tligebtion, the pentavalent arsenic is reduced to the trivalent state in order to increase tlie length of stain and uniformity of results, using the method of Davis and Maltby (20) n-itli very slight modifications. Tliree series of quanarid 0.2 gram of sodium bisulfite were tities of 0.02, 0.075, 0.1, tried in this procedure. One-tenth gram was chosen as the standard quantity for future 11-ork since the differences, though slight, were consistent. Potassium metabisulfite was as satisfactory as sodium bisulfite in this reduction procedure. Hydrazine sulfate and potassium iodide gave low results. The use of bisulfite, hydrazine, or Alohr's salt as a reducing agent in the concentrated digest \vas T-ery unsatisfactory. Since the arsenic in arsenic trioxide and neoarsphenamine added t o "normal" (containing less than 0.Oly of arsenic per milliliter) blood v a s fully recovered, the lengthy process of dry ashing (fusion) was not tried. Determinations n-ere made on arsenic which had been isolated from the products of digestion by distillation as the trichloride (5, 7 , 8, 81, 42, 63) and by adsorption on ferric hydroxide (46,47,5 4 ) . S o increase in amount or uniformity of recovery could be obtained.
Apparatus The generator consists of a 23 X 150 mm. Pyrex test tube, H , with an inner tube, I , as shonn in Figures 1 and 4. The height of this inner tube should be so adjusted that when the tube containing a piece of zinc 20 nim. long and 8 mm. in diameter is placed in the generator tube the lowest point of the meniscus of 40 ml. of n-ater will be 4.0 to 4.3 mm. above the top of the inner tube. The gases are passrd through two 10 X 65 mm. wash tubes, the first containing a plug of cheesecloth (Figure 1, F ) , made by rolling a 9 X 90 mm. quadruple thickness strip of the matrrial; the second is a glass trap. Both tubes G and E are illustrated in Figure 1. The absorption unit is completed bv the string tube, which consists of a capillary tube of 1.3-mm. bore, 7.5 mni. in outside diameter, and 60 mm. long (Figure 1, C), n-elded to a tube of the same length and outside diameter, but of 4.3-nlni. bore (Figure 1, D). I t is essential to avoid constriction of the capillary tube at the weld. The generator and vash tubes are immersed in a water bath maintained at 30" C. The string tube projects into a closely fitting copper tube (Figure 1, .4) which is surrounded by Tvater a t 25' C. Khen less than 4-f of arsenic is to tle expected, a quartz 23 X 150 mm. test tub? is used as the digestion flask. When the amount of arsenic is likely to exceed i y , a Pyrex micro-Kjeldahl flask of ahout 12-ml. capacity, of the newer '' is used.
Tn-o Pyrex glaa3 beads 5 mni. in diameter are placed in cach digrstion flask to prevent bumping. Rends are unnecessnry when opaque quartz tubes are used. A piece of opal glass or other n-hite background should be provided as a. aid to the nieasu1,ement of the black stain. An excellent mold for casting the alloy sticks can be made 1)y clamping two carbon plates (welding plates) 15 mm. thick face to face and drilling 8-mm. holes at - 2 5 mm. intervals along tlie junction as a midline. Several porcelain (or quartz) casseroles or evaporating dishes of 50- and 200-ml. capacities should be cleaned and seasoned.
Reagents Concentrated sulfuric acid. The digestion mixture consists of 60 per cent perchloric, concentrated nitric, and concentrated sulfuric acids iri the proportion 1 to 1 to 4.
230
INDUSTRIhL AND ESGINEERING CHEMISTHY
VOL. 10, NO. 4
Sodium bisulfite in 0.1-gram amounts. or in an opaque quartz tube, and boil until fumes of sulfur triLead acetate solution, 2 3 ,acidified with acetic acid. oxide appear. Continue boiling about 5 minutes. Cool, 2. Transfer the digested mixture to a 50-ml. Erlenmeyer Hydrochloric acid, 1 to 3. flask and rinse the Kjeldahl flask with three or four small portions Alloy consisting of zinc, tin, lead, and iron in the following of distilled water, making the volume up to about 15 ml. Cool proportion by n-eight: 99.5 to 0.5 to 0.01 to about 0.0028. The and add 0.1 gram of sodium bisulfite. Cover the flask with a exact proport'ion of iron must be determined by actual trial in the glazs bulb and heat for 30 minutes in a water bath maintained at laboratory. The alloy is cast into sticks 8 mm. in diameter and 80 to 85' C. Then hoil the contents of the flask for 2 or 3 cut into 20-mm. lengths. These are immersed in 1 to 3 hydrominutes or until no trace of sulfur dioxide can 'ne detected in the chloric acid for 5 minutes, washed with distilled water, allowed vapor. to dry on a clean glass plate, and stored in a glass-stoppered 3. Transfer contents of flask to a generator tube, increasing bottle. These sensitized pieces of alloy should be handled only the volume to 40 ml. If steps 1 and 2 can be omitted, dilute by galvanized forceps. the sample to 36 ml., and acidify T\-ith 4 ml. of concentrated sulSilver nitrate, 2 per cent in ordinary dilute (1 to 10) animofuric acid. Place the tube in the 30" xvater bath for 15 to 20 nium hydroxide. minutes. The string used \vas Morse and Kaley KO.8 knitting cotton. 4. To prepare the absorption unit, place 5 drops of distilled Some care must be exercised to secure even deposition of the water in the trap, moisten the gauze plug in the other tube with mercuric chloride. The most successful procedure used in this 5 drops of the lead acetate solution, and connect the two as shown laboratory is as follows: in Figure 1. Connect the capillary end of the string tube to a The string is wound as loosely as possible in spiral form on a vacuum line, draw in the string, and disconnect the tube from glass tube 5 cm. in diameter and about 35 cm. long, with a space the line, leaving about 1 cm. projecting a t the capillary end and of 1 to 2 mm. between adjacent turns, and is placed in a glass 3 t o 4 cm. at the lower end. Cut off about 2 em. of this lower cylinder about 6.5 cm. in diameter and 45 cm. long. The inner end (handle only this loxer end when inserting string in tube), tube is held in place by removable glass spacers at each end of the and dralv the string up into the tube until the freshly cut end tube, which keep the string from touching the wall of the cylinder. is 1 em. above the weld. Cut off the projecting length of string, The cylinder is filled to a height of about 5 cm. above the top of insert the lower end of the tube into a stopper, and connect to the. inner tube with a 5 or 0.25 per cent alcoholic solution the trap to complete the absorption unit. of mercuric chloride, stoppered, and allowed to stand 15 to Remove the generator tube from the water bath, insert I). 20 hours. The excess solution is then pressed from the string the inner tube which already contains the piece of alloy (Figure 1, by passing it through a wringer made of seamless rubber tubing J ) , and immediately stopper xith the absorption unit. Submerge on glass rod and driven at moderate uniform speed by a small the generator and lower scrubber tube in the 30 ' C. nater bath, laboratory motor. The string is fed through the n-ringer diwith the absorption tube in the copper jacket maintained at rectly from the tube, which is not removed from the cylinder 25" C. After 45 minutes remove from the water bath and disfull of solution. After passing through the wringer, the string connect generator and absorption unit, should be spiraled lightly on a solid drum across the room or, 6. Remove string from absorption tube and immerse for a still better, at the far end of a draftless hallway. If the building moment in silver nitrate solution. Then place the developed is equipped with an elevator, excellent results may be obtained string on a spot plate or piece of opal glass and use a vernier by allowing the wrung string to hang down the closed shaft caliper to measure the length of the stain. until dry. Avoid drafts! When the string is completely dry it is cut in 18- to 20-cm. lengths and stored in a 23 X 250 mm. tightly stoppered glass tube which has been covered by black Discussion pa er. In these operations the string should be exposed to day,,Et as little as possible. Each step in the foregoing procedure, each reagent, and The stock solution of arsenic for standardization is made as each piece of apparatus was adopted only after comparison described by Lachele (/ti).-4bout 25 ml. of 20 per cent sodium with existing methods and various modifications of these hydroxide are used to dissolve 1.3208 grams of arsenic trioxide. methods. Since selection was made on the basis of reproThis solution is saturated with carbon dioxide and diluted to 1 ducibility, the modifications of method were compared by liter with recently boiled distilled water. A 5-ml. portion of this stock solution is diluted to 15 ml. in a series of three to ten (usually five) determinations. 50-ml. Erlenmeyer flask and acidified with one drop of conThe absorption tube s h o m in Figure 1, C, D, gives deficentrated sulfuric acid. One-half gram of sodium bisulfite is nitely more uniform results than a single length of capillary then added and the flask is heated in a water bath to 80" to 85" C. tubing. This construction was suggested by the thought for 30 minutes. It is then boiled for 2 or 3 minutes or until no trace of sulfur dioxide can be detected in the vapor. The rethat. the gases would pass more slowly through the wide porduced solution is then diluted to 500 ml., a concentration of 1 0 ~ tion, and would therefore be more uniformly cooled than arsenic per ml. More dilute solutions are made from this the when forced through the capillary with greater velocity. day they are used. The position of the lower end of the string a t 1 em. above the weld in the absorption tube was almost arbitrarily chosen Procedure after a great number of determinations had been made with strings a t various heights. 1. Add 6 ml. of the digestion mixture to the sample in a micro-Kjeldahl flask containing two 5-mm. Pyrex glass beads, Strings which were first cut in the required lengths and then soaked in solutions of mercuric chloride were not nearly so satisfactory as string prepared as described above. TABLE I. MICRODETERMINdTION O F ,kRSENIC The exact strength of the silrer niP E. of trate solution is not important, but a P E . binple greater concentration than that specified is unnecessary-even undesirable above 10 per cent. However, the color 026 1 4 Q . l 0 2 6 0 2 5 0 2 6 0 25 0 . 1 7 is not as black if the solution contains .. 056 1 3 0 . 3 0 60 0 . 5 6 0 . 3 4 0 3.5 0 34 less than 0.5 t o 1 per cent of silver ni0 5 0 81 0 . 8 5 0 8 1 0 8 8 0 83 .. 0.82 1 2 0 7 1 2 2 1 1 1 1 0 8 1 1 5 1.21 . .. 1 15 1.1; trate. . 1 5 7 1 1 1 . 0 1 51 1 . G 3 1 GI 1 63 1 .i8 .. . It was found t h a t a w a t e r - t r a p . .. 2 05 0 ti; 1 . 5 2 01 2 . 1 0 2 06 2 1 0 2 no .. . , 2 74 2.0 2 . 7 7 2 71 .. , . scrubber tube of the dimensions given r t i i n g l q , f r o m 5 Per Cent HgC1, should not contain more than 5 drops 5 0 43 n 42 n an n 4 4 n 44 0.13 1 2 2 6 of water because of the danger of wet10 0.56 0.66 0 6i 6 . 6 I 0 an 0 G I 0 6 6 0 62 0 . 6 2 0 66 0 6a 1 1 3 5 20 1 08 0 9 4 1 05 1 O!J 1 0 3 1 02 1 02 1 06 1 OD 1 . 0 0 1 04 0 90 2.8 ting the string. The number of drops 40 1 79 1 72 1 7 8 1 '33 1 OS 1 . 8 t i 1 . 8 2 1 . 7 9 1 . 8 1 1 7 8 1 So 0 8 3 2 7 60 2 60 2 6 5 2 i 0 2 64 2 31 2 311 2 44 2.i0 2 49 2 . 6 4 2 a , 0 89 2 8 should ne\-er be varied, because the 80 3 . 0 9 3 68 3 41 3 24 3 27 3 03 3 1 5 3 06 3 13 . 3 23 1 4 character of the stain is so intimately 100 3 . 8 7 3 S i 3 89 3 98 3 91 . .. related to the moisture content of the ,
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ANALYTICAL EDITIOS
APRIL 15, 1938
gases. This last consideration also applies to the amount of lead acetate solution used in t h e lower tube. Blood digestions clear very rapidly and after water and nitrogen oxides have been driven off the solution assumes a deep canary-yellow color. The temperature rises, and dense white perchloric fumes appear. K h e n the perchloric acid has been eliminated the liquid retains only a n extremely faint straw coloration. This color change is sharp; and the boiling need be continued only 2 or 3 minutes after it occiirs. The fluid become4 ~vater-clc~ir on cooling
931
umn is about 1 per cent for each string. The average probable error of a single determination is for each string slightly less than 2.8 per cent. Adding these values :iceording to the gives a n average string probahle error of formula 43 per cent for single t1etermination.j of unknowns. Table I1 shows results obtained through the use of thia zinc alloy describecl under Reagents. Here the erroi has been further reduced. These data, liowever, represent con.tancy of stain lengths and not preci-ion of arsenic determination. Because the straight-line extrapolations of the curves do not pass through the oiigin. a n additional error is involred when stain lengths are tran-foxmeti into micrograms of arsenic. This becomes serious only in the minimal tleteiniinations. Thus the probable error for 0.17of arsenic is increased to 5 per cent, but 0 . 2 ~may be estimated with a n error of only 3.5 per cent. This decreases to 3.0 per cent for 0.7?, and reaches a minimum of 1.8 per cent a t 1.57. T h e marked change of slope causes a n increase of error to 2.2 per cent a t 2 . 5 ~and 2.8 per cent a t 3.07. Similarly, an error of 6 per cent for 5-f by 5 per cent mercuric chloride string drops to 4 per cent for 207 and to 1.6 per cent for 1007
20 40 60 8.0 100 MICROGRAMS ARSENIC FIGURE 5
Pyrex beads are used to prevent bumping, because a meaeurable amount of arsenic is dissolved from common beads during t h e course of a n ordinary digestion. It is possible to digest the sample and reduce the arsenic in the tube used as a generator, but i t is very difficult to prevent bumping when the 15 ml. of solution are boiled to remove the sulfur dioxide. A volumetric portion of a stock solution 6 months old was acidified, heated with sodium bisulfite, boiled, diluted, and compared with a similarly treated portion from a freshly made stock solution. Stains produced by like quantities of the two solutions were identical. Such a reduced solution, after standing 3 months, was compared with a freshly reduced portion from the same stock solution. Stains were identical. An attempt mas made to use the Gutzeit stick of zinc to displace arsenic from neutral solution. T h e zinc was then used in a determination with only the low-arsenic reagents. It mas possible to detect as little as 0.57 of arsenic in this way, but the results were too irregular to be of any value in a quantitative procedure. Since variations of stains could not be correlated with variations of atmospheric pressure, no attempt was made to control the pressure of the gases from the generator as practiced by Bird ( I O ) . Table I is self-explanatory, b u t attention must be called to the fact t h a t these d a t a were compiled before the extremely pure zinc was acquired, and t h a t the zinc-tin alloy used had a n uncontrolled rate of dissolution. T h e average probable error of graphs drawn from the values in the “average” col-
TABLE11.
h f I C R O D E T E R M I S A T I O S O F ARSESIC
:‘tv.iiig 2 0 , f l o m 0.23 per Leiit
As
y
Length of Stain
I
Cm. C m . 0.1 0.22 0 . 2 2 0 . 3 0 43 0.46 0 . 7 0 85 0 8 4 1 . 5 1 72 1 . 7 0 2 . 5 2 60 2.51 3 . 0 2 80 2 . 9 1
HeCii
Cm.
Trii.
022 0 46 0 47 0 89 0 8 3 1 . 6 6 1 71 2 . 5 3 2’ 5 2 2 $14 2 so 0 2 2
Cin.
C‘m.
oao
0.20
0 45 0 83 1.73 2 67 2.84
0 49 0.89 1.66 2 . 2
2.91
Cm. 0.21 0 45 0.88 1 68
2
j?
2 80
-
C‘ni.
L‘m.
C m
Cin.
0.21 0.47 0 88 1 63 2 G1 2 82
0.22 0 46 0 90 1 70 2 57 2.90
0.20 0 43 0 8G 1 67 2 51 2 85
021 0.46 0 87 1 DO 2.53 2 86
I 2 MICROGRAMS ARSENIC FIGURE 6 Figures 5 and 6 are drawn from the tabulated data on strings 19 and 20, respectively. It will be noted t h a t t h e curve for string 19 consists essentially of two straight lines, intersecting near the ordinate yalue t h a t represents a stain length of 2.5 em. This effect has been noted 011 other heavily impregnated strings, but is not so marked B-hen the amount of mercuric chloride present is small, as illustrated by Figure 6. Observation of this abrupt change of slope might be of value in a determination of the nature of the qtepwise reaction between arsine and mercuric chloride, Gut an investigation of its possibilities cannot be undertaken in ~illis laboratory. It has been impossible to eliminate a certain small amount of fading a t the P. E . P. E. oi termination of the stain. This causes of Ninele a reading error when measurement’s are 70 % carried to 0.01 cm. The arerage read0.0; 3 3 ing error should not be more than 0.01 2.6 0 86 cm. and is of little importance when 0 83 2 8 0 48 1 3 the stain is longer than 1 cm. When 0 47 I 3 rl . i 2 1 7 a stain shorter than 1 em. is to be measured, this error may be practically
232
INDLSTRIhL I N D EXGINEERISG CHEhlISTRP
eliminated by taking the average of a series of about fiye independent measurements of the length of stain. Using the very pure zinc alloy, it is possible t o detect as little as 0.017 of arsenic. This is slion-n by string 12 in Figure i , where 1 S ~ O T T - Sa blank deterniination, 2 s1ion-s the stain obtained through addition of 0.017, and 3 reprwents 0 . 0 5 ~of arsenic. Figure 8 pictures typical 0.1. 0.3. 0.7, 1.5, and 3.07 btains on string 20.
,
I
3
FIGURE 7
FIGURE S
Stress has been laid on uiiiforiiiitv-iinit'oriiiitg of reagents, procedure, and apparatus. Pirice the character of the stain is influenced by nearly every miiable in the tleterniination, it is essential for accurate work that strict adherence to standard conditions be observed.
Literature Cited (1) Allcroft and Green, Biochem. J . , 29, 824 (1935). (2) Allen and Palmer, O r i g . Coni. 6th Intern. Corqr. -4ppl. Cheni., 1, 9 (1912).
( 3 ) Amati, Biocliim. t e m p . sper., 20, 523 (1933). (4) d t k i n s and Wilson, Biochevi. J . , 20, 1223 (1926). ( 5 ) Bang, Biochem. Z . , 161, 195 (19251. (a) Barnes and hfurray, ISD. ESG. CHEX.,Anal. E d . , 2, 29 (1930). (7) Billeter. Helv. Chim. Acta, 1, 475 (1918). (8) Ihid.. 6, 258 (1923). (9) Billeter and Marfurt, Ibid., 6, 771 (1923). (10) Bird, Analyst, 26, 181 (1901). (11) Bolley, Ann. Chern. Pharm., 95, 294 (1555), 112) Busquets, Anales soc. espail. fis. quim., 34, 557 (1930). (13) Cahill, dissertation, Catholic University of America, 1936. (14) Clarke, J . Assoc. Oficial A g r . Cheni., 10, 425 (1927). (15) Collins, J. IKD. ESG.CHEM.,10, 362 (1918).
c
(16) (17) (18) (19)
I-OL. 10. KO. 4
Cox, Analyst, 50, 3 (1925). Cribb, I b i d . , 52, 701 (1927). Crossley, J . Soc. Chem. Ind., 55, 272-6T (1936). Curtmann, Phernz. Rundschau, 9, 175 (1891); .4nalyst. 16, 237
(1891). (20) Dax-is and Maltby, Ibid., 61, 96 (1936). (21) Deemer and Schricker, J . dssoc. OficiaZ B g i . Chem., 16, 226 (1933). (22) Don-zard, J . Chenz. Soc., 79, 715 (1901). (23) Evans, A n a l U s t , 45, 8 (1920). (24) Fellenberg, yon, Biochem. Z . , 218, 283 (1930). 125) Flnk, J . B i d . Cheni., 72, 737 (1927). (26) Fliickiper, -iwh. Piiarvi.. 227, 1 (1869). ( 2 7 ) Ford>-ce, Rosen, and Myers, . l n i . J . .Wed. Sei., 164, 492 (1922). ( 2 8 ) Gang1 and Sanchez, Z.n i i n l . Chenz., 98, 81 (1934). ( 2 % Goode and Perkin. J . SOC.( ' i i m In(!., 25, 507 (1906). (30) Griffon a n d nuisson, Bull. soc. c h i m . , 53, 1518 (1933). (31) Hackford a n d Sand, T r u n s . Chem. Soc., 85, 1018 (1904). ( 3 2 ) Harkins, J . A n i . C'heiu. Soc.. 32, 518 (1910). (33) Harl-ey. .J. SOC.Ciiem. IM!.. 26, 1226 (1907). (34) Heidenhain. ,J. dsroc. Oficial i l g r . C'iieni., 11, 107 (1928). (35) Hiinerliein, Chem.-Zfg., 48, 380 (1924). ( 3 6 ) Hynds. S.1'. State Dept. A q r . & M a r k e t s , Ann. Rept., 1931, 98 (1932)). ( 3 7 ) Jandcr a n d H a r m s , 2.angelc. Chsm., 48, 2 6 i (1935). (3s) Karanovich. T r a n s . Inst. Picre Ciiem. Reagents ( C . S. S . E ) , KO. 14, 9 3 (1935). (39) King and Ilrown, ISD. Eso. CHEN.,Anal. Ed., 5, 165 (1933). (40) Klein, J . ASSOC.Oficinl d g r . Cheni., 3, 512 (1920). (41) Kleinmann and Pangritz. Biociieni. Z., 185, 15 (192i). (42) Ihid., 185, 14 (1927). (43) Koreninan and .Auhroch, J f i k r o c h e m i e , 21, 60 (1936). (44) Lachele, ISD.ESG. CHEY.,Anal. E d . , 6, 256 (1934). (45) Lawson and Scott, J . B i d . Cheni., 64, 23 (1919). (46) Lockeniann, Biochem. Z., 35, 478 (1911). (4;) Lockemann, Z.anyew. C h e w . , 48, 199 (1935). (45) hlaechling and Flinn, J . Lab. Clin. M e d , 15, i7Y (1930). (49) hfai and H u r t , 2. ITrstersucii. Lehensin., 9, 193 (1905j. (50) Martin and Pien. BidZ. S O C . chim., 47, 646 (1930). (51) Minot, J . Cancer Research. 10,29:3 (1920). ( 5 2 ) Monier-\Tilliams, d m l y s t , 48, 112 (1923). (53) Miilsteph, Z. anal. r h e i n . . 104, 333 (1936). (54) Myers, U. S. Pub. Health Seri-ice. I'uh. Health R e p t . , 34, 581 (1919). (55) Seller, J . Assoc. Oficinl.- 1 g r . Cliem., 12, 332 (1929). (56) Pribyl. Biociiem. Z., 159, 276 (1925). ( 5 7 ) Qunickr and Schnetka. Z. L-ntersiicii. Lehensni., 66, 581 (1933). (58) Sanger and Hlack. J . S o c . C h e m . I d . , 26, 1115 (1907). (59) Thompson. Royal Cornmiasion on .krscnical Poisoning, Final Report. Tol. 11, p. 58, London, Eyre and Spottiswoode, 1903. (60) Trotnian, J . S o c . ('hem. Inn'., 23, 177 (1904). (61) l o u n g b u r g and Farber, J . Lah. Clin. -Ifen'., 17, 363 (1932). (62) Kinkier, Z. anjlew. Chena., 26, 143 (1913). (63) Tinterfeld. Diirle, and Rauch, d r ~ l Phi'?%, ~ . 273, 457 (1935).
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C o u r t c s y , L E UR. Yano-oaki and T. -1. I € $ I L Z J
\-IT.i>l?S FRCILI QATL-RATED .kQVEOUh S O L C T I O N Probably contaminated. a i t h oxidation products. Origiiial iiiagnification X 1 9 0
