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Vol. 19, x o . 4
Ouantitative Methods for the Determination of Mercury Vapor' By Birger W. Nordlander GENERALELECTRIC COMPANY,
SCHENECTADY,
N. Y .
A study of Turner's methods o j testing the concentration qf mercury gapor in the air during an inuestigation of the amount which must be inhaled to cause mercurial poisoning. URNER2 has recently reported on cases of chronic PRocEDuRE-In order to check up the method, a current mercurial poisoning among men operating high- of air containing a known amount of mercury was passed frequency induction furnaces in a metallurgical labo- through the machine at a certain rate, and for a certain ratory of the Bureau of Standards. The cause was traced time. The mercury dissolved in the aqua regia spray was to the escape of mercury vapor from the discharge gaps of determined by analysis and compared with the total calcuthe high-frequency converters. The physical examination lated amount of mercury carried by the air entering the and clinical histories of the persons exposed to the vapor are apparatus. As a further check the outgoing air from the given in detail and constitute a valuable contribution to the apparatus was analyzed for mercury. knowledge of the symptoms of chronic mercurialism. The experimental arrangement was the same as that used In order to estimate the amount of mercury vapor which for studying the reaction between mercury vapor and selehad been inhaled by the persons affected, and to be able to nium ~ u l f i d e ,the ~ only difference being that the Palmer mastate the concentration of vapor necessary to cause acute chine was inserted into the system between the baths I and poisoning, Turner tested the air from the laboratory for 11. The air saturated with mercury at the temperature of mercury by two different methods. A fairly good agreement bath I was passed through the Palmer machine,6 charged was obtained and Turner could conclude from the results with diluted aqua regia. The air-pressure line instead of that daily exposure to an atmosphere containing as small the blower in the machine was used to maintain a steady a quantity of mercury as 0.0007 mg. per liter for a period of flow through the system. The rate of flow through the 2 or 3 months would cause poisoning. This concentration Palmer machine was measured by means of a carefully calicorresponds to one volume of mercury vapor in 12,700,000 brated venturimeter attached to the machine. The outvolumes of air, or to 0.000054 per cent by weight. going air from the machine was analyzed for mercury by Experience gained during the extensive development work means of the sensitive selenium sulfide indicator developed of the mercury boiler gave grounds for the belief that this by the writer.' concentration was far too low to account for the severe nature It is possible to make use of this indicator for quantitative of the cases of poisoning reported. An investigation has estimation of mercury vapor if certain conditions are maintherefore been made of the adequacy of the two methods tained constant. A small portion of the outgoing air from employed by Turner-namely, the aqua regia spray method the Palmer machine, first passed through a U-tube filled with and the amalgamation method. soda lime to remove traces of acid from the aqua regia spray, was led into the reaction system in bath 11, the temperature Aqua Regia Spray Method of which was maintained at 70" C. The velocity of the air This method consisted in passing the air through a Palmer current against the paper was maintained a t one meter per dust machine charged with a dilute solution of aqua regia. second. PRELIMINARY EXPERIMENTS-Air from the line was passed This machine,3 developed as a sampling apparatus for the determination of aerial dust, consists of a 375-mm. high, through the reaction system in bath I1 for some time. Abpear-shaped glass bulb, with a widest diameter of 100 mm., sence of blackening of the paper even after one hour's exposure vertically disposed, with 25-mm. tubular elongations at top indicated that the system was free from mercury. Air and bottom. The lower tube is U-shaped and when filled saturated with mercury a t 20.' C. was then passed through with a liquid will form a trap through which the air must pass. the Palmer machine, charged with 40 CC. of dilute (1:3) The upper and larger end of the bulb is connected to a blower aqua regia, and then through the mercury tester. A blackenwhich will draw a current of the air rapidly through the ap- ing appeared on the selenium sulfide paper, indistinguishabIe paratus. The liquid in the trap is thus blown out of the from that obtained when the air from bath I was passed U-tube and on reaching the bulb enlargement breaks up into directly into the tester, showing that very little mercury was a spray. As long as the air is drawn through, this spray taken out by the spray. Since it is possible by this method formation is maintained and is supposed to be effective in to detect changes in the concentration of mercury in the air as close as 5 per cent, it could be concluded that less than removing suspended matter from the air. This apparatus was used by Turner to analyze the air. 5 per cent of the total mercury content had been absorbed He substituted diluted (1:3) aqua regia for the water used by the aqua regia spray. INVESTIGATION OF COLORIMETRIC METHOD FOR MERCTJRYin ordinary dust determinations. The machine was placed close to the furnace a t about the level of the furnace operator's In order to obtain more definite data as to how much merface, and the air was drawn through a t a rate of about 113 cury was removed an analysis of the acid drawn off from the liters (4cubic feet) per minute for 30 minutes. The mercury apparatus was required. The method used by Turner for this purpose was that dedissolved in the aqua regia spray was determined quantiscribed by Lloyd and Gardner.8 Originally developed for tatively by a colorimetric method.
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Received August 13, 1926 U.S. Public Health Repts., 39, 329 (1924),also published as Reprinl 90s; compare also Jordan and Barrows, THIS JOURNAL, 16, 898 (1934). 3 Palmer, A m . J . Pub. Health, 6, 54 (1916). 1
2
1
THISJOURNAL, 19,518 (1927).
* Procured from the U. S. Public Health Service through the courtesy of the surgeon in charge. 6 J . SOC.Chem. Ind., Si, 1109 (1912).
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1927
the analysis of mercury in textiles where other heavy metals might also be found, it is a colorimetric method substantially a reversal of the Nessler test for ammonia. Instead of the usual Nessler solution, a solution containing ammonium chloride, sodium hydroxide, and potassium iodide is added to the unknown solution containing mercury chloride. A brown coloration is produced which is compared colorimetrically with a solution containing a known amount of mercury to which the same amount of reagent has been added. If other heavy metals are present they may interfere with the test. For this reason the solution of mercuric chloride in aqua regia is made slightly alkaline with caustic soda, saturated with hydrogen sulfide, and then acidified with acetic acid. The sulfides are filtered off, washed with water until all chlorides have been removed, washed with warm, dilute nitric acid, and finally dissolved in warm aqua regia. The last solution is made neutral with caustic soda and then compared colorimetrically with a standard mercury solution. I n applying this method to the present problem, where only mercury should be present, it was thought that the method might be simplified. The final solution obtained from the separation of mercury is very similar to the original solution, except that a considerably larger amount of salts (sodium chloride and nitrate) is present than in the latter case. A few experiments were made to ascertain if the reagent could be added directly to the original solution after neutralization-in other words, to study the effect of neutral salt on the coloration produced by the reaction. Solutions containing varying amounts of sodium chloride, 5 cc. of reagent solution, 0.2 mg. of mercury, and diluted to 50 cc. were made up and their colors compared after 15 to 30 minutes' standing, using a colorimeter. The results were as follows: GRAMS NaCl
0 0.5 1 2 4
COLOR 100.0 70.9 56.7 27.9 17.7
It is thus seen that the coloration was largely suppressed by the amount of salt present. Sodium nitrate was found to have a similar, although somewhat smaller, effect. One cubic centimeter of dilute aqua regia solution (1.3) will on neutralization give 0.3 gram of salt. Since very little acid was found to evaporate in the Palmer machine on a 30-minute run, although the water of the acid solution evaporated a t approximately 0.5 cc. per minute, a neutralization of the acid drawn off from the bulb after a run would give about 12 grams of salt in a volume of 50 to 75 cc. It was thus evident that (1)adding reagent solution directly to the original neutralized solution would give erroneous results, and therefore only the mercury should be precipitated from this solution and redissolved in a smaller quantity of aqua regia; ( 2 ) in order to get exact results with this final solution, the standard for comparison must contain the same amount of salt as is found in this solution when neutralized. An attempt was made to utilize the color produced on passing hydrogen sulfide into the original solution, made slightly alkaline, as a means of estimating the mercury present. It was found, however, when investigating the effect of neutral salt on the coloration produced, that the acids and alkali used, although C. P., contained a sufficient amount of heavy metals (especially iron) to interfere seriously with the color from the mercury. It thus proved necessary to return to the original method of Lloyd and Gardner. Owing to the impurities in the acids and alkali used, it was found important to wash the sulfide precipitate thoroughly with warm nitric acid to remove the
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foreign sulfides. In this connection the writer wishes to draw attention to that part of the paper by Lloyd and Gardners where proof is offered as to the accuracy of the method there described. The authors took 20 cc. of their standard mercury solution (HgCI2 solution containing 0.1 mg. mercury per cubic centimeter) and added 0.01 gram of copper sulfate in a flask containing 4 grams of alpaca wool, extracted by acid, etc., filtered, separated the mercury sulfide, and finally dissolved in aqua regia. The solution was made up to 100 cc., and 20-cc. portions were compared with standard mercury solution in Nessler tubes. A mean value of 7.86 cc. standard mercury solution to match these portions was obtained, whereas theory was stated to be 8.0 cc. A recalculation, however, shows that this figure should have been 4.0. This indicates (if no misprint is involved) that all the copper sulfide had not been removed before the mercuric sulfide was dissolved. The method finally adopted for the analysis of the aqua regia solution was as follows: The aqua regia was neutralized with concentrated sodium hydroxide, precipitated with hydrogen sulfide, and the precipitate treated a s outlined by Lloyd and Gardner. Sodium hydroxide solution from a buret was added t o the solution of mercuric sulfide in aqua regia until neutral. Another solution was prepared by neutralizing the same amount of alkali with aqua regia. A certain volume of the standard mercury solution was added and both solutions were made up t o same volume. The reagent was then added and the colors were compared.
CHECKOF METHOD-A few runs with the Palmer machine were now made at two different concentrations of mercury. Since the machine was placed in the room, no higher mercury vapor concentration than that corresponding to saturation a t room temperature (25" C.) could be used. The conditions of the experiments were similar to those maintained by Turner. Table I shows the results obtained. Table I-Check Results on t h e Aqua Regia Spray Method (Air flow through Palmer 113.2 liters per minute; total volume of air, 3396 l i k : time of run. 30 minutes) BATH I
HgIN AIR
Hg
THROUGH
PALMER ~~
C.
M g . ?er ltter
TOTAL
TOTAL
TEMPERATURE CONCN. NO.
HBYZ ANALYSIS
Hg
TAKEN OUT
~
Mg.
Mg.
40 cc. DILUTED (1.3)A Q U A REGIA 21.40 0.05 0.0063 26.40 0.05 0.0075 59.2 0.10 0.0174 59.2 0.10 0.0174 C H A R G E , 40 cc. DILUTED (1:l)NITRIC ACID 0.0174 59.2 2.5
Pe7 cenl
CHARGE,
1 2 3 4
12.8 14.5 24.0 24.0
5
24.0
0.23 0.19 0.17 0.17 4.2
Amalgamation Method This method consisted in drawing the air through a weighed glass tube filled with gold leaf and glass wool in alternate layers, the glass wool serving to support the gold leaf and to prevent packing and clogging. For each test about 991 liters (35 cubic feet) of air were drawn through the tube during a period of about 2 hours. The increase in weight gave the quantity of mercury that had amalgamated with the gold. Blank tests were run to get correction for suspended dust which accumulated on the filler. Results obtained with this method checked fairly well those with the aqua regia spray method, although they were slightly lower. Since the aqua regia spray method had been shown to give wrong results, it was evident that this method also should have an error of the same magnitude. To verify this a 25-mm. glass tube was filled with alternating layers of gold leaf and glass wool, forming a 200-mm. active layer in the tube. One book of gold leaf was used to make up the filling. The experiment was of a qualitative nature, and was carried
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out similarly to the first experiments with the aqua regia method, the tube being substituted for the Palmer machine. Air saturated with mercury vapor at 21.5’ C. was passed through the tube and thence led through the reaction system in bath 11. The rate of flow of air was kept a t 8.65 liters per minute, giving the standard velocity of one meter per second a t 70’ C. for the selenium suliide test. This rate is practically the same as that used by Turner which amounted to 8.27 liters per minute (35 cubic feet per 2 hours). Before the run was started the system was tested and found to be free from mercury. For the same time of exposure no difference in degree of blackening of the selenium sulfide paper could be seen when the air was first passed through the tube or led directly through the tester. This proved again that under these conditions very little of the mercury in the air was amalgamated with the gold leaf and that therefore this method was as useless as the aqua regia spray method. Discussion of Results The experiments have thus proved that both methods give erroneous results, the discrepancy in case of the aqua regia spray method amounting to as much as 50,000 per cent. Neither diluted aqua regia (1:3) nor gold leaf is under the conditions of the test capable of reacting with the mercury vapor sufficiently rapidly to remove more than a very small fraction of the vapor in the air. From a chemical viewpoint the use of aqua regia as a solvent for mercury is already condemnable. If a drop of mercury metal is acted upon by aqua regia a white coat of mercuric chloride will soon form, protecting the drop from further attack by the acid. Something similar might also take place in the gas phase. At any rate, nitric acid alone is a better solvent, as can be seen by experiment 5, Table I. The spray obtained by the Palmer machine is also very coarse and no intimate contact with the air passing through is obtained. Turner’s conclusions must therefore be rejected. The question then arises as to whether it is possible, by using the data given by Turne? and the results from this investigation, to get an approximate estimation of the mercury vapor concentration in the cases recorded by him. This would be of
Vol. 19, No. 4
interest since the clinical side of Turner’s work is of unquestioned value. The experiments indicate that with the concentrations concerned and under the conditions for the test only about 0.2 per cent of the mercury content is taken out of the air. Assuming that this figure would hold for a wider range than actually tested for, and taking the maximum value 0.0007 mg. per liter, found by Turner for total mercury in the air, we can calculate that the concentration should have been equal to 0.350 mg. mercury per liter air, or one volume of mercury vapor in 23,000 volumes of air, or 0.029 per cent by weight. This concentration would correspond to saturation of the air with mercury a t about 65” C. The temperature of the air close to the furnace where the samples were taken is not given, but probably it was much below this figure. That the initial concentration of mercury a t its source, the high-frequency converter, must have been very high, and thus also the temperature, can be concluded from the results of the analysis of dust taken from various places close to the furnace. According to Turner’s report, as much as 3 per cent mercury was found in a sample of dust taken 7 feet from the furnace, and a t places close to the spark gap were deposits of fine gray powder which on rubbing showed small mercury globules. A fine fog of mercury must be formed when the mercury escaping from the hot converter meets the cooler atmosphere. If dust particles are a t hand they will act as nuclei for the condensing mercury. It is possible that some of this fog was drawn into the Palmer machine where it was caught by the spray, thus raising the result above that which would correspond to saturation a t the temperature concerned. Other sources of error, although of much less magnitude, may have existed in the method for analyzing the aqua regia solution. On the whole, in the absence of sufficient data, it is scarcely possible to arrive at a correct estimation of the concentration of mercury vapor in Turner’s case, which is much to be regretted. It can be safely said, however, that it must have been several thousand per cent higher than that found by him. That the figures given by Turner as to the concentration necessary to oause poisoning must be as many per cent below the actual concentration needed to cause the severe symptoms described is an inescapable conclusion.
Distillation of Formaldehyde Solution’ By A. Zimmerli RHODIA CHSMICAL COMPANY, NBWBRUNSWICK, N. J.
Previous Work
HE preparation of formaldehyde solution of commer-
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cial strength (37 to 38 per cent by weight) by distillation of weaker solutions has been attempted from time to time. Kraut2 states that by fractional distillation of crude formaldehyde methanol accumulates in the most volatile, formaldehyde in the least volatile, fraction. Delysine3 has made a thorough study of the thermochemical properties of formaldehyde. He, too, states that on boiling a formaldehyde solution a liquid poorer in formaldehyde than the original solution distils over. Auerbach4 took up the question once more, only to confirm the h d i n g s of earlier investigators. He states that this behavior of formaldehyde is in contradiction to physical laws Received January 25, 1927. 2 A M . , am, 195 (isgo). 8 Bull. SOC. chim., [3] 17, 849 (1897). 4 Arb. kais. Gesundh.. 22, 584 (1905). 1
and accounts for it by assuming that formaldehyde solutions contain several monomolecular and polymolecular association products with water. Orloff indicates that the only way to concentrate formaldehyde solutions that are not quite up to standard is to add 100 per cent methanol to the weak solution and distil. The methanol carries water with it and leaves behind a stronger formaldehyde solution. Recently, Wilkinson and Gibsons found that Auerbach’s statements were not correct for weak solutions. They were able to concentrate solutions weaker than 8 per cent. Stronger solutions, however, followed Auerbach’s rule. Theoretical Monomolecular formaldehyde is stable only in absence of traces of water.’ It is to be expected that a t higher tem5
0 7
“Formaldehyd,” J. E. Barth, Leipzig, 1909. J . A m . Chem. Soc., I S , 395 (1921). Trautz and Ufer, J . p r a k f . Chem.. 113, 105 (1926).
