Polarographic Microdetermination of Chromium in Dusts and Mists PAUL F. URONE, MARY LOUISE DRUSCHEL, AND HANNS K . ANDERS Division of Industrial Hygiene, Ohio Department of Health, Columbus, Ohio
A polarographic method for the determination of chromium in dust and mists is described. The chromium may exist in the form of its ores, oxides, and various inorganic salts. Samples analyzed are of a micro or semimicro size. The amounts of chromium found vary from a few micrograms to several milligrams. A number of factors influencing the accuracy of the determination have been studied. Results obtained with the Droposed method are compared w-ith results obtained with standard methods. Inalyses of several National Bureau of Standards samples are also given.
T
HE determination of chromium in the air of a plant manufacturing chromate salts and liquor concentrates from chrome ore presents a variety of problems. The manufacturing process begins with the crushing and grinding.of the ore to less than 200-mesh size. The pulverized ore is mixed with sodium or potassium carbonate and some dolomitic lime. The mixture is heated to a high temperature in rotary kilns where most of the chromium is converted to the chromate form. The kiln products are transported to large leaching tanks, and the soluble components are removed with water. The highly alkaline leach is gradually neutralized with sulfuric acid; aluminum, silica, and various sulfates are removed by pH control, concentration, and filtration. The chromate is converted into the dichromate a t a pH of approximately 4,and the product is shipped either as a concentrated liquor, or after further concentrating, crystallizing, filtering, drying, and bagging, as the dichromate salt itself. Some of these operations are dustier than others; some may not be as dusty but may contain greater percentages of chromium in one form or another. I n plants where most of the operations are carried on under one roof, the air may contain the products of all the operations, with possibly a larger proportion of the products of the operation nearest a t hand. I n this investigation, it v a s desired to determine the total amount of chromium in the air and to get estimates of the water-soluble chromium compounds, the chromium in which was mostly hexavalent with traces of the tri- and divalent forms. Of the various analytical methods available, it was thought that a polarographic method (6) would probably be the more easily adaptable to a rapid routine procedure.
and independent of the chromate concentration, and that the diffusion current is directly proportional to the concentration of the chromate ion. I t remained to develop techniques to analyze chrome ore and chromium compound mixtures and to determine much smaller quantities of chromium. The procedure as finally worked out consists of two parts: one for solutions and one for residues, the latter essentially a fusion method. REAGENTS AND SOLUTIONS
Unless otherwise specified, all reagents are of analytical quality. Standard Chromate Solution. Potassium dichromate obtained from the National Bureau of Standards (2.829 grams) is dissolved in double-distilled water and made up to 1 liter. One milliliter of this solution is equivalent to 1 mg. of chromium. Solutions containing 100 and 10 micrograms of chromium per milliliter are made by diluting this st,ock solution with double-distilled water. Sodium Hydroxide Solution. Depending upon the assay, 42 to 45 grams of reagent quality sodium hydroxide per liter of final solution are dissolved in freshly boiled double-distilled water, cooled, and made up to volume. This should be stored in a bottle (preferably of borosilicate glass) from which carbon dioxide and water are excluded, and from which the solution is withdrawn without disturbing the carbonateq silicates, and other insolubles which settle to the bottom of the bottle. The solution may be standardized and adjusted to exactly 1 .V; however, slight variations in normality do not greatly affect the results. To obtain a uniformly consistent blank, it is desirable to make several liters of this solution a t a time. Fusion Mixture. Four part,s by weight of magnesium oxide to one part of sodium carbonate are mixed thoroughly, dried, and stored in a dustproof bottle. I t is mixed thoroughly each t>ime before using. Distilled Water. Ordinary distilled water does not contain noticeable amounts of chromium. However, there is always a possibility that, the n-ater may contain free chlorine, organic matter, or other volatile substances which may react with the chromate ion. Once the chromium is osidized it is necessary to use either freshly boiled distilled water or double-distilled water that has been well kept. Mercury. The mercury is first cleaned by letting it fall several times through a column of 10% nitric acid. After rinsing and drying it is triply distilled in an all-glass apparatus a t a pressure of 20 mm. with a verv fine stream of air sweeping through and over the mercury throighout, the distillation.
Thanheiser and Willems ( I O ) reported a polarographic method for the determination of chromium in steel. A 0.1- to 0.2-gram sample was dissolved in hot concentrated perchloric acid, neutralized, oxidized in alkaline solution with hydrogen peroxide, boiled, and then run polarographically in approximatelp 1.6 iV sodium hydroxide. Determinations of 0.5 mg. and less of chromium in 50 ml. of final solution were said to be run only with great difficulty. Molybdenum, tungsten, vanadium, and aluminum were reported as noninterfering. Stackelberg, Klinger, Koch, and Krath (9) also reported a method for analyzing for chromium in steel by use of the polarograph. In their method a 0.2-gram sample was dissolved in 1 to 1 hydrochloric acid, oxidized with nitric acid, dried, fused xith 4 grams of sodium peroxide, dissolved in hot water, and diluted to 100 ml., and a portion of the supernatant liquid was run polarographically. The final solution was between 1.5 and 2.0 JV in sodium hydroxide a t the time the polarogram \vas run. No standardization figures mere given, but comparative analyses were shown on a number of steel samples.
~I
IN STRUMENTATION
-1Model XXI Sargent pen recording polarograph was used, which had an ultimate sensitivity of 0.003 microampere per millimeter scale deflection. Diffusion currents, id, were obtained,by taking the averages of the oscillations of the residual and the limiting diffusion currents. The supplementary damping provided on the instrument was not used even a t high concentrations of chromium. One capillary was used throughout the entire problem. I t had a drop time of 5.80 seconds, and the amount of mercury, tn, passing through the capillary in a 1 iY sodium hydroxide solution with an open circuit was 1.06 mg. per second. Kith a closed circuit and an imposed e.m.f. of -0.85 volt us. the saturated calomel electrode (S.C.E.),the drop time WVBS 5.52 seconds
Lingane and Kolthoff ( 7 )studied the reduction of the chromate ion a t the dropping mercury electrode in unbuffered, buffered, and strongly alkaline media. They found that in a 1 ,V sodium hydroxide solution the half-n-nve 1)otrntial is practically constant 472
V O L U M E 22, NO. 3, M A R C H 1 9 5 0
473
and m was equal to 1.09 mg. per second. The mercury colum: height was kept a t 20 inches (50 cm.) and the temperature a t 25 * 0.2' C. ( 1 ) . All samples were bubbled for a t least 15 minutes with high purity nitrogen gas passed through 1 iV sodium hydroxide before passing through the samples. Small, simplified polarographic cells (Figure 1) were used. They required small mercury pool anodes, and anodic contact was made by means of a beaker partially filled with mercury and suspended in a constant temperature bath. These cells were designed to be easily changed after an analysis. Although the use of mercury pool anodes in place of the usual saturated calomel electrode requires a relatively large amount of triply distilled mercury, the possibility of contamination of one sample by another and the bother of cleaning and keeping salt bridge3 are e1imin:tted.
50mrn.
T 20mm.
Figure 1.
IOmm.
6mm.
Simplified Polarographic Cells
Glassware. All glassware RBS cleaned Kith Calgonite, rinsed with tap water, rerinsed in 1: 1: 8 solution of nitric and sulfuric acids in water, and finally rinsed with distilled water. The common sulfuric acid-chromic acid cleaning solution was rigorously avoided. COLLECTION OF DUSTS AND MISTS
A sufficient quantity of air is drawn through one of the sampling devices recommended for dust and/or mists ( 3 , 5, 8) to give a sample containing 5 or more micrograms of chromium. A 15cubic foot sample of air is usually ample. During the course of this investigation several sampling devices were used. These included the Greenburg-Smith impinger with 100 ml. of either distilled water or 1 N sodium hydroxide, midget impingers with 10 ml. of distilled water, a 5.5-cm. Eaton-Dikeman Xo. 623-026 filter paper in a Casella filter paper holder, a 15-cm. E. & D . No. 623-026 filter paper in a special filter head, and a Mine Safety Applianre Company (M.S.A.) electrostatic dust and fume samplrr. The M.S.A. midget impinger hand pump set t o draw 0.1 cubic foot per minute was used for the midget impingers. A Wilson portable electric vacuum pump, drawing air a t the rate of l cubic foot per minute, was used for the impingers and the sinall filter paper holders, and a Crowell pump, size 2A, driven by x gasoline motor and drawing air a t a maximum of 13 cubic feet per minute, was used for the large filter paper holders. Air volumes of all except the midget impingers were measured by means of specially adapted orifice mrtcrs \vhirh had heen previously c:ililirated against a wet gas meter.
Table I.
Filtered
ANALYTlCAL PROCEDURE
Solutions. When the contaminated air is sampled with an impinger device (5, 8) distilled water is preferred as a collecting medium. The solution is filtered through a fine-textured filter paper such as Whatman KO.42, and the filtrate is collected in a 250-ml. beaker. A small amount of acetone should be used to free the impinger of particles of grease or oil adhering to the sides. The filter paper and the container are thoroughly washed with a fine stream of distilled water. The filtrate, which contains most of the soluble and slightly soluble chromium compounds, is neutralized, if acid, with 1 N sodium hydroxide solution. At this point the volume of the solution should be between I50 and 175 ml. One milliliter of 1 iV sodium hydroxide to excess is added, the solution is stirred, and 1 ml. of 30% hydrogen peroxide is added The beaker is covered with an elevated watch glass, and the solution is brought to a quick boil, boiled strongly for a few minutes, and then evaporated to near dryness. The beaker is removed from the hot plate, the cover and sides are washed down with freshly boiled distilled water, and the solution is taken just to dryness without visible boiling-preferably with an infrared heater. After cooling, 10.0 ml. of 1 N sodium hydroxide are added with a pipet and allowed to soak with intermittent stirring. If the solution is too cloudy it is centrifuged; otherwise some of the supernatant liquid is poured into a polarographic cell, a few drops of mercury are added, and the solution is bubbled for a t least 15 minutes with purified nitrogen or hydrogen and run polarographically between 0.5 and 1.2 volts us. the saturated calomel electrode. The half-wave potential rarely varies from -0.85 volt. Residues. The chromium compounds remaining in the filter papers are analyzed by ashing the paper in a platinum crucible, preferably using a muffle furnace, starting from room temperature and heating to 550 O C. After the paper is completely ashed, 100 to 200 mg. of fusion mixture are added and carefully and thoroughly mixed with the ash by means of an elongated glass rod drawn to a diameter of 1 to 2 mm. and fire-polished into a rounded point. The crucibles are covered and heated for 10 minutes between 825" and 850" C. After cooling, 10.0 ml. of l N sodium hydroxide are added with a pipet and allowed to soak for 0.5 hour with intermittent stirring. The supernatant liquid is centrifuged for 5 minutes, and some of the clear solution is added to a polarographic cell and run as above. When the collecting medium is an alkaline solution, it may be treated similarly, except that precautions should be taken to keep the final concentration of the alkaline solution near 1 N. No additional sodium hydroxide need be added, and the solution is brought to near dryness rather than just to dryness. Other strengths of alkalinity may be used, provided that the standard curves (or the IlkoviE equation constants, 4 ) are derived under similar conditions. The authors have on numerous occasions taken solutions containing 25 me. of sodium hydroxide almost to dryness and diluted with 25 ml. of water without causing too great a deviation from results obtained from a 1 N solution. The carbonates and silicates formed during the evaporation process do not noticeably interfere.
Diffusion Current Values (Microamperes)Obtained by Using Routine Laboratory Procedures
Method Serirs A Untreated 1 .\'"itOFI solutions Series B Treatedasfiltrates Serii..
Sampling time varied from 2 to 30 minutes within the plant and from 2 to 4 hours a t distances up to 1 mile from the plant. A number of bulk samples were obtained by means of a householdtype vacuum cleaner run continuously for periods up to 72 hours. Settled rafter dust samples were also collected. However, the usual sample was obtained i n 15 minutes a t a rate of 1 cubic foot per minute.
r
a n d treated as filtrates Series D Treatedasresidues
-_
0
_
5
_
~
10
Chromium T a k e n , Micrograms ~ 25 50 100 2ZO
300
780
1000
0.01 0.01
0.09 0.09
0.16 0.15
0.41 0.40
0.79 0.80
1.48 1.5-%
3.86 3.92
8.00
7.78
11.3 11.3
15.4
0.02 0.03
0.07 0.08
0.15 0.15
0.44 0.39
0.82 0.79
1.52 1.46
3.92 3.88
7.76 8.00
11.4 11.4
15.2 15.4
0.01 0.03
0.06 0.04
0.22 0.09
0.28 0.31
0.64 0.65
1.28 1.40
3.40 3.32
7.52
10.7
15.0 15.2
0.01 0.01
0.08 0.11
0.16 0.15
0.36 0.34
0.85 0.79
1.54 1.54
4.11 4.20
8.20 8.10
.. .
.., .., ...
15.0
16.2 16.2
If filter uauer is used as a collectine: medium (i)a n d it is desired to find the amount of soluble chromium compounds, the filter papers are macerated in a beaker using a small volume of water. The water is filtered through a finctextured filter paper by decantation. More water is added, stirred, and decanted. This process is repeated three to four times to obtain the greater part of the soluble compounds. The macerated paper is then transferred to the filter funnel and washed thoroughly a t least six times with a fine stream of distilled water. The filtrate and residue are analyzed as given above. Should only the total chromium be wanted, the paper may be ashed directly.
ANALYTICAL CHEMISTRY
474
Ii the samples are collected by means of an electrostatic precipitator ( 5 , 8 ) ,the dust in the tube is Rashed down with acetone or 95% alcohol, dried, picked up with water, and filtered if desired. 0: the dried dust may be fused directly and run for total chromium.
I
5a I N2
mi.
200 mm.
STANDARDIZ 4TION
Table I gives the diffusion currents obtained with four series of standards, each run by a different method. Series A shows the diffusion currents, zd, obtair:tJd from increasing concentrations of chromium, as the chromate ion, in 10.0 ml. of 1 N sodium hydroxide solution. These solutions were untreated except for bubbling with purified nitrogen before running polarographically. Using the m and t values obtained a t the halfwave potential in 1 N sodium hydroxide solution, the calculated value for id/C is found to be 8.35 microamperes per millimole per liter. The observed average value obtained from Series A, Table I, after corrections for the blank, is 8.01 * 0.16 microamperes per millimole per liter. Series B shows the id values obtained by running the given amounts of chromium through the procedure as given for solutions, except that the filtration step was eliminated. The chromate was added to a 250-ml. beaker and diluted with 150 to 175 ml. of distilled water, 1 ml. of 1 N sodium hydroxide and 1 ml. of 30% hydrogen peroxide were added, and the solution was boiled, dried, etc. In Series C the given amounts of chromium were added to an air-Mmpling filter paper (5.5-em. E. & D. S o . 623026) contained in a 150-ml. beaker. The paper was soaked for 0.5 hour and then filtered and run as outlined in the procedure. In Series D the given amounts of chromium were added to an airsampling filter paper. The paper was ashed and fused as outlined in the procedure for residues. The results are approximately, but not exactly, the same. dome difficulty is to be expected, and was encountered, in attempting to filter such small quantities of chromium from a comparatively large amount of paper. The results, however, are all of the same order of magnitude as the amount of chromium taken, and in the higher concentrations the losses are comparatively small. The limit of sensitivity under carefully controlled conditions is approximately 0.05 microgram per milliliter of final solution, Under routine conditions a more practical lower limit is of the order of 0.2 microgram per milliliter of final solution. The usual final volume was 10.0 ml., but whenever small amounts of chromium were expected the final solution was reduced to 5.0 ml. of 1 N sodium hydroxide or less. Figure 2 shows the increased wave height which may be obtained from 5 micrograms of chromium by reducing the volume of the final solution. I t must be recognized that the effects of other constituents in the sample are also increased. Nevertheless, amounts of chromium less than 1 microgram may be estimated in this manner.
Table 11.
150mm.
Table I1 gives comparative analyses of various dusts and the National Bureau of Standards sample of chrome refractory 103. The polarographic samples were weighed on an Ainsworth micro balance, mixed with fusion mixture, and run as outlined in the procedure for residues. Table I11 gives comparative analyses for some steel samples. The steels were dissolved in platinum cruci-
I
100mm.
I.
OLTAGE--
I
Figure 2. Relative Wave Heights Obtained from 5 Micrograms of Chromium in 2, 5, and 10 M1. of 1; N Sodium Hydroxide Solution Sensitivity 0.003
pa./mm.
Table 111. Polarographic Analyses of Steels Bureau of Standards Sample No. 32c
Wt. of Sample, Mg. 5.44 6.28 6.03
% Cr Found 0.64 0.60
.4v.
0.68 0.64 2.07 1.86 2.16 2.03 18.3 17.6 19.2 18.4
Av.
0.09 0.11 0.05 0.08
Av. 115
4.26 3.66 3.78
121a
1.02 1.05 1.60
Av.
Comparison of Analyses of Dusts and Chrome Refractory
Polarographic Macro NazOz Fusion Wt. of SamKMnOd-FeSOh Titratio;, Sample ple, mg. % Cr % Cr Air-borne dust 1 0.83 17.6 17.46 Air-borne dust 2 0.84 14.6 14.98 Rafter dust 1 0.81 15.6 15.50 Rafter dust 2 0.97 16.7 16.28 Bureau of Standards 103 0.92 25.5 2 5 . 30a 1.39 25.9 25. 30a 2.37 25.7 25.30" a Certificate value converted to per cent chromium.
I
l52b
b
9.61 9.25 9.78
% ' Cr PresentD 0.654
2.17 2.17
18.69
0.05
Certificate values Tin bearing.
bles with 1 t o 1 hydrochloric acid plus a drop of concentrated sulfuric acid, brought to fumes of sulfur trioxide, oxidized with nitric acid, dried, and fused with magnesium oxide-sodium carbonate (4 to 1) fusion mixture. EFFECT OF SODIUM HYDROXIDE CONCENTRATION
Thanheiser and Willems (IO) studied the effect of sodium hydroxide normality on the height of the chromium diffusion current wave in 0.5, 1, 2, 4,and 6 N sodium hydroxide solutions. It was desirable to know the effect of a slight variation in con-
V O L U M E 22, NO. 3, M A R C H 1 9 5 0
475
centration from the usual 1S sodium hydroxide solution. Figure 3 shows the results obtained with 50 micrograms of chromium in 10.0 ml. of alkaline solution. The general trend agrees well with Thanheiser and Willems' results. It shows that in the vicinity of 1 N sodium hydroxide, a slight variation of normality has little or no effect on the wave height. Higher well formed waves are obtainable in 0.1 to 0.4 S solutions, but the change in height of the wave with small variations in normality is much greater. It is possible that better Raves may be obtained in buffered solutions. However, a t low normalities the polarograms of chromate solutions show an additional wave a t 0.3 volt us. the saturated calomel electrode. At normalities greater than 0.2 N this wave tends to disappear. INTERFERENCES
The effects of increasing amounts of various substances upon the recovery of 50 micrograms of chromium are shown in Table
__
Table I \ .
Chromium Recovery
(AIicrogram of chromium rerovered from 50 m i c r w r a m added) 1 50 10 100 Substances Added 0 mg. mg. mg. mg. 45 47 EN08 48 49 57
HIP04 HCl
HClOi NaiVOa NasSOl NasPO4
NaCl NazCOs BIZ C d (as CdSOb) C o [ae Co(NOa)z]
Cu (as CuSOd
Fe [as Fe(NH4)r(SOl)zI M n [as Mn(NOa)i] Mo (as MOO,
+ NHdOH)
Xi (as NiSOdj Pb ( a s PbSOd
5
51
53
51 50 48 4i 52 48 53
50 46 45 48 48 50 51 53 200 200
54 51
..
194
194 6 .i 85
51 49
47 49 52 48 49 33 32 45 41 50 50 51 48 53 53 48 53 53 53 50 51
45 53 57 53 52 39 40 41 23 52 48 47 48 56 57
47 46 51 48 44 58 35 35 52 48 45 44 1200 1150 950 950 72 102
6
b
3
h
51
53 53 47 1035 552 40 36 25 37 48 49 47 49 52 55 52
48 55 58 49 53
253
::
4
14 14 43 47 2 2
'I
50 43 h
2800 225;
-~
________._
CRUCIBLES
Practically all the fusions were carried out in platinum crucibles. There is some loss of platinum, but the cost of the platinum lost is small in comparison to the ultimate cost of other types of crucibles. Satisfactory results may be obtained by fusing the chrome ore in porcelain crucibles with sodium peroxide. Care has to be taken to let the melt solidify on the sides of the crucibles, lest the crucibles be cracked. The magnesium oxidesodium carbonate fusion mixture cannot satisfactorily be used with the porcelain crucibles; the melt vitrifies upon solidifying and becomes very difficult t o dissolve. Unsatisfactory results were also obtained upon using nickel crucibles with the magnesium oxide-sodium carbonate fusion mixture; the chromate pitted the sides and bottom of the crucibles and gave low, erratic results. Electroplating the insides of the nickel crucibles with new nickel surfaces did not help. Sodium carbonate-sodium nitrate fusion mixtures worked well for chromium oxides and compounds, but could not satisfactorily fuse chrome ore. Iron crucibles were not tried for small samples because of the possible formation of large amounts of ferric hydroxides. SUMMARY
Interference due t o adsorption, absorption, a n d l o r entlapment Interference from maxima or high residual currents.
__ 1.0
%
49 48 54 57 47 49 52 50 49 49 57 62 51 50 51
j0
P b [as Pb(N01)zl
Zn as ZnSOdj
47 54 50 48 47 50 49 35 35
47 49 50 47 53 51 49 50 54 4i 47 49 50 53 51 48 49 50 49 53 54 48 52 53 50 53
&SO4
IV. The chromium and the foreign substance were niixed in 250ml. beakers, diluted with 150 to 175 ml. of distilled water, and run through the procedure for filtrates. The acids were either dried or fumed for 0.5 hour before diluting with water. Sulfuric and nitric acids haxe little effect, Phosphoric acid has no effect in small quantities, but in larger quantities it renders the waves unmeasurable. Hydrochloric acid shows a peculiar tendency to cause low results when present in amounts ranging from 10 to 100 mg. When concentrations of 500 to 1000 mg. of hydrochloric acid are used the effects are absent. Check analyses give the same results. Perchloric acid gives low and erratic results a t all times. The authors have not been able to use this acid successfully. Chapman et al. ( 2 )reported similar effects in the treatment of chromium compounds with perchloric and hydrofluoric acid solutions. The sodium salts of the common acids do not seem to interfere. Of the metals studied, lead, zinc, and cadmium interfere the most. The lead wave comes just before the chromium wave, and in the procedure, the lead and chromium waves merge, giving high results for chromium. The zinc, cadmium, and copper curves interfere to the extent that their maxima or high residual currents, due to the comparatively large amounts being used, tend to merge or flow into the'chrornium wave. Cobalt, iron, manganese, and nickel interfere only to the extent that their bulky precipitates adsorb or entrap the chromium ions. As a rule, if the precipitate is not too bulky, there is no interference.
-
A polarographic method for the determination of chromium in dusts and mists is described. It is essentially a micro or semi-
I
I
u)
W
a w a I 4 0
a
0 I
0.6 0 NoOH
Figure 3.
1.0 NORMALITY
2.0
Effect of Sodium Hydroxide Concentration on Diffusion Current SO micrograms of chromium in 10.0 ml. of alkaline aolution
micromethod and consists of two Darts: one for solutions and one for residues. The solutions are oxidized with hydrogen peroxide in alkaline solution, dried, and run polarographically in 10.0 ml. of 1 N sodium hydroxide solution. The residues are fused with a 4 to 1 mixture of magnesium oxide and sodium carbonate, soaked with 10.0 ml. of 1 N sodium hydroxide solution, centrifuged, and run polarographically. The lower limit of sensitivity is 0.2 microgram of chromium per milliliter of final solution. Theoretical and observed diffusion current values agree within expected limits. Results obtained with the proposed method are compared to those
416
ANALYTICAL CHEMISTRY
obtained by standard macromethods. A number of Sational Bureau of Standards samples were also analyzed. Results fall within an average deviation of *4% of the amount present. A number of factors influencing the accuracy of the determination have been studied and their effects tabulated. More than 1000 routine analyses have been made with t,hemethod described. LITERATURE CITED
(1) Buckley, F., and Taylor, J. K., T r a n s . Electrochem. Soc., 87, 463 (1945). (2) Chapman. F. W., Jr., Marvin, G. G., and Tyree, S. Y.. J r . . ANAL.CHEY.,21, 700 (1949). (3) Ege, J. F., Jr., and Silverman, L., Znd. H y g . Quart., 8 , 12 (1947). (4) IlkoviC, D., Collection Cz~choslov.Chem. C o m m u n . , 6 , 498 (1934). (5) Jacobs, M , B., “.lnalytical Chemistry of Industrial Poisons,
Hazards and Solvents,” pp. 18-89, New York, Interscience Publishers, 1941. ( 6 ) Kolthoff, I. M., and Lingane, J. J., “Polarography,” pp. 215-54, New York, Interscience Publishers, 1946. ( i )Lingane. J. J., and Kolthoff. I. M., J. Am. Chem. Soc., 62,852 (1940). (8) Silverman, L., “Industrial Air Sampling and Analysis,” Chernistry and Toxicology Series, Pittsburgh, Industrial Hygiene Foundation, Bull. 1, p. 8 (November 1947). (9) Stackelberg, M. v., Klinger, P., Koch, W., and Krath, E., A r c h . Eisenhiittenw., 13,249 (1939). (10) Thanheiser, G., and Willems, J., Mitt. Kaiser-Wilhelm Inst. Eisenforsch., Dusseldorf, 21,65 (1939). RECEIVED August 29, 1949. This project was supported by a cancer control g r a n t from the National Cancer Institute, U. 5. Public Health Servire CS-887, Thomas F. hlanciiso. hl.D., project director.
Microdetermination of Knock Characteristics of Motor Fuels f
JULIAN ALEXANDER, JR., A N D CARL PFEIFFEH lloudry Process Corporation, Marcus Hook, Pa.
A method is described whereby motor and research octane numbers can be determined on 20-ml. samples of fuel with an accuracy approaching that of the standard knock engine test procedures. This method employs the customary knock engine and involves two alterations in design and one change in procedure: The amount of waste fuel in the fuel inlet system is reduced from 60 to 1 ml. by the use of a specially designed glass capillary inlet system. The running time necessary for the standard knock
T
lIE standard methods for the determination of the knock characteristics of motor fuels which have been developed during the past decade by the petroleum industry and have recently been standardized by the American Society for Testing Materials ( 1 ) have proved of considerable value in studying the products of petroleum research and in controlling the manufacture of motor gasoline. The primary need behind the development of these methods was for a plant control test, with the result that very little thought was given to the size of the sample required for an octane determination. Samples of a t least 300 ml. are generally considered necessary for a normal determination. During recent years, in which research on motor fuels has expanded considerably, this minimum requirement for each octane determination has placed a considerable strain on the laboratory and pilot plant equipment used to prepare experimental batches of gasoline. It has often led to many unnecessary hours being spent repeating an experiment several times in order to build up a sufficient supply of product for the various engine tests which are needed. The desire of chemists and engineers to overcome this difficulty has led to the unofficial use of two types of modifications of the standard engine test methods. The first of these has involved blending the unknown sample Kith a fuel of known octane value, determining the octane number of the blend, and then correcting for the contribution of the diluent by means of empirical convep sion tables or charts. The weakness of such procedures lies in the fact that different blending charts must be used for different types of fuels. This leads to uncertain results when unknown samples are being evaluated. The second type of modification has involved reducing the holdup of sample in the carburetor and float bowl of the test engine by the use of a small-scale commercially available carburetor, or a specially built substitute fuel container. A number of such modifications have been tested by various engine groups ( 2 , 3 )and
meter to reach equilibrium is eliminated with negligible loss in accuracy by its replacement with an instantaneous recording knock meter. The running time necessary for the adjustment of the compression ratio and the fuel-air ratio is reduced by the use of several manipulative shortcuts including a continuously variable fuel-air ratio. The standard deviation for 42 research octane determinations on different fuels was 1.42, and that for 38 motor octane determinations was 1.15.
information has been circulated throughout the industry, although no published papers are available, according to the best information of the authors. The use of such modifications as those just described reduces the minimum sample size to 80 to 100 ml. The Houdry Laboratory has employed one of these modifications for over a year with considerable success, but has felt that for research purposes the fuel requirement was still greater than desired. It therefore undertook the study described in this paper, and has succeeded in developing a method whereby octane num-
50-ML
PIPET
3
STOPCOCK
2
J DRAIN
Figure 1. Modified Fuel Inlet System for Octane Microdetermination
