V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8 price paid for greater protection is generally a n increase in the length of the interval said to cover the true value. Thus, in the present instance, lower bounds on the true amount of added cobalt recovered a t the 99% confidence level are given by 3 2.567 Sz. For each of the amounts of cobalt added these values are less than the lower bounds given above. CONCLUSIONS
A method for the determination of cobalt in atmospheric dust samples has been developed and applied successfully to a large number of environmental samples secured in the cemented tungsten carbide industry. During this investigation there vas developed a means of “opening” the carbide dust, removal of the carbon, and solution of the metallic components as a n initial stage of the analysis. The nitroso R salt method has been examined for its accuacy in estimating the cobalt content of atmospheric dust samples. Recoveiies of known amounts of cobalt added to a qeries of atmospheric dust samples reveal that the perccntage of added cobalt recovered tends to increase with increasing aniounts of added cobalt, a lower limit on the percentage recovered approaching 95 as the amount of added cobalt approaches 0.1 ing. Thus, if the amount of cobalt in an atniojpheric duqt sample is of the order of magnitude of 0.1 nig. or greater, it may be concluded that a t least QZm0 of the cobalt is recovered by the method described. LITERATURE CITED
(1) Bellucci, I., Gaze. chim. ital., 49, 294 (1919). (2) Black, I. A., Soil Sci., 51, 387 (1941). (3) Cartledge, G. H., and Nichols, P. M., IND.ENG.CHEM.,ANAL. ED., 13, 20 (1941).
1241 Clarke, W. W., I r o n Age, 150, 45 (Dec. 3, 1942). Cronheim, G., IND.ENG.CHEM.,ANAL.ED., 14, 445 (1942). DeGray, R. J., and Rittershausen, E. P., Ibid., 14, 858 (1942). Ellis, G. H., and Thompson, J. F., Ibid., 17, 254 (1945). Fisher, R. A., “Statistical Methods for Research Workers,” 8th ed., p. 167, New York, G. E. Stechert & Co., 1941. Haywood, F. W., and Wood, A. A. R., J . Soc. Chem. Ind., 62,37 (1943). Ilinski, M., and Knorre, G. van, Ber., 18, 699 (1885). Kidson, E. B., and Askew, H. O., N u Zealand J . Sci. Tech., 21B, 178 (1940). Kidson, E. B., Askew, H. O., and Dixon, J. K., Ibid., 18, 601 (1936). King, .4., “Inorganic Preparations,” p. 103, New York, D. Van Nostrand Co., 1936. Klooster, H. S. van, J . Am. Chem. Soc., 43, 746 (1921). McNaught, K. J., Analyst, 67, 97 (1942). McNaupht, K. J., N e w Zealand J . Sci. Tech., 18, 655 (1937). I b i d . . 26 14 (1938). Moss, M. L., and Mellon, M. G., IXD. ENG.CHEY.,A N ~ LED., . 15, 71 (1943). Overholser, L. G., and Yoe, J. H., I b i d . , 15, 310 (1943). PutschB, H. M., and Malooly, IT’. F., ASAL. CHEW.,19, 236 (1947). Sandell, E. B., “Colorimetric Determination of Traces of Metals.” p. 206, New York, Interscience Publishers, 1944. Scott, W.IT.,“Standard Methods of Chemical Analysis,” 5th ed., N.H. Furman, ed., p. 315, New York, D. Van Nostrand Co., 19.39. Seifert, H. E., Kecnan, 1%. G., and Fairhall, L. T., P u b . HeaZth Rapts., 60, 441 (1945). Snell, F. D., and Snell, C. T . , “Colorimetric Methods of Analysis,” 1701.I. u. 324, New York, D. Van Nostrand Co., 1936. (25) Stare, F. J., and Elvehjem, C. A , J . Bid. Chem., 99, 473 (1933). (26) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” 1701. 11, 9th ed., p. 199, New York, John Wiley & Sons, 1945. (27) Young, R. S.,Pinkney, E. T., and Dick, R., IND.ENG.CHEM., -4h-a~.ED., 18, 474 (1946). RECEIVED June 30. 1948.
Semi-Self-Filling Micropycnometers Drainage of Micropipets HERBERT H. ANDERSON Harvard University, Cambridge, M a s s . A new- self-adjusting, or semi-self-filling, Ostwald type of micropycnometer has increased the accuracy fivefold, while capillary action completes the filling; a 1-ml. model has a reproducibility of 1 in 40,000. Satisfactory micropipets from 3.7 to 0.7 ml. in size conformed to drainage values derived from the equation (Robs. - R i ) ( T ) = k; previous investigators have found a similar relationship to apply to larger equipment. Studies on still smaller equipment include those on a self-adjusting, self-filling 0.002589 j=0.000006-ml.micropipet. A brief comparison of these new results and those of certain other investigators is presented.
D
U R I S G sustained inveetigations on inorganic liquids, the author has often found it highly advantageous to be able to manipulate rather moderate volumes of liquids ( 1 ) . I n the present article a reproducibility of 1 part in 10,000 v a s attained in a 0.25-ml. micropycnometer of a new semi-self-filling design which may be considered an improvement on the Ostwald pycnometer ( 1 4 ) . MICROPYCNOMETERS
Pycnometers of 1-ml. size show a considerable variation in both design and accuracy; Ostwald-Luther (14, p. 241) Presents a diagram of a 1-ml. pycnometer consisting of a tiny round-bot-
tomed flask with a ground stopper bearing a capillary hole. Such a model is, in the opinion of the present author, unsuited for liquids such as phosphorus trichloride, and unsuited for accuracy better than 1 to 1000 on ordinary organic liquids. It seems advisable to avoid even a single stopcock, such as is present in a completely linear model ( I C ) , as well as an excessive length of small connecting tubing ( 7 ) . SELF-ADJUSTING OSTWALD PYCNOMETER
Bulb F (Figure l), of appropriate size, is blown from a 300mm. length of Pyrex capillary tubing of 0.5-mm. inside dianleter and 5-mm. outside diameter; the taper must be smooth, with a
ANALYTICAL CHEMISTRY
1242 conical flare in region G. Next, bend D is made, and then bends C and H are made in such manner that ABC and HZK lie in a straight line. Thereafter, a narrow capillary, AB-about 25 t o 30 mm. long, 1.5 to 2.0 mm. in outside diameter, and 0.5 mm. in inside diameter-is prepared by blowing a slight bulb in the 0.5-mm. capillary and then drawing out the region; slight constriction of a portion of A B gives the tip, A, approximately 0.15 mm. in inside diameter and 0.8 mm. in outside diameter, which is then fire-polished. Thereupon, capillary Z J is made by blowing a slight bulb in the original 0.5-mm. tubing and drawing out; fire-polished tip J should be not much larger than 0.15 mm. in inside diameter and 0.5 t o 0.7 mm. in outside diameter. The whole capillary line, G H J must nowhere exceed 0.6 mm. in inside diameter, in order to maintain automatic filling; tip A must be smaller than tip J in order to prevent leakage a t A . Kext, a guard serving also as a handle is attached by means of a ring seal a t Z, with slight tapering before cutting off and fire-polishing a t K , the open end; Z should never cool until after complete anneal!ng: Pyrex tubing 5.5 mm. in outside diameter and 4 mm. in inside diameter is the starting material for the handle.
C H
I
two calibration marks 10.0 mm. apart (Figure 2) has been designed. The author fills to an intermediate point, x i t h visual estimation of the extent of filling to the closest millimeter, the limit set by parallax and eyestrain. Tip A is of smaller internal diameter than the unaltered portion of the capillary; careful annealing is done. An error of 1 mm. in reading betxeen the two marks involves an error of 0.0004 ml. in volume with capillary 0.7 mm. in internal diameter; an error of only 0.0002 ml. in the usual type with 0.5-mm. tubing; an error of only 0.0001 ml. was achieved with 0.38-mm. capillary, despite difficulty in construction and operation. A 1-ml. model had a reproducibility of 1 bo 6000. There are no serious limitations on the Ostwald micromodel with calibration marks, save for the accuracy, which is less than that of the self-adjusting type. I n the self-adjusting type the overflow is a serious problem if the filling is made a t a temperature less than that of the room, whereas the less accurate type with calibration marks has a tiny bulb to serve as a reservoir. MICROPIPETS
*TK
Figure 1
Operation. Using a syringe with an adapter of gum rubber tuhing, the micropycnometer is filled a t will up t o the narrow capillary above point G; thereupon, capillary action fills the relatively narrow diameter completely to the end of J . It is advisable t o use a 1OX eyepiece to check the completeness of filling a t A and J, and to confirm absence of microbubbles of air along GHJ. h platinum or bare copper wire is convenient for suspending the system from points just below the line ABCH l K during weighings, in which a counterpoise is used. After filling, A is wiped from the direction B to '4, using tissue paper; a cap (or ground joint) a t A is needed nhen the liquid has an appreciable vapor pressure a t room temperature. It is always advisable to have HZK and A B as nearly level as possible during the filling of capillary GZJ. Precision. The balmce room, protected by a double-door system, and controlled by a thermostat, varied not more than 0.4" C. in temperature; the balance was used in weighing to 0.001 mg., always with a counterpoise. Temperatures of water which had stood covered in the balance room for a t least 5 hours wivere measured to 0.01 O with the 1.59516-m1,. and the 0.49236-m1. models, and to 0.1 with the 0.24613-m1. size. The assumption that the temperature of the water represented the temperature of the system as filled cannot introduce much error in the values for these pycnometers; all volumes were corrected to 25.00" C. I n the two better models the reproducibilities mere favorable: 0.24616, 0.24610, 0.2611, and 0.24613 ml., average 0.24613 * 0.00002 nil.; 1.59520, 1.59513, and 1.59514 ml., average 1.59516 * 0.00003 ml. These give respective reproducibilities of 1 t o 10,000 and 1to 50,000; as both have anuncertaint,yof 0.000025 f 0.000005 ml., it seems justifiable to estimate the reproducibility of a 1-ml. modcl as 1.000000 =t0.000025 ml., or 1 to 40,000.
It is believed that this self-adjusting Ostwald pycnometer will be satisfactory up to 10 ml. in size; it is unsatisfactory for a 50-ml. size because of air pressure difficulties in filling. A model without a ring seal or ground joint at Z (Figure 1) contained 0.49236 * 0.00008 ml. (0.49236, 0.49246, and 0.49226 ml.), and was less satisfactory than the above two pycnometers. This self-adjusting Ostwald pycnometer has not been used on liquids like silicon tetrachloride because of corrosion. OSTWALD PYCNOMETER WITH CALIBRATION MARKS
To meet the need for a pycnometer for work on phosphorus chlorodiisocyanate and other corrosive liquids (I), a type with
Pipets of 1-nil. size show several different designs; the construction of the customary transfer pipet of 5- to 100-Inl. capacity is probably least suitable for the measuring of small volumes. The shape of a cylindrical tube without bulb, which delivers contents aithin 40 seconds, has been advocated (4). It is generally conceded that any pipet that uses the drainage betneen two marks is inaccurate. Ostnald-Luther (Id) gives an accuracy of 1 in 2500 on the old, long-form, macrodesign of 1-in1 pipet, while the present author has attained the value of 1 in 10,000 with the short pipet shown in Figure 3, if the time of drainage is made 30 seconds. The pipet has two bulbs, one serving as a safety reservoir. I n contrast to this, Alika (12, p. 68) and Benedetti-Pichler (2, p. 240) give models with only one bulb, both called type a. hlika (12) in type c and Benedetti-Pichler in type d present washout micropipets of a design that the present author has never seen in use; R hen one is using solutions M hieh are necessarily dilute because of limited solubility it is advisable to avoid the dilution that would arise from use of a washout pipet instead of one which drained suitably.
Figure 2
I n the truly self-filling class, the pipet shown in Figure 4 is a more elegant version of a small type b of Benedetti-Pichler (a) and may also be considered an improvement on Clemo and RIcQuillen's micropycnometer (3) in some nays, as they had a tip of too large a n external diameter for a given internal diametera ratio of 10 to 1. Constrictions of larger tubing to give selffilling capillary are mentioned by Mika (12) and by Sicloux ( I S ) in models that are less desirable than that of the present author in the 5-microliter range a t least. It is possible t o make a pipet of 1-ml. size with the self-adjusting capillary (zone CDEF, Figure 4), with a bulb a t BC, although the pipet is unsatisfactory. MICROPIPET WITH CALIBRATION MARK AND SAFETY BULB
A syringe is employed for controlling the flow of liquid or solution; the tip is allowed to touch the side of the container into which the contents are being delivered. As it is advisable to have the rate of drainage as nearly constant as possible, one handles the controlling syringe with care, estimating the time of drainage
V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8 K h e n the pipet (Figure 3) is allowed t o drain of its own accord in vertical position, the tip is finally blown out against the side of the container such as a beaker.
’ MARK 100 MM. Figure 3
/
This type is self-draining (although a syringe is nearly always used) but not self-filling; the better range of size is from 3.7 to 0.5 ml., but it is useful-although less accurate-from 0.5 to 0.020 ml. ”or good reproducibility t!ic pipe1 should have a narroiv tip. In a b:i::aule 0.75-1n1. model the tip was 0 2 mm. in inside diameter, preceded by 20 mm. of 0.6-mm. capillary, whereas the undesirable 1-ml. models had tips 0.5 in inside diameter preceded by a 20-nini. length 1.8 mm. in inside diameter. Thus drainage depends both on the internal diameter of the tip and on the internal diameter of the capillary leading to the tip. Coninicrcial 0.500- to 0.020-ml. micropipets appeared to be variable in performance, and this general type becomes inaccurate for still smaller sizes.
1243 in 10,000 would remain in the pipet after a delivery of 36 seconds time, or that 3.0 mg. of water would remain in a 2.0-ml. size. These results are in very good agreement with the equation (Robs - R1)( T ) = k , in which Robs is the observed fraction of residual liquid, Ri is a constant factor related t o the amount of liquid needed to wet the glass, T is the time of drainage in seconds, and k is a constant. As determined through the principle of least squares Ri was 0.0011 and k was 0.0152 * 0.0003 seconds; 0.0011 gram of water would remain in a 1-ml. pipet in an infinite time of drainage. If an accuracy of 1 in 1000 is desired in using large micropipets, of 4- to 0.5-ml. capacity, a slow drainage of approximately 10 seconds, but definitely in the range from 7 to 15 seconds, will be satisfactory. Micropipets between 0.5 and 0.0020 ml., less accurate, should be used with caution. Still smaller sizes are accurate if made self-adjusting and if rinsed tn-ice. .A pipet of this smallest size contained 0.002589 * 0.000006 ml. when filled with water, or 0.002579 * 0.000002 ml. when filled with mercury; the wetting of the tip by water and a slight lack of filling by mercury undoubtedly are factors in the difference. Numerous past standardizations have established the accuracy of a 10microliter micropipet as 1 in 1000, or possibly better.
I
4
1
F
B C
D
E
Figure 4 SELF-ADJUSTING MICROPIPET
.In idealized model of 5.00-microliter (0.00500-ml.) capacity, calibrated by weighing either ~ a t e or r mercury, is 110 nim. long; the self-filling capillary length is 65 mm., divided as follolvs: length A B , 30 mm., average outside diameter 1.5 mm., internal diameter 0.2 mm., and outside diameter only 0.5 mni. a t the tip: length BC, 10 mm., 5.5 nim. in outside diameter and 0.6 mm. in inside diameter; length CD, all vithin the ring seal, 25 mni , average outside diameter 1.5 mm., inside diameter 0.2 t o 0.3 mni. Length CEE’, 70 mm. total, serves as a handle and as an adapter for cleaning nhen in a n inverted position, using suction; a ring seal is used at C (Figure 4),and a moderate taper a t EF fits into a small 60 a filter funnel. This ielf-adjusting micropipet is best in the range 1 to 10 microliters; the capacity may even be reduced to 0.5 microliter by applying a similar method of construction t o a narron-er capillary. On the other hand, models a‘ large as 15 microliters are self-filling if filled a t an angle of 30” t o 45’ from the horizontal. PRECISION OF MICROPIPETS
This investigation studies the relationship of time of drainage and completeness of drainage, as based on neighing the individual pipet, using a comparable counterpoise.
A 2.O-ni1. pipet left the following amounts of water undelivered after various times of drainage: self-drainage, 32 seconds, 0.13 * 0.OlC;: 11 seconds (steady flow using syringe), 0.22 * 0 . 0 1 7 ; 6 seconds, 0.38 * 0.01%; 2.5 seconds, 0.67 * 0.03y0. Micropipets of 0.3-nil. capacity and smaller were handled similarly, with weighings on a special microbalance to 0.001 mg.; a bpecial0.200-nil. model left the following less satisfactory amounts undelivered: self-drainage, 10 seconds, 0.53%; 3.5 seconds, 0.90cc; 1.5 seconds, 1.67y0. d 0.0025-ml. model, probably typical of all self-filling (self-adjusting) micropipets, left 2’3 undelivered after a brief forced drainage. Sineteen experimental points taken on five pipets, 3.7, 3.0, 2.0, 0.9, and 0.75 nil. in size, n-ere plotted on millimeter graph paper (not duplicated herein) mith the follo\\ing values lying on the curve which best represented the points as a whole: 0.0015 a t 36 seconds, 0.0018 a t 20 seconds, 0.0021 a t 15 seconds, 0.0026 a t 10 seconds, 0.0042 a t 5 seconds, 0.0058 a t 3 seconds, and 0.016 calculated for 1 second. Here 0.0015 means that 15 parts
Other investigators have studied the drainage problem on pipets of the macrodesign larger than 2 ml., and with burets. Mika (12, p. 32) wisely observes that a long period of drainage of a microburet is better than a short time of drainage followed by a long time of waiting before reading the meniscus. Others have studied the connection between liquid drainage and times up to 60 minutes in burets (5, 10, 11, 15). For pipets and viscometers some important related information is available (6, 8, 9). The present author knows of no experimental evidence coinciding exactly with that presented herein; special evidence of relationship between time of drainage and completeness of drainage of larger equipment was found by Jones and eo-workers (8,9). LITERATURE CITED
Anderson, H. H . , J . Am. Chem. Soc., 67,2176 (1945). h typical paper. Benedetti-Pichler, A . A , , “Microtechnique of Inorganic Analysis,” pp. 238-40, 257, Kew York, John Wiley & Sons, 1942. Clemo, G. R., and McQuillen, A,, J . Chem. Soc., 1935, 1220. Conwav, E.J., “Microdiffusion Analysis and Tolumetric Error,” pp. 28-32, 270 f f , London, Crosby Lockwood & Son, 1947., Freudenberg, K., and Weber, E., 2 . angeic. Chem., 38, 280-5 (1928). Geilmann, IT., and Holtje, It., 2 . anorg. Chem., 152,69 (1926). ED., 9, 479 (1936). Hennion, G. F., ISD.ESG. CHEM.,.INAI.. Jones, G., and Ferrell, E., J . Chem. Soc., 1939,325. Jones, G., and Stauffer, R. E., J . Am. Chem. SOC.,62, 335 (1940). dlso references therein. Lindner, J., and Haslwanter, F., Z.angeu. Chem., 42, 821-5 (1929). Loscalso, -1. G., and Benedetti-Pichler. -4. d.,ISD. EXG.CHEM.. ANAL.ED.,17,189 (1946). Mika, J., “Die exacten Methoden der 5Zikrornassanal?se,” pp. 32, 68, F. Enke, Stuttgart, 1939. Nicloux, 31.. Bull. S O C . chim. biol., 7 , 750-2 (1925). OJtwald-Luther, “Hand- und Hilfsbuch sur iiusfiihrung physikochemischer Messungen,” pp. 225-41, Leipsig, C. Drucker, 1931. Rank, V., 3likrochemie,21,231 (1937). Yuster, S. T.. and Keyerson, L. H . , ISD. ENG.CHEM..AKAL. ED.,8, 61 (1936). RECEIVED January 2 , 1948.
