V O L U M E 26, NO. 6, J U N E 1 9 5 4
1099
adjustable to the demands of the bridge, high internal resistance which permits high resistances to be measured, relatively simple construction, and precise null indication a t relatively high minimum signal a t bridge balance. The voltmeter must be reasonably sensitive to achieve good precision. With a meter that has a full scale sensitivity of 0.01 volt, it has been found possible to compare resistances from about 100 to greater than 106 ohms with a precision within 0.1% with a Leeds & Northrup 7651 potentiometer a t 1000 cycles. When resistances of 5 X lo4 ohms or greater are compared a Wagner ground should be used to reduce null reading and increase precision. A kit-type vacuumtube voltmeter has given excellent service without any changes in circuitry.
-
TO VACUUM TWE VOLTMETER
few changes for conductivity work. Most of these devices cannot be directly connected to a low-resistance slide-wire. Much better operation will be achieved if a small, adjustable “line-tovoice coil” radio transformer is used to match the signal source and bridge. The transformer should be adjusted for maximum bridge voltage. If a low-resistance slide-wire is used and the signal generator has much less than 0.02-watt output, i t may be necessary to add a power-output stage as described by Sulzer [Electronics,25, 95 (January 1952)] ahd shown in Figure 2. This may replace the existing output circuit if 25 ma. a t 200 to 300 volts and 6.3 volts at 0.45 ampere are available. Instrunients of the type described have been found useful in laboratory work [Phillips, J. P., J. Chem. Educ., 30, 70 (1953)l. Assembly is facilitated by the comprehensive directions t h a t accompany the instruments and by the fact that both are relatively simple. The accuracy possible with their use depends on the accuracy of other bridge components. These two instruments, however, are useful with apparatus capable of very precise determinations. Thus, accuracy is easily obtainable a t reasonable cost.
Use of Surfaceactive Agents in Breaking Emulsions Formed in Vegetable Extractions. C. L. Dunn and R. H. Earle, Jr., Hercules Experiment Station, Hercules Powder Co., Wilmington, Del.
Figure 1
insecticide residues in various foodstuffs, the Istatematerial to be analyzed is frequently extracted in a finelydivided with a suitable solvent. An effective device for this opera-
The less sensitive multipurpose vacuum-tube voltmeters can be used if a conventional amplifier such as is shown in Figure 1 is also employed. This will allow the sensitivity of the voltmeter to be increased by a factor of 100. This amplifier requires a meter of lo5 ohms or higher internal resistance as an indicator. Because the vacuum-tube voltmeter responds to 60-cycle as well as 1000-cycle voltages, relatively high precision can be obtained if the bridge is energized by one half of a small, centertapped, 2.5-volt, radio filament transformer. A series of conductance determinations made a t this frequency checked those made a t 1000 cycles to within the limits of precision of the determination, if an alternating current vacuum-tube voltmeter was
tion is a Waring Blendor or similar apparatus. Many materials when so treated form very stable emulsions, and solvent extract recoveries are poor. Large quantities of salts, such as sodium sulfate, are sometimes effective in breaking these emulsions, b u t frequently the emulsions are resistant to such treatment. It has been found in this laboratory that surfactants can be used in some cases to break the emulsion. Preliminary information is given here for the use of other workers faced with similar problems. The use of surfactants in breaking oil-water emulsions was reported by Lawrence and Killner [J. Inst. Petroleum, 34, 821-56 (1948)l. In light of their findings the following surfactants were evaluated for demulsification properties:
*=do Mzd
x DETERMINING
UUS
UL llESlSloR5
K)%tNESS MARKED
Used.
Anionic. Allied Chemical’s Kacconol N.R.S.F., Monsanto’s Santomerse 3, Xinol Laboratories’ Toximul 300, and Atlantic Refining’s Ultrawet K. Cationic. -4rmour’s Armac HT, Arquad 14, and Arquad 2HT. Nonionic. Hercules’ Synthetics C-71. The chemical compositions of the surfactants used are not readily available. In general, the anionic detergents are alkyl aryl sulfonates; the cationics are quaternary ammonium salts; and the nonionic is an alkaryl-alkylene oxide adduct.
M-18
I’IO’, WMS 6 3 V HEATER NOT SHOWN
Figure 2
If a higher or variable bridge energizing frequency is required, an audio-frequency signal generator can be used to energize the bridge. These generators usually have low harmonic content in their outputs; thus the bridge is easier to balance because of loxer residual null voltage. They are also rapidly variable over a wide frequency range, if this is required. These instruments are available unassembled a t reasonable cost, and umally require
In most cases separation was obtained to a greater or lesser degree. Ultrawet K with sodium sulfate was most satisfactory under the conditions tried. Typical data are given in Table I. Further investigation might have revealed a more efficient demulsification system, but needs were met by this system. Separation was obtained in 5 to 10 minutes with a 40 to 75% recovery of extraction solvent. Longer standing did not materially increase the recovery. Centrifugal separation of untreated emulsions gave low and variable recoveries in addition to being time-consuming and laborious. Extraction and Demulsification Technique. The emulsionbreaking technique was applied to three different foods which formed stable emulsions during extraction: potatoes, canned field peas, and peeled oranges. The samples were pulped in an appropriate solvent using a stainless steel stirrer fitted with sharp vanes and driven by a 1/30-hp. Eclipse Model ARL air stirrer. With the stirrer on,
ANALYTICAL CHEMISTRY
11m Table I. Dernulsifioation Usinn- S u r f a c t a n t s U1trsrvet (25%
Sample, Grams
nfat&i
Solvent
canne. field peas
500
Benzene
oran ges
1500
Heptane
Peeled
Potatoe~
Solvent. N a r S 0 ~ . Solids). hii. rems hii. 1
1 2
Benrene.
1000
Extract Recovered. Ml.
anhydrous sodium sulfate was added and thoroughly disnersed. The stirrer was removed and the Ultrawet solution gently blended in with a spatula. (Vigorous Stirring resulted in formation Of an unbreakable emulsion.) After standing for several minutes. the emulsion hroke. The clear supernatant was decanted. washed with distilled water, and dried over anhydroufi sodium sulfate. The amount of Ultrawet needed to break the emulsion appeared to be somewhat critical and had to he determined experimentally for each material. Preparation of Ultrawet Solution. As total chlorine determination was to he used as a measure of insecticide content qf the extracts, it was desirable to use a chloridefree solution of Ultrawet X. The most. satisfactory method of preparing this material was by passage through an ion exchange colum~coiitniiiingIRA 400 (Rohm and Haas). Using a column 4 em. in diameter and 45 cm. long, two passes (the column regenerated between passes) were suffirientto purify 1 liter of Ultrawet K solution containing 25% solids. After the second pass, the column could be used without rcgeneration for preliminar,v chlorido removal from a n untreatrd hatch of solution.
Precision Metal and Glass Syringe with Teflon Gaskets. TI n . L :
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U--,"..A
TT .*-.
r--..A-..
u^"":*"l ?2""&"..
C
...nn
was developed for use with special vacuum sysTtemsvrrnge .for' the assay of radioactive water [Robinson, C. V., a
HIS
Rev. Sei. Imtr., 22, 353 (1951)l. I n the procedure it is necessary to deliver 0.2 ml. of water into z partial vacuum dmp by drop with good control. It is also desirable to deliver 88 nearly B.R possible t,he entire amount and to he able to clean and dry the syringe readily. The ordinary type of syringe retains 60 to 70 mg. of wrater in the huh of the needle and is difficult to control unlem it is heavily greased; in this case it is difficult to clean and decontaminate. The svrinee . I described here has a smooth. controllahle action of adjustable resistance, delivers nearly the entire sample except for what is in the bore of the needle, requires no grease, and may be readily taken apart, cleaned, and dried. The syringe consists of three principal parts: the barrel, 1, the eomhined needle and plug, 2, and the plunger, 3 (see Figures 1 and 2). In the syringe shown, the barrel is a piece of precision-bore borosilicate glass tubing (manufactured by Fbcher & Porter Co., Hatboro, Pa.) of 0.1875 inch bare and 2 inches long. The useful capacity is 0.7 ml. and the motion is 0.221 inch per 0.1 ml. The needle-and-plug assembly consists of part 2A made hv silver-soldering together the stainless steel tube of ..~ ..~ ~"
~~~~~
I
~
~~~~
serted into the barrel and screwed together tightly enbugh en that the oxoansion of the gasket holds the plug firm1.y in place. Reasonable-care must be ewciaed, as overtightening will break the glass. The plunger, 3A, similarly has a sleeve, 3.8, nut, 3C. and Teflon easket. 3 0 . and is tightened to eive the desired &aunt of resiSt&e. ' T h e plug or p h g e r is tightened by 8.ppl.Ving small wrenches to the nut and to the flat surfaces which have been filed a t the end of the threaded portion (see Figure 1). Teflon is suitable as the gasket material because it is readily deformed, is hydrophobic and chemically inert, and has a greasy quality, so that i t slides smootbly over glass. Several tests were m d e to compare the properties of this type of syringe with the standard 1-ml. tuberculin syringe. The pre-
.
Figure 2.
Precision Syringe
Over-ell length 17.5 cm.
cision sJ-ringe was found to deliver all but 16 mg. of water, 14 ma. of which *-ere in the bore of the needle. The tuberculin syringe with the same size of needle, 3-inch No. 22, was found to have a SO-mg. residue a t the end of delivery, 13 mg. of which were in the syringe, 62 mg. in the huh of the needle, and 15 mg. in the hare of the needle. A plunger-stop was provided for one of the precision syringes to test the reproduoihility of delivery. A series of six deliveries gave an average of 0.4071 3~0.00023 gram, with a standard deviation of 0.5T mg. for a single delivery. Teflon needle caps are provided far these syringes, so that samples may be stared in them (see Figure 2). Evaporation rat,es a t ronm temperature were measured on a pair of capped precision syringes and on a pair of tuberculin syringes whose tips were waxed shut. Each syringe conbined about 0.4 ml. of water. The average rate of evaporation for the tuberculin Syringes was found to he 2 mg. per day. When the precision syringes were adjusted for moderate plunger resistance, the average rate was found to he 0.6 mg. per day and, when adjusted for high resistance, less than 0.1 mg. per week. A number of syringes of this type have been in use in this Isboratory over the past year, not only for use with the tritium assay system but dso for directly sampling cerebrospinal fluid from catheters connected into the subarachnoid space and for Storing these samples prior to assap. The syringes may be autoclaved with no other ill effect than loosening of the gaskets, which can he corrected by subsequent tightening under sterile conditions. Refrigeration Seems to offer no difficulty even when ice crystals are formed. Stainless steel Construction and handtightening could he incorporated into the design without major difficulty. Although only three sizes have been built to date0.7-ml., l.O-ml,, and 5mI. capacity-it should he practical to extend the range of sizes to smaller and larger syringes. Thr plunger and barrel parts of the 0.7-ml. syringe provide a nice17 controllahle pipetter which requires no grease and is sufficiently air-tight even when adjusted for moderate resistance. For pistons of 3-mm. diameter or less it is preferable, because of the simpler construction, to make the plunger a solid metal rod and use a looefitting metal barrel with a Teflon packing and a packing nut to make the sed. A stainless steel piston 2.3 mm. in dizmeter made in this way [Chapman-Andresen, C., and Robinson, C . V., Cmnpt. rad. trav. Lab. Carlsberg, Ser. chim., 28, 343 (1953)l has proved to he satisfactory for fairly rapid delivery of several microliters of water, adjustable to 10.01 PI. In this type of construction a much smoother action is obtained if the plunger is polished lengthwise than if i t is polished around.
