V O L U M E 2 2 , N O . 9, S E P T E M B E R 1 9 5 0
1169
For routine control work the above forinu1:r may he simplifimi without serious error as follows: Specific gravity of the separated oil may be assumed to be that Per Cent Oil Found of the sample--e.g., 125/130" F. American melting point = 0.900 Filtering -143/150°F . Micro Solid specific gravity. Temperature, 125/130° F. AMP AMP, Point on Grams of methyl ethyl ketone in the mixture at the tiine of F. Sample 1 Sample 2 sample 3 Oil. Av.. F, filtration may be assumed to be 80.5 and grams of sample 36.0. 0.54 0.76 25 - 12 0.43 The correction of the aliquot for thevolume of oil it eoot:iins 0.53 0.73 12 - 20 0.42 (0.002A) may be disregarded. 0.51 0.71 6 - 25 0 41 0.40 0.69 -8 - 30 0.38 The formula then simplifies to: - -~ ~- When C = 10. Oil, % by weight = 0.05.4 T h e n C = 5. Oil, % by weight = O.1OA Table 11. Oil C o n t e n t of Wax Samples Table I.
Filtering Temperatures us. Pour P o i n t
O
.I.S.T.M. D 721-47 AMP,
O
% oil
F.
125/130 125/130 125/130 143/150 143/150 143/150
0.26,0.22
0.21,0.17 0.48.0.58 0.29,0.22
0.63,0.70 0.45,0.49
Deviation 10.02 t0.02 tO.05
t0.04 t0.04 t0.02
Bisulfite Method DeviaYooil tion 0.30,0.30 0 0.20,0.20 0 0.55.0.58 *0.02
0.30,0.30 0.73.0.73 0.53,0.55
0 0
*o.oi
~
B from A.S.T.M. f0.06 +0.01 +0.04 +o 0.1 +0.06
+o.n7
.4verage deviation between methods +O 03 ~-
__
___~ -
mometer iii the beaker, and, if the sample is not completely di+ solved, heat on a steam hot plate (or equivalent), while stirring, until it just dissolves. Transfer the beaker to the -50" to - 100' F. bath, and stir with the thermometer, scraping off the wax coat Ivhich is formed on the sides and bottom as completely and rapidly as possible. Continue stirring until the mixture has a slushy consistency, after which stirring may be intermittent. When the temperature drops to -20" F., stir continuously until i t drops to -25' F. Transfer the beaker to the -25" F. bath, place the precooled filter tube in the mixture nearly to the bottom of the beaker, and attach a vacuum line t o the filter tube. Turn the vacuum on slightly, and when an estimated 15 ml. of filtrate are in the filter tube, detach the vacuum line, remove the filter tube from the beaker, and transfer the filtrate to a test tube or other suitable container. Pipet 5 ml. of the filtrate, if it is estimated the oil content is over 2.57&,or 10 ml. if under 2.5%, into a skim milk test bottle. Add 5 nil. of distilled water and 20 ml. of a saturated solution of sodium bisulfite to the skim milk test bottle. Press the end of a rubber t u t x attached to the vacuum lin:. against the open end of the capillary neck, and adjust the vacuum so that maximum agitation occuri without loss of liquid from the bottle. Continue the agitation for 3 minut>es,then detach the vacuum line. Add water to the bottle until the surface is in the upper one third of the calibrated portion of the capillary neck. Centrifuge the bottle at approximately 1500 r.p.m. for 5 minut'es and read the number of divisions of separated oil t o the nearest half-division (smallest division = 0.002 ml.). Calculat,e the oil content by means of the following formula:
~
~
~EXPERIMENTAL ~ ~DATA
~
e
For the purpose of this investigation, oil is considered as "zero pour point oil," determined by filtering three samples of various melting point wax a t four different temperatures, with the results shown in Table I. A wire is constructed with about a 0.5-inch (1.25 cm.) length at right angles t o a small loop. Microsolid point is determined by placing a small amount of oil on the end of a thermometer bulb and embedding the small loop of wire in the oil. By chilling the embedded wire in the vertical position and allowing it to warm slowly in the horizontal position, the temperature a t which the wire starts to fall from t,he horizontal position is recorded as the micro solid point, and is correlated with the pour point of the oil. With the wire used zero pour oil has a micro solid point of 6' F. Each wire must be standardized against zero pour oil before use.
It was concluded from the data in Table I that a filtering temperature of -25" F. would give zero pour point oil. In order to obtain data on the accuracy of the outlined method, a number of samples were run in duplicate by A.S.T.M. D 721-47 and this method. The duplicate results in Table TI were determined by different operators. The average deviation between methods is better than the repeatability of the A.S.T.M. method. No experimental work was done with microcrystalline or high melting point waxes. coYcLusloYs
The oil content of paraffin wax, as determined by this method, agrees closely with results obtained by A.S.T.M. D 721-47 on commercial waxes of the usual low oil content. The advantages of the method are simplicity, reliability, and speed. Approximately 0.5 hour is requited for completing the test. Another advantage is that it avoids evaporation of solvent, and thus includes light oils if present in the determination. LITERATURE CITED
(1) -hn.Soc. Testing Materials, A.S.T.M. Committee D-2, "Stand-
where A = number of 0.002-ml. divisions of separated oil, B = specific gravity of separated oil, C = ml. of aliquot treated with sodium bisulfite solution, D = grams of methyl ethyl ketone added to the sample, and W = weight of sample. (0.805 is the specific gravity of methyl ethyl ketone at 20" C.)
ards on Petroleum Products and Lubricants," Method D 72147, Philadelphia, Pa. (2) Wiberley. J. S., and Rather. .J. B., J r . , ANAL. CHEY..20, 972 (1948). R E C E I V EJanuary D 27, 1QA0.
Improved Potentiostat for Controlled Potential Electrolysis JAMES J . LIKGANE AYD ST.4NLEY L. JONES, Harcard University, Cambridge 38, M a s s .
P
OTESTIOSTATS, which automatically perform the function of maintaining the potential of an electrode constant during an electrolysis, have not yet become commercially available, so that those who wish to exploit the planifold analytical applications ( 4 ) of the controlled potential electrolysis technique must construct for themselves the necessary apparatus. In recent years a number of different types of potentiostat have been described (1-5), whose relative merits are best assessed by reference to the original papers.
The inst.rument desvribed herein, whose operating principle is indicated schematically in Figure 1, is an improved version of an instrument previously described ( 4 ) . The chief improvement is the use of a rectifier and filter circuit to enable operation from an ordinary 110-volt alternating current line and the concomitant employment of Variac autotransformers to control the alternating current input and hence the output direct current voltage applied to the elec.trol,vsiq rrll.
ANALYTICAL CHEMISTRY
1170
An improved potentiostat is described, with the following characteristics: complete operation from the 110-volt alternating current line, except for one ordinary 1.5-volt dry battery; either cathodic or anodic potential control with sensitivity of 10.01 volt; output capacity up to 5 amperes at 6 volts, and voltage output up td 25 volts with smaller electrolysis currents; and no preliminary calibration or adjustments required before use. Because all components have long life expectancy, long periods of use without servicing may be anticipated.
The first Variac transformer provides a convenient means of manually controlling the input to the automatically operated Variar t,o provide the optimum direct current output range for a particular electrolysis experiment. The motor-operated Variac is followed by a stepdown trmsformer, a full-wave selenium rtvtifier, and a conventional indurtance-capacitanre filter circuit to smooth the rectified direct current. A voltmeter across the output indicates the total voltage applied to the cell and theelectroly& current is read on a multirange ammeter. High precision is not required of either of these meters, and ordinary panel-type iiistruriients with an accuracy of the order of *2y0are adequatr. The control circuit comprises an ordinitry radio-type potentionirtrr powered by a 1.5-volt dry cell to provide the reference voltage, whirhis read directly on a voltmeter, R \Veston I\.lodel 30 galvaiionirattBr relay, and a dual elwtronic relay to control t,he reversible sh:Ltird-pole motor which drives the Variac autotransformer. FVlitxii the potential of the working electrode against the referenct’ c,lec*trode (usually a saturated caloniel electrode) differs from the opposing referenre voltagr, the galvanometer relay makes contact right or left and activates the double elect,ronic relay, which in turn causes the motor to operate the Variac in the appropriate direction either to increase or decrease the total voltage applied to the cell until the potential of the Lvorki,ig electrode returns to the value of the reference voltage. Because the referencc voltage is read directly on a meter, the control circuit requirrs 110 preliminary adjustment or calibration before use, and the rderence voltage can be changed instantly during the cnurse of R I I c~lertrolysisby mercly readjusting thr potentiometer The complete rircuit of the instrument, with specifications of all components, is shown in Figures 2 and 3, and Figures 4 and 5 sho\v the chassis arrangement and completely assembled instrunitant, To put the instrument into service it is only nevessary to close switches 5-1, S-2, and S-5, adjust R-6 to the desired referenve voltage, adjust Variac 7’-1 to provide the desired direct currt,ut output range, and finally close the output circuit and se1rc.t the optimum range of ammeter hf-1 by snitrh S-3. Sone of the components is requirqd to function critically, and all are conservatively rated to provide long life expectancy, so that’ extended periods of use without servicing m a y he antiripated. The, control sensitivity depends primarily O I L the sensitivity of t l i r J\.cJston galvanometer relay (extreme top right on panel in Figurts 5 ) . This iustrunieut hits it rated sensitivity of 1 1 5 microaml)(*rtss per m.ni., and ail internal resistancbe of 1100 dhnis, rorrespiiding to a voltage sensitivity of approximately *16 mv. per nim. By adjusting t.he contarts to minimal clearance the net sriisitivity can be adjusted to somewhat better than * l o mv. w . h r 1 1 the rrsist,aricne in the caontrol circuit does not exceed about 1000 ohms. LInst of the resistance in the caontrol circuit orcurs in t h c s salt Ixitige betFveen the rrxfercnce elwtrode (usually a saturatrd r:tlomel elevtrode) and the t>lectrol> solution, and it is a simple matter so to design the salt hricigc. thitt the total resistance i i i thts vontrol rircuit aniouiits t o only :i fe\v hundred ohms. A voiitrol sensitivity smaller than about =t10 mv. usually is of no priwfical use, and control to *60 mv. is adrquate for many purposes. The potential of a tvorking electroGe usually undergoes rapid erratic fluctuations \vhic.h may vary from a fey to as murh as 10 or 20 mv., depending on the types of elrc,trode, ronditions of stirring, and similar factors. Thcb us(’ of a control sensit,ivity smallel, than thew natural flurtuations only Ieatis to o1)jwtion:it110 “hunting” :ind t i w r r i w t i voritrnl efic*ieiicy.
w ln- U SHADED POLE MOTOR
O e 3 RPM
STE PO OYN
TRANS.
ELECTRONI C
S E L f N l UM RECT.
MAX. MAX. C. I .5
V.
REF. ELECTRODE Figure 1. Schematic Circuit The rate of rotation of Variav 1’-2 is :ti1 important f’dctor for optimum control action. It should he great enough so that the voltage applied to the cell is promptly corrected and yet not so great that overshooting and consequent hunting occur. A rate of rotation of approximately 0.3 r.p.ni. is optimum for most purposes. The shaft of Variac T-2 is extended to the rear and provided with a large spur gear which is driven via a reducing spur gear train by the drive motor. Two normally closed microswitches in the shading coil circuit of the drive motor, actuated by a peg on the spur gear attached to the shaft of Variac T-2, serve as limit switches to prevent damage to 1’-2. Pilot lights 1-2 and 1-3 (right-hand side of panel below Wrston relay) in the shading coil circuit of the d r i w motor indicate the direction of operation of the motor. Hecause t,hese 6-vnlt pilot lamps opefate on a current (0.15 anipcrc) approximately equal to that in the motor shading rnils, they do not affect the operationof the motor. The function of the dual electronii~w1Ry(Figure 3) is to r d u w t o an infinitesimal magnitude the current handled by the collt:icts of the galvanometer relay. The elecatroriic wl: operate on an input, current (2 microamperes) that any contact resistance in the g:ilvaiiomr portant and feathrr-light contacting is sufficaieut for positive, operation. The input terniinds, 1, 2, anti 3 (Figurv 3), are connected to the three output terminals of t,he galvariometer rel:iy. When the galvanometer rrlay makes contact i n one diwction tcrtninal 1 is shorted to the common terminal 2, and the constquent abrupt increase in the left-hmd plate current of thtfi 6557 twit1 triodv
V O L U M E 22, NO. 9, S E P T E M B E R 1 9 5 0
1171
I
1
11
Figure 2.
Complete Circuit
1.5-volt dry hsttery C-1, C-2. 6000-rfd. electrolytic e,apaoitora 8-1.
(50 volts)
1-1. pilot lamp (115 v01i a,
