1213
V O L U M E 22, NO. 9, S E P T E M B E R 1 9 5 0 covery tests on acetone-alcohol solutions of cholesterol, however (Table IV), gave better results when the samples were compared to standards containing approximately the same quantity of cholesterol. The discrepancy resulting from the use of a single standard was greater when the photoelectric colorimeter was used. It is therefore suggested that three different standards containing 0.250, 0.500, and 0.750 mg. be prepared for each series of photoelectric determination of plasma (or blood) cholestcsrol. It was necessary for the authors’ purposes to extract 5 ml. of plasma (or whole blood), but total cholesterol can be determined in 0.25 ml. and free cholesterol in 0.50 ml. of plasma. In this case the extraction should be done as recommended by Sotiel and hlaycr (b),and from then on the procedure continued as described here. SUIMLMMARY
The Schoenheimer-Sperrv method of cholesterol determination has been adapted to photoelectric instruments. The iiitroduc-
tion of glass-stoppered centrifuge tubes obviates the use of stirring rods and preserving jars recommended in the original method. Cholesterol may be determined accurately by photoelectric methods in samples containing 0.125 to 0.750 mg. of cholesterol. The accuracy of the Schoenheimer-Sperry method may be. enhanced by the use of serial standards; calculations are based on the comparison of the unknown t o the standard of approximating concentration. LITERATURE CITED
(1) Schoenheimer, R.,and Sperry, W. M., J . B i d . Chetn., 32, 745 (1934). (2) Sobel, A. E., and Mayer, A. M., Ibid., 157,255 (1945). (3) Sperry, W.M.,,fim. J. Clin. Path., Tech. SuppZ., 2, 91 (1938). (4) Sperry, W.M., Schoenheimer-SperryMethod for Determination of Fholesterol,” New York, 1945. (5) Sperry, W.M.,“Technical Manual for Laboratory Technicians,” War Dept.. TM-8-227, r-. S.Government Printing Office, 1946. R E C E I V E Dneceiiiber 6 , 1949.
Dropping Mercury Electrode Apparatus W. CHARLES COOPER’
A N D M . M . WRIGHT* Princeton llniversity, Princeton, N . J .
4RIOUS dropping mercury electrode assemblies have been Vduggested (1-7). In the majority of these, rubber tubing with its inherent disadvantages is used to some extent. For polarographic analysis based on standardized diffusion current constants, the apparatus of Lingane ( 4 )with the stop-clock circuit for the rapid, automatic determination of the rate of flow of mercury is probably the most advantageous. The apparatus drscribed in this paper is particularly well suited for routine polarographic analysis by comparative methods. Designed entirely of glass and having ground-glass connections, there is n? contact between mercury and rubber and hence no possibility of the mercury being fouled from this source. The three-piece assembly is compact, is more readily disassembled and cleaned than the glass-rubber or all-glass apparatus currently in use, and is not considered by the authors aa extremely complicated or fragile. These advantages offset the initial expense. Figure 1 illustrates the entire apparatus, an enlarged view of one section of the stand tube, and a hand-drawn capillary. The apparatus is made up of three pieces: A , the capillary; B , the mercury reservoir vessel; and C, the stand tube with adjoining stopcock and blood pressure bulb and valve. In A a 5.5-cm. length of Corning marine barometer tubing is joined to 6-mm. soft- lass tubing have a 7/25 J female joint. Electrical contact to &e mercury in B is made by rpeans of the tungsten contact and mercury well, D. The details of the construction of the stand tube, C, in the region of the ground-glass joint, E, 12/30 J , are shown in Figure 1, center. The operation of the apparatus is simple. By means of the blood pressure bulb and valve, F , mercury can be raised to any desired height in the stand tube. The air forced in when the bulb is squeezed (valve is closed and stopcock is open during this operation) enters the ieservoir vessel through a hole, G , close to the lower inner seal. As a result of the pressure which is built up in the reservoir vessel, the mercury proceeds up tube H. The stopcock is closed when the mercury has reached the desired level. When the polarographic analysis has been completed, the capillary is washed carefully with a stream of distilled water and immersed in either distilled water or ure mercury. Then the mercury column is lowered by opening t i e stopcock and the valve, whereupon the pressure in the vessel is returned to atmospheric.
capillaries have been discussed by Novak (8). The major disadvantage in their use is the fact that they are easily breakable. Figure 1, right, shows a hand-drawn capillary for which this disadvantage has been overcome. Such a capillary has been used
C
I
$7125
P B
$12130
In polarographic work hand-drawn capillaries can be used as well as marine barometer tubing. Various features of such 1 Present address, Department of Chemistry, University of Wisconsin, Madison, Wis. 1 Preaent address, Chemistry Division, National Research Council, Ottawa, Canada.
Figure 1. Dropping Mercury Electrode Apparatus Left.
Entire apparatus Enlar ed view of section of s t a n d tube Hand-&awn capillary w i t h protective shield
Center.
Right.
1214
ANALYTICAL CHEMISTRY
successfully for many months and the operator has been relieved of any fear of losing Iiis capillary through breakage.
draws distilled water through the capillary, then a little acetone, and finally a little air.
The capillary is protected against mechanical shock by a shield of 7-mm. tubin Six holes, three on opposite sides, 11the shield near the tip of %e rapillary permit circulation of the solution. A hole in the shield near its upper end permits air to escape when the ca illary is immersed. During routine analysii a stream of distilkd water from a wash bottle directed through one of the ports in the shield sprves to wash the capillary free of any solution. Adhering water is removed readily by wiping the capillary and shield with a small piece of filter paper.
The hand-drawn capillary with the protective shield can be cleaned as easily as the same capillary without the shield or a capillary of marine barometer tubing, but it may be a little more difficult to determine whether or not liquid is passing up the capillary.
The following is a very efficient way to clean the capillary, either the hand-drawn model or one of marine barometer tubing.
Having removed all mercury from the capillary (by carefully jarring the rxipillary or by suction supplied by an aspirator), one first draws hot. concentrated nitric acid through the ca illary, using an aspirator. Whether or not the acid is passing tErou h the capillary can be determined by lifting the capillary out of t i e acid and draining any excess off the tip. If the passage is free, a column of liquid will be seen moving up the capillary. Next one
LITERATURE CITED
(1) (2) (3) (4) (5) (6) (7) (8)
Kahan, G..J., IND.ENG.CHEM., ANAL.ED., 14, 549 (1942). Kolthoff, I. M., and Lingane, J. J., Chem. Revs., 24, 1 (1939). Langer, A.. IND.ENG.CHEM.,ANAL.ED.,13, 794 (1941). Lingane, J. J.,Zbid., 16, 329 (1944). Lingane, J. J., and Laitinen, H. A., Ibid., 11, 504 (1939). McReynoIds. R. C., Ibid., 14,586 (1942). Mue1ler.E. F., Zbid., 12, 171 (1940). Novak, J. V. A., Collection C w c h o s h . Chem. Commune., 12, 237 (1947).
RlcrrvrD December 29. 1949.
Rate-Indicating Mariotte Bottle F. A. SCHWERTZ Koppers Company, Znc., Mellon Institute, Pittsburgh 13, Pa.
MARIOTTE bottle is frequently employed in the laboratory If the liquid is used to displace a gas, this bottle serves also to meter the gas a t rates far below the threshold of the ordinary wet-test gas meter. Its utility suffers, however, from the fact that it is not an indicating instrument and must be set a t the required flow rate by a method of trial and error. The present note deals with some simple additions to the Mariotte bottle that transform it into a rate-indicating device. These additions are based on the approximate theory of the operation of the bottle.
Putting this value in Equation 4 gives
A to obtain low but constant rates of liquid flow.
THEORY OF MARIOTTE BOTTLE
ud
=
d2g(h, - hb)
or vc =
d2ghl
The velocity of discharge is therefore proportional to the square root of the hydrostatic head between the points of liquid discharge and air entry. The rate of discharge may accordingly be regulated by changing the distance between these points. An alternative way of controlling the ,rate is to insert a resistance in the air-intake .line.
A typical Mariotte bottle, shown in Figure 1, consists essentially of a jar provided with a liquid-exhaust line, A , and an airintake line, B As the liquid flows out of the jar a t C it is relaced by air which enters a t B and bubbles up through the iquid level, D. The ratre of discharge of the liquid, considered as ideal and nonviscous, may be calculated from Bernoulli's equation, which requires that the energy per unit volume of liquid a t any two oints along a streamline must be constant. Its familiar form or an incompressible liquid is
P ' P
u2
+ P + dgz = constant
(1)
where d is the density of the liquid, u its velo'city a t any point, P the hydrostatic pressure a t the same point, and z the vertical distance of the point above some arbitrary datum line. If this datum line is taken a t level C , it is necessary that
tu2
+ Po = K
(2)
where us is the velocity of discharge, Po is atmospheric pressure, and K is a constant. Similarly, because the velocity a t liquid surface D is negligibly small, Pi
+ dgho = K
where Pi is the air pressure above the liquid surface. combining Equations 2 and 3,
or
+ dgha
I
(3) Hence, on
(4) I t is also necessary that P o = Pi
_ _ _ _ _ _ - -I
(5)
I
*Is
Figure 1. Typical Mariotte Bottle
