tioned on top of the first during assembly. The top window used was thinner than the bottom and was about 1.5 mm in thickness. This sandwich arrangement was held together tightly by two clamp bars in the center of which were 3/82-inchholes, into which were threaded metal tubes for attachment to external vacuum. Solvent tight seals were obtained by use of amalgamated lead washers between the top window and the bars and at the allen head screws. Even pressure on the windows could be achieved by adjustment of the four binding post nuts, with the pieces of heavy paper serving to relieve minor inequalities and strains. When in use, one of the tubes leading from the cell was immersed in the liquid to be drawn into the cell. The other was attached via a stopcock and glass joint to a vacuum system. Slow evacuation then enabled liquid to be drawn into and through the thin cell without leaving residual air bubbles in the cell. Closing of the stopcock followed by withdrawal of the tubing from the test solution then prevented drainage of liquid from the cell. Experiments performed with this device revealed it could readily be filled and emptied by evacuation and would remain leak free over an extended period. For quantitative studies, a matched pair of such cells were employed using the same approaches taken for the simpler demountable cells.
RESULTS AND DISCUSSION
Both types of cells were used to obtain calibration curves of absorbance at specific frequencies us. concentration for solutions of calcium nitrate and nitrite in water and calcium sulfate, sulfite, nitrate, and nitrite in saturated aqueous tetrasodium EDTA solution. The curves for several aqueous solutions obtained with the demountable cells are shown in Figures 3 and 4. As can be seen, there is little scatter and good reproducibility. Similar quality data have been obtained with sealed cells. Data of this quality demonstrate the overall feasibility of using this method for conducting infrared studies for analyses of aqueous solutions. With these more resistant thin cells, it should be possible to develop more methods of analyses for several inorganic ions of the type which was recently given for sulfate in EDTA solutions (7). Such cells may also prove to be of use in applying the infrared method to studies of kinetics of reactions in aqueous solutions.
RECEIVED for review January 25, 1971. Accepted October 26, 1971. This work was performed pursuant to contract CPA 70-63 with the Air Pollution Control Office, Environmental Protection Agency. The mention of any tradenames in this paper does not imply endorsement by the Government of any specific product.
Improved Capillary Direct Current Cell Suitable for Conductometric Titrations Otto Hello School of Chemistry, Hobart Technical College, Hobart, Tasmania 7000, Australia
A STABLE AND SENSITIVE conductivity cell can be made by measuring current flow as limited by a small capillary tube. In such a cell, practically all the potential drop will be across the capillary and polarization effects will be reduced to a low level where it becomes practical to use direct current for conductivity measurements. As very small direct currents can be measured accurately, the sensitivity of the cell is well above that feasible with conventional alternating current methods. The generally accepted improved ac bridge method is accurate for conductivity measurements over 200 ohms ( I , 2 ) . Direct current methods using conventional conductivity cells are limited to resistances over 1 Mohm (3),or to halide solutions by using reversible Ag/Ag-halide electrodes ( 4 ) which has been improved by use of a special cell design (5). The cell described here provides a method more suitable for conductometric titrations than ac bridge techniques, but which provides stability and range comparable to that achieved by the more complex Bipolar Pulse Technique (6J A typical capillary-limited conductivity cell (Figure 1) was constructed from two 100-ml vessels (hard polyethylene centrifuge tubes) connected by 2 cm of 0.16-mm i.d. glass tubing, sealed into the vessels with epoxy resin. Each elec(1) F. Kohlrausch, W e d . An!?,69, 249 (1893). (2) G. Jones and R. Joseph, J . Amer. Clzem. SOC.,50, 1049 (1928). (3) R. M. Fuoss and C. A. Kraus, ibid.,55, 21 (1933). (4) A. R. Gordon et al., ihid., 75, 2855 (1953). ( 5 ) D. J. G. Ives and S. Swapoora, T r a m Faraday SOC.,49, 788
(1953). (6) D. E. Johnson and C. G. Enke, ANAL.CHEM., 42,329 (1970). 646
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
-1 I +
Figure 1. Apparatus for dc capillary conductometry A 2-cm long 0.16-mm i.d. capillary between two 100-ml vessels. Electrodes 6 sq cm bright Pt mesh. Liquid levels over 5 cm apart
trode consisted of 6 sq cm of bright platinum mesh. By filling vessel B somewhat above the level of vessel A, the conductivity measurement is controlled primarily by the solution in vessel B, as only liquid from B will flow through the capillary. If the level difference is more than 5 cm and the voltage gradient across the capillary is more than 10 volts cm-l, polarity or level change has relatively little effect on the conductivity of the cell. Testing this cell with 0.1N KC1, the E us. Z graph (not shown) is a straight line from 5 to 100 volts (approximately 5 to 100 PA). Below 5 volts the current readings are unsteady, while above 100 volts the graph develops a positive deviation due to ohmic heating culminating in bubble formation at 700 volts (1.3 mA), where the current decreases rapidly to a low value.
I
.
.
. .
1
.
.
. .
I
*
'
.
.
. 0.1 ml 0.1 0.2M E.D.T.A. 0.3
Figure 2. Graph showing a conductometric titration of 100 ml equinormal mixture of HCI and acetic acid with lNNaOH
Od
0.5
Figure 4. Conductometric titration of a mixture of Ca and Mg ions Prior to the titration the conductivity was measured as 0.5 ml of 0.05N CaCI?,1 ml of 0.05N MgSO, and 1 ml of NH8/NH4CIbuffer were added to 100 ml of water
Table I. Liquid Flow through a Capillary 1 cm Long in 1 min for a Liquid Head of 10 cm and a Temperature of 15 "C for Various Inside Diameters ID, mm 0.250 0. I25 0.063
01
0
I
I
m l 0.1 M E.D.T.A
I
I
5
Figure 3. Conductometric titration of 10 ml of 0.04M Zn(N03)2and 20 ml of NH3/NH4CIbuffer in 100 ml of solution with O.1MEDTA The conductivity was measured as the solution was prepared. Counter current (c.c.) was used to offset the titration graph The application of this cell to conductometric titration is shown in Figures 2-4. A conventional conductometric titration of a weak and strong acid mixture is shown in Figure 2. The hyphenated line demonstrates the stability of the cell when the chart direction was reversed for 10 min (at 0.6 inch per min), after stopping the flow of titrant. Figure 3 shows an EDTA titration of zinc under the same conditions used in the Bipolar Pulse Technique (6), except that 100-ml total solution volume was used here and the offset current was only 80% of the total cell current. The end point as the first of the two inflections shows up clearly though it was found that a sudden increase of conductivity could be observed before or after the expected end point
Loss, ml per min 0.49 0.03 0.0019
when lessthan 10 ml of buffer was used with 10 ml of 0.04M Zn(N03)2. This could be due to the dissociation of Zn(OH)z which was formed initially on adding the buffer. The two end points on the calcium and magnesium salt mixture shown in Figure 4 indicate that the method will show up relatively small changes of conductivity with sufficient sensitivity to show the calcium equivalence point as a separate end point without the use of two complexing agents as titrant (7). A slight delay shown at the start of most of the titration curves is caused by the mixing time and the finite flow time through the capillary. On the other hand this delay provides a natural damping effect resulting in very smooth titration graphs. All titrations were recorded on a Beckman Electroscan Model TM 30, while the titrant was added through a peristaltic pump from a self-filling buret. The peristaltic action on a plastic tube does cause a periodic charging current which can show up on the graph as a slight ripple. During the titrations the solution was stirred rapidly with a centrally mounted stirring rod (a small Mini-stirrer was ideal). Magnetic stirrers gave unsatisfactory results with the above cell. (7) C. 0. Huber, K. Dahnke, and F. Hinz, ANAL.CHEM., 43, 152 (1971). ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
647
Solution 0.1N KCI 0.1N HCI 0 . 1N NaOH 0.1N Na2S203 0.01N HCI 0 . 0 1 N NaOH
Table 11. Comparison of AC and DC Resistances of Some Inorganic Solutions at 25 "C R in ohms" RIR, R in ohmsb RIR,' 1 6887 ( R 8 ' ) 1 29.59 (Rs)
9.92 0.34 17.24 0.58 0.72 21.28 3.13 92.51 5.39 159.49 a Measured with Cambridge Instruments Conductivity Bridge with a blackened * Measured with dc cell at 30 volts, measuring cell positive.
All titrations were done at an ambient temperature of approximately 15 "C; however, to avoid sudden variations the cell should be protected from draughts. As some liquid will flow through the capillary during a titration, a correction factor should be used to obtain the most accurate results. Table I gives calculated losses in milliliters per minute for distilled water passing through a 1-cm capillary for three different diameters, with a level difference of 10 cm and a temp of 15 "C. The correction factor to be used for different circumstances can be calculated from the following formula : correction =
+ 1/2 X
lOO/V X
h/lO
x
ljl
x
(d/d')4 x v'
x
0.35 0.59 0.73 3.28 5.51 Pt dipcell with a cell constant of 0.38.
+3 +2 +1 +5 1-2
been titrated to 5 0 z of its end point. The correction to be applied for the cell used in above titrations with a level difference of 7 cm, a temp of 15 "C, and a titration time of 10 min would be 0.15 For very small capillaries, especially at high potential gradients, electrokinetic phenomena could probably modify this result to some extent. An all glass cell with a capillary, 2.5 cm long and 0.7 mm i.d., was used to compare this method with the ac bridge method for absolute conductivity determinations. Table I1 lists some results on inorganic solutions at 25 "C. The readings obtained with the dc capillary method were stable as soon as temperature equilibrium with the bath was reached.
+
z.
t,
where V = volume of cell in ml, h = difference of level in cm, I = diameter of capillary in cm, d = diameter of capillary in mm, d' = diameter from Table I, v' = corresponding flow loss from Table I, and t = time in min. The 1/2 factor at the beginning of the formula is used because, assuming a constant titration rate, the liquid lost has
648
2378 4046 5055 22585 37939
Diff.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
ACKNOWLEDGMENT
The author is grateful to R. L. Gunther of this college for his assistance rendered in this project. RECEIVED for review June 28, 1971. Accepted October 14, 1971.
