milliliter samples of dimethyl sulfoxide containing from 18 to 82 pg. of water were analyzed; the absolute standard deviation was 1 0 . 3 pg. of water. The number of samples of dimethyl sulfoxide analyzed in the same electrolysis cell solution reagent had a considerable effect on the reaction time necessary in step 4 for 100% recovery. For 3minute reaction times only four 0.1ml. samples could be analyzed per cell solution reagent, This number was increased to eight by increasing the reaction time in step 4 to 5 minutes. This effect was not observed for methanol and from ten to twenty 0.1-ml. samples were analyzed for each cell solution reagent. The dimethyl sulfoxide appears to have a greater affinity for water than methanol. This demonstrates the advantage of a back-titration type of procedure for solvents which release water slowly to the Karl Fischer reagent and shows that with such solvents the ratio of sample volume to reagent volume should be kept as small as possible.
CONCLUSIONS
Other determinations developed on the basis of the Karl Fischer reaction typically show an uncertainty of =t2 pg. of water when tens to hundreds of micrograms of water are determined in 1- to bgram samples. The large sample size accentuates the influence of interfering side reactions given by the solvent and the Karl Fischer reagent. The proposed controlled potential method, employing 0.1-ml. samples, permits water determinations at the 10- to 80pg. level with an absolute standard deviation of k0.2 pg. of water. The small sample size minimizes the effect of undesirable side reactions and is particularly significant when little sample is available. Furthermore, the method is not limited by the sensitivity of a conventional end point detection system. By permitting excess iodine to remain in the system a predetermined time, complete reaction is assured in t.he case of solvents which slowly give up the last traces of water. The development of a rapid electrolysis system allows the
method to be competitive with existing procedures &s far as over-all time of analysis is concerned. LITERATURE CITED
(1) Bard, A. J., ANAL.CHEM.35, 1125 (1963). (2) Bastin, E. L., Siegel, H., Bullock, A. B., Ibid., 31, 467 (1959): (3) Kolthoff, I. M., Elving, P. J., “Treatise on Analytical Chemistry,” Part 11, Vol. 1, Sec. A, p. 69, Interscience, New York, 1961. (4) Meites, L., Moros, S. A., ANAL.CHEM. 31, 23 (1959). (5) Meyer, A. S.,Jr., Boyd, C. M., Ibid., 31, 215 (1959). (6) Otterson, D.A., Ibid., 33, 450 (1961). (7)Pribyl, M., Slovak, Z., Mikrochim. Ichnoanal. Acta, No. 6, 1097 (1964). (8) Scott, F. A,, Peekema, R. M., Proc. 8nd U . N . Intern. Conf. Peaceful Uses At. Energy 28, 583 (1958). (9) Swensen, R. F., Keyworth, D. A., ANAL.CHEM.35, 863 (1963). RECEIVEDfor review July 6, 1965. Accepted September 21, 1965. M. R. L. is indebted to the Shell Companies Foundation, Inc., for financial assistance in the form of the Shell Fellowship in Chemistry.
Continuous Back-Titrations with Direct Readout Application to EDTA Systems W. J. BLAEDEL and R. H. LAESSIG Department o f Chemistry, University of Wisconsin, Madison, Wis.
A continuous automated potentiometric titrator with direct readout has been adapted to the back-titration of metal ions with EDTA. The metals studied were those which are commonly back-titrated because of slow reaction with the chelate. After calibration of the titrator with standard samples, readout of sample concentration is both digital and direct, requiring no computation to obtain the analytical result. The average standard deviation for a typical series of samples was 0,0001 mole per liter with sample concentrations ranging from 0 to 0.035M. The precision of the titrator was obtained for the following titrations at pH 9.7 or 4.8: Zn(ll), Ce(lll), La(”, Pr(lll), Cu(ll), Th(lV), In(lll), Zr(lV), Cr(lll), and AI(II1). For Zr(lV), Cr(lll), and AI(III), 1 minute on stream at 98” C. was used to promote reaction.
A
AUTOMATED continuous titrator with direct readout has been described (2) and applied (3, 4) to (ethylenedinitrilo) tetraacetic acid (EDTA) titrations with mercury-EDTA electrodes, using the potentiometric
N
1650
ANALYTICAL CHEMISTRY
methods developed by Reilley and Schmid (5, 6). Without sacrificing direct readout of sample concentration, these studies introduced the concept of automatic correction for end point error caused by asymmetry in the titration curves. In the present study, the titrator has been adapted to the continuous EDTA back-titration of metal ions with direct readout, overcoming the disadvantages usually associated with back titrationthat is, the measured addition of two standard solutions and computation of the analytical result. The continuous back-titration technique permits automated determination of metal ions (Al, Cr, Zr, etc.) which react so slowly with EDTA that direct titration is unfeasible. THEORY AND DESIGN OF TITRATOR
I n the titrator, a continuously pumped stream of sample is combined with a buffered excess of reagent EDTA. After a reaction period, an excess of reagent metal ion is added [Zn(II) or Cu(II), whose reaction with EDTA is rapid], consuming the surplus chelate and leaving a small concentration of easily titratable metal ion. The excess
Zn(I1) or Cu(I1) is then titrated with another EDTA stream to a preselected end point potential by the addition of EDTA through a variable speed pump. This system gives rapid response time (1 to 2 seconds) and a linear relationship between titrant EDTA pump rate and sample concentration. The excess reagent EDTA could be titrated with the Zn(I1) or Cu(I1) stream, dispensing with the second titrant EDTA solution, but this procedure would result in a disadvantageous inverse relationship between sample concentration and Zn (11) or Cu(I1) reagent pump speed. When the formation constants of the sample metal ion-EDTA complexes are comparable to or larger than those of Cu (11)-EDTA or Zn (11)-EDTA, the final titration is a simple direct one. The effects of stability constants, buffer concentration, etc., have been investigated by Reilley and Schmid (6, 6) and applied to continuous titration (S,4). Figure 1 is a block diagram of the titrator. I n the chemical system, separate streams containing excess EDTA (0.0131M, 5.74 ml. per minute), buffer (0.2M in base and its conjugated acid, 9.93 ml. per minute), and sample (metal ion being titrated, 0 to 0.035M,
-
I!-----
I
I
I
I
EXCESS EDTA
5.74 ml./MIN.
I
--_-------
- -- - - - - - - - - - - - - SYSTEM (SEE REF,)
Figure 1.
I
Figure 2.
SYSTEM (SEE REF.)
Flow diagram of titrator
2.01 ml. per minute), are pumped a t constant speeds through peristaltic pumps to mixer 1, producing a stream containing the metal ion in excess EDTA. After reacting for a fixed time in a delay line, a t room or elevated temperatures, the stream is combined in mixer 2 with reagent Zn(I1) or Cu(I1) (0.0178.V1, 6.90 ml. per minute) and titrant EDTA (O.OL11, 4.8 to 12.3 ml. per minute). The reagent Zn(I1) or Cu(I1) is pumped a t constant speed with a Yew Brunswick pump, and the titrant E D T S is pumped with a variable-speed pump. From mixer 2, the main stream of the titrator flows through the indicator and reference electrodes. A servo unit, reacting to the electrodes’ potential, adjusts the titrant EDTA flow rate to keep the main stream a t the end point, a t which the titrant EDTA pump rate is proportional to the sample concentration. With suitable calibration, direct digital readout of the sample concentration can be obtained. Equipment, materials, and construction techniques mere similar to those described previously (2-4), with the following changes. The mixers were modified as shown in Figure 2. The housing was machined from Plexiglas stock (Rohm and Haas Co., Philadelphia, Pa.). End and vertical clearances for the Teflon-covered magnetic stirring bar were 0.005 inch. A raised ring 0.010 inch high and 0.015 inch wide ( A , Figure 2 ) was machined on the wall of the mixer a t a height corresponding to the horizontal axis of the stirring bar. A similar ring, 0.70 inch in 0.d. was machined on the bottom plate (B, Figure 2). These rings provide bearing surfaces to reduce the friction and maintain the bar’s position. With less stringent tolerances, the bars frequently decoupled from the magnetic stirrers (E. H. Sargent Co., Chicago, Ill., Catalog XO. 76490). The mixer inlet and outlet tubes were 3/s2 X 7/32 ,inch Tygon tubing, press-fitted into slightly undersized holes. X heated delay coil was used in some systems to promote more complete reaction. Conditions for complete reaction were determined empirically for each ion being titrated. The heating coil consisted of 3-mm. i d . glass tubing immersed in water in a 1-liter three-
Magnetic stirrer
necked flask maintained a t 98” C. by an electric heating mantle. Water loss from the bath was prevented by a reflux condenser mounted in one of the side necks. At the elevated temperature of the bath, liberation of dissolved gases caused bubbles in the main stream and erratic electrode potentials. The bubbles, along with 5 ml. of solution per minute, were removed from the main stream by pumping through a tee placed upstream from the indicator electrode. Small volume losses did not affect the potentiometric measurements. The readout system (4) measures the titrant EDTA pump rate in terms of photoelectric tachometer pulses accumulated by a scaler over a precisely timed interval. The interval is controlled by a preset scaler that counts cycles of line voltage. The total pulses accumulated are automatically corrected for systematic error due to any discrepancy between the titrator end point and the true equivalence point and are presented digitally. With calibration, the final numerical readout can be made to equal the sample concentration. Sample concentration, S, may be determined as a function of the titrant EDTA pump rate, S,measured as the number of tachometer pulses accumulated over a fixed time interval, and directly proportional to the volume flow rate of titrant EDTA in milliliters per minute, At the equivalence point,
(I:s
Total millimoles titratable minute metal of)
2.01 (S)
s =mS+
K
(4)
To calibrate for direct readout, two or more known sample solutions are titrated with a correction of zero and a convenient counting interval, say 700 cycles (700/60 seconds). A calibration plot of concentration vs. pump rate allows m and K to be determined from the slope and intercept. When the 700-cycle interval is used, m S is proportional to the sample concentration, but not equal to it. To make the corrected readout numerically equal to the concentration, the titrator is calibrated by selecting a new counting interval of I cycles such that I
=
(700) (m7oo)
(5)
K i t h the new counting interval, the value of the intercept will be changed to
where KI is the value which is set into the corrector, so that it is subtracted from the total number of counts accumulated. The corrected value which is read out-Le., total counts accumulated during the counting interval I , less KI-is numerically equal to the sample concentration.
total milliEXPERIMENTAL
=
+ 6.90 (R) = 5.74 ( E )
+ s (T)
(2)
In Equation 2 , E is the molarity of the excess EDTA stream, R is the molarity of the reagent Zn(I1) or Cu(II), T is the molarity of the titrant EDTA stream, and the numbers represent flow rates of the various streams. Rearranging Equation 2,
S=
When the constant terms in Equation 3 are combined and replaced by arbitrary constants m and K , the linear relationship between the readout, N , and the sought-for sample concentration, S,becomes apparent.
(5.74 ( E ) - (6.90) ( R ) 2.01
+
Reagents. Stock buffer solutions were 1.031 H M T X (hexamethylenetetramine) made 0.5-11 in nitric acid (pH 4.7), and 1.0J1 ~ H 4 ~ 0 3 - 1 . 0 A 1 1 K H 4 0 H (pH 9.7). Stock E D T A solution was 0.05-TI in the disodium salt. Stock reagent metal ion solutions were 0.1M copper nitrate and 0.1X zinc nitrate. Samples were prepared from stock solutions of the following salts: ZrOC12, La(S03)3.6H20, Th(K03)4 ,4H20, .kl(s03)3 9Hz0, Ce(X03)3.6H20,In(C104)3.8H20,PrC13, and Cr(N03)3.9H20. llfetal ion and EDTA solutions were standardized by methods outlined ( 1 , 6). For the performance of titrations, solutions as specified in Table I were prepared by dilution of the stock solutions with VOL. 37, NO. 13, DECEMBER 1965
1651
distilled deionized water. Samples were prepared by dilution in 100-ml. volumetric flasks. Titration Conditions. Solution compositions and flow rates were fixed by the following requirements: Ionic strength and p H of the main stream had to remain constant despite variations in the sample concentration and titrant E D T A flow rate. T o this end, the buffer stream ( 0 . 2 M ) was pumped a t 9.93 ml. per minute into the main stream, giving a combined buffer, sample, and reagent Zn(I1) flow rate of 18.84 ml. per minute, and a resultant buffer concentration of 0.105M. Both EDTA streams were also 0.105M in buffer, and therefore variations in titrant EDTA flow rates did not affect the pH and ionic strength of the main stream. The mass flow rate (millimoles per minute) of the excess EDTA had to be made somewhat greater than that of the most concentrated sample. On the other hand, to permit titration of a sample of zero concentration, the mass flow rate of the reagent Zn(I1) or Cu(I1) had to be made equal to that of the combined excess EDTA stream and that of the titrant EDTA stream ;it minimum pump speed.
u
Volume
Stream Sample (metal ion) Excess EDTA" Buffer Reagent Zn(I1) or Cu(I1) Titrant EDTA a
ml./min. 2.01
ka
d2 0.01
I 700 900 I100 1300
COUNTS PER 700-CYCLE INTERVAL Figure 3. Calibration plots for Cr(lll) titration
Conditions as specified in Table I, except for excess EDTA concentration
from the deviations between known and determined values of the sample concentrations. The metal ions investigated a t pH 9.7 all reacted rapidly enough to be titratable with minimum delay time (10 seconds) and without heating. The calibration plots (concentration us, counts per 700-cycle interval) were linear over the range of the titrator. The standard deviations obtrained for Zn(I1) and Cu(I1) describe the precision of the titrator for metals that react rapidly with EDTA. Since the other more slowly reacting metals are
I. Titration Conditions Concn. of sample or reagent species, mmole/ml. 0 000 to 0.035 0.0131 I
5.74 9.93 6.90
0.00 0.0178
4.8 to
0.010
12.3
i
> 0.02
RESULTS AND CONCLUSIONS
flow rate,
0.03
8
Several metal ions were titrated a t pH 9.7 and 4.8 with results summarized in Table 11. The standard deviation for each type of titration was computed
Table
t
-
Mass flow rate of sample or reagent species, mmole/min. 0.000 to 0.070
Buffer concn., mmole/ml. 0.00
0.075
0.105 0.20 0.00
0.048 to 0.123
0.105
0.00 0.123
Also 10-6M in Hg(I1).
Table II.
Metal ion in sample Zn(I1)
Ce(II1) La(II1) Pr(II1)
Results of Continuous Back-Titrations"
Delay Sample concn. No..of time, Toemp., C. range, mole/liter titrations seconds Ammonia Buffer, pH 9.7, Zn(I1) Reagent 0.000 to 0.030 10 10 25 0.000 to 0.035 0.000 t o 0.035 0.000 to 0.030
9 16
19
10 10 10
25 25 25
HMTA Buffer, pH 4.8, Cu(I1) Reagent Cu(I1) 0.000 to 0.038 13 10 25 Th(1V) 0.000 t o 0.035 12 10 25 In(II1) 0.000 t o 0.035 12 10 25 Zr( IV) 0.000 to 0.030 14 60 98 Cr(II1) 11 60 98 0.000 to 0.025 Al(II1) 0.000 to 0.030 14 60 98 Conditions specified in Table I.
1652
ANALYTICAL CHEMISTRY
Standard deviation, mole/liter 0.00010 0.00016 0.00012 0.00013 0.00011
0,00009 0.00012 0.00010
0.00009
0.00014
titratable with about the same standard deviation, it may be concluded that the back-titration procedure and equipment are adequate for these metals. At pH 4.8, Th(IV) and In(III) were also accurately titratable with a minimum delay time a t room temperature. However, attempts to titrate Zr(IV) with a 10-second delay a t room temperature gave a calibration plot with a slope less than half of that for rapidly reacting metals, and a pronounced curvature a t high sample concentrations. With a heating bath and a 1-minute delay time, a linear calibration plot was obtained whose slope was comparable to that for other metals. After calibration for direct readout, Zr(1V) samples gave the results reported in Table 11. Cr(II1) and Al(II1) reacted even more slowly than Zr(1V). In fact, with a 10-second delay a t room temperature, the purple color (6) of the Cr(II1)EDTA couple did not begin to form. At elevated temperature, and with a 1minute delay , the calibration plots were linear over the range of sample concentrations given in Table 11, and precise titration with direct readout was possible. For high Cr(II1) or Al(II1) concentrations, however, reaction became less complete, and deviations from linearity began to occur, as shown in the upper curve of Figure 3. The extent of reaction and the range of linearity could be increased by using a higher concentration of excess EDTA [O.O306M, which also required a higher concentration of reagent Cu(II), O.O349;M], as shown for the titration of Cr(II1) by the lower curve of Figure 3. With the higher excess of EDTA, the precision of the titrations improved, even though potential fluctuations and excursions in the end point region were increased. In the linear portion of the calibration curves in Figure 3, reaction between Cr(II1) and EDTA could be incomplete, but as long as it was consistent, direct readout of sample concentration would still be achieveable. To find the extent of the reaction, the titrator was cali-
brated for direct readout with Cr(II1) samples, and then a known sample of rapidly reacting Cu(I1) was run. The molarity of the Cu(I1) sample was read to be higher than its true value, indicating that the reaction between Cr(II1) and EDTA was incomplete even in the linear portion of the calibration plot. From the discrepancy between the correct and observed concentrations of the Cu(I1) sample, the extent of the reaction between Cr(II1) and EDTA was cal-
culated to be 93% for the titration conditions of Table 11. Similar measurements gave 91% for Al(II1) and 99% for Zr(IV).
(3) Ibid., 37, 332 (1965).
[:; )$:ye;:
y k , , Schmid, R.~ ~ . ,
30, 947 (1958).
(6) Reilley, C. X.,Schmid, 13. IT,, LamIbid., 30,953 (1958). son, D. W.>
LITERATURE CITED
(1) Barnard,
A. J., Broad, W. C., RECEIVEDfor review August 4, 1965. Flaschka, H., "The EDTA Titration. hccepted September 14, 1965. Work supported by an academic vear felloaPiature and Methods of End Point ship from the Minnesota llining and Detection," J. T. Baker Chemical Co., Phillipsburg, N. J., Sovember Manufacturing Co. (1964-65) and a wmmer fellowship from the National Science 1957. (2) Blaedel, W. J., Laessig, R. H., - 4 ~ 4 ~ . Foundation (1965) ( R . H. L.), both of which are gratefully acknoaledged. CHEM.36,1617 (1964).
Electrochemical Preparation of Thin Metal Films as Standards on Pyrolytic Graphite BASIL H. VASSOS, FRANCIS J. BERLANDI, THOMAS E. NEAL, and HARRY 8. MARK, JR. Department o f Chemistry, The University o f Michigan, Ann Arbor, Mich. Gold, silver, copper, and cobalt have been deposited quantitatively in microgram to milligram amounts on disks of pyrolytic graphite. Electrochemical plating conditions and the electrode pretreatment procedures that provided for the most uniform and strongly adhering metal film deposit are described. The procedures employed make use of both potentiometric and amperostatic electrochemical deposition techniques. The linearity of plating deposits, as a function of time, concentration, and current, is discussed together with other parameters affecting the deposition. With the use of operational amplifiers in the control circuits, the film deposits prepared showed an internal precision that was when checked by a within *2% number of independent methods, including activation analysis. Potential application of such quantitatively prepared disks to neutron flux monitoring and to other analytical methods i s discussed.
E
and neutron activation analysis (S-LI) are two of the common methods employed in trace analysis (8). Results of both of these techniques demonstrate satisfactory precision and accuracy in the microgram and, in some cases, even in the nanogram range. Also, by combining the quantitative aspect of electrochemical techniques in the trace regions with the qualitative identification abilities of NL4, an efficient system can be devised for handling, characterizing, and determining small quantities of metals (11, 1 2 ) . As very few trace analytical methods LECTROCHEMICAL TECHNIQUES
are used strictly on a n absolute basis, it is generally necessary to use some known amount of material as a comparator sample. I n the present study, known amounts of high purity metal were electrodeposited on a substrate of pyrolytic graphite. This high purity carbon material is relatively inert to thermal neutron activation, high temperatures, and to both chemical and electrochemical attack. These properties, coupled with its inherent mechanical strength, cause the prepared samples to behave essentially as a thin film of metal which can be employed as a standard comparator with various typesof trace methods. Furthermore, in this form the metal films are easily handled. Metal films (in milligram to microgram quantities) of four elements were prepared. Gold, silver, cobalt, and copper were chosen for this study and in no way represent the only metals with which this technique is applicable. As virtually every metal in the periodic table and a number of nonmetals can be electrodeposited from solution (1 , S ) , this method of quantitatively preparing trace amounts of metal films could be extended to other elements not specifically mentioned in this paper. Pyrolytic graphite in the form of a thin disk is employed as the electrode substrate. Because of its high purity, only a low amount of activity can be induced in the disk itself upon thermal neutron bombardment (9, 11). Therefore, the amount of metal deposit can be readily calculated and independently checked by S A A . The size and shape of the disks are identical and lend conformity to counting techniques as far as geometry is concerned.
Both conventional ampero3tatic and potentiostatic electrodeposition techniques were employed. Potentiostatic techniques were used when ,>mall amounts of metal (microgram) were to be deposited from dilute solutions. L-nder these solution conditioiis the background current n-ould he an allpreciable portion of the total current in an amperostatic deposition, thus limiting the quantitative reliability of the deposition. Potentiostatic deposition of microgram quantities of known and reproducitle amounts, however, have been reported by several investigators (2, 7 , 14-1 6 ) . EXPERIMENTAL
Reagents. -111 chemicals used were analytical reagent grade and further purified by standard methods before use. The calcium chloride, employed as the supporting electrolyte in the cobalt, gold, and copper electrodepositions, was purified by three recrystallizations from acidic aqueous solution. Purification \vas aided by the addition of S a O H t o t h e solution until precipitation of calcium hydroxide. This, in turn, coprecipit'ated the heavy metal impurit'ies as hydroxides. After the final filtration and recrystallization, the calcium chloride was dissolved and an excess of calcium carbonate n-as added t'o buffer the solut,ion at' p H 6. The resulting solution \vas then electrolyzed wit'h stirring under a constant current of 50 to 100 pa. for a period of 48 hours. Pyrolytic graphite (large surface area) was employed as both the anode and cat'hode. The anode was in a separate solution and made electrical contact through a salt bridge of the type described below. After electrolysis, the solution was again filtered VOL. 37, NO. 13, DECEMBER 1 9 6 5
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