Anal. Chem. 1993, 65, 181-182
181
A New Low Conductivity Standard Solution Truman S. Light' 4 Webster Road, Lexington, Massachusetts 021 73
Edwards S. Atwood and John Driscoll HNU Systems, Inc., 160 Charlemont Street, Newton Highlands, Massachusetts 02161
Stuart Licht Department of Chemistry, Clark University, 950 Main Street, Worcester, Massachusetts 01610
A new conductlvlty standard solutlon Is descrlbed that may be used for accurate determlnatlon of cell constants. Thls lowconductlvlty, hlgh-reslstlvlty solution Is partlcularly appllcabk for callbratlon of cells that are used for measurement In the ultrapure water domain (0.0550 pS cm-l, 18.18 MQ cm, 25 "C). The low conductlvlty Is achkved by the addltlon of 4 5 4 0 % sucrose whlch Increases vlscoslty IO-fold and correspondinglydecreasesthe conductlvlty. A recommended concentratlonIs 0.010 00 N acetlc acld and 45.00% sucrose. The conductlvlty of thls solution Is 23.06 i 0.1 pS cm-l (rerlstlvlty of 43 370 S2 cm). The solutlon 1s stable for several months and Is accurately prepared by common analytlcal methodology.
INTRODUCTION The routine measurement of low-conductivity solutions, such as ultrapure water (0.0550 p S cm-l, 18.18 Ma cm, 25 "C), is important in the semiconductor, power, and nuclear reactor industries as well as in ordinary laboratory practice.'!* Slight differences in resistivity from that of pure water may be interpreted as ionic impurities a t the fractional parts per billion leve1.3.4 For this reason, cell constants need to be accurately calibrated with an accuracy of the order of 0.1 7%. Standard solutions for obtaining cell constants are available in the ASTM literature for higher conductivity domains.5 Employing these commonly used standard solutions, a lack of constancy in cell constants has been demonstrated for cells made of materials other than platinum.lv3 The difficulty has been attributed to measurement problems inherent in the low electrical resistance of the solutions, the addition of extraneous resistance due to oxide coatings on the electrode surfaces, and other problems ascribed to alternating current errors at low resistance due to cable and electrode capacitances and shielding.6 Many of these problems are mitigated if lowconductivity, high-resistance standard solutions are made available. Search for reproducible low-conductivity organic liquids has not been successful because of the inability to control the trace concentration of ions and the variability of impurity ions. Special solutions for standardization of cells
* To whom correspondence should be addressed.
(1) Th0rnton.R. D.; Light, T. S.Ultrapure Water 1989 (July/August), 14-26. (2)Light, T. S.;Licht, S. Anal. Chem. 1987,59, 2327-2330. (3)Light, T. S.;Thornton, R. D. Ultrapure Water, 1991 (April), 5966. (4)Pate, K.T. Ultrapure Water 1991 (JaniFeb), 26-33. (5)American Society for Testing and Materials. Standard Test Methods for theElectn'cal Conductivity and Resistiuity of Water;ASTM D1125-91;ASTM Philadelphia, PA, 1992. (6)Braunstein, J.; Robbins, G. D.J. Chem. Educ. 1971, 48, 52. 0003-2700/93/0365-0181$04.00/0
used in low-conductivity applications have recently been s~ggested.3,~ In 1990NIST (NationalInstitute for Standards and Technology) introduced the 25 $3 cm-l standard7 which is composed of approximately 5.88 X M hydrochloric acid solution. However, that solution is costly and due to its low concentration is particularly sensitive to environmental and other impurities. A 14.9 p S cm-l standard suggested in 1991: composed of 10-4 M KC1, is also quite dilute, is sensitive to environmental impurities, and has a relatively large blank correction. This paper presents a procedure that permits preparation of standard solutions with unusually low-conductivity, highresistivity, that are less susceptible to environmental contaminants and have an accuracy suitable for calibration of conductivity cell constants. Aqueous solutions with macroconcentrations of sucrose are utilized. Sucrose substantially increases the viscosity of the solution and, from the relationship illustrated by the Walden product: decreases the diffusion coefficient and, correspondingly, the conductivity.
EXPERIMENTAL SECTION Apparatus. Conductivity measurementswere obtained with Thornton conductivity cells and a conductivity/resistivity meter, Thornton Model 750 LM, with resistance measurements verified with standard 0.1 % resistors. A thermometer of 0.1 "C accuracy traceable to NIST was employed. Reagents. All chemicalswere reagent grade. Deionized water was used with a conductivity of less than 1.0 p S cm-1 at 25 "C when measured in air. The sucrose employed was tested to give a low blank. The 50% sucrose solution measured 2.0 p S cm-l at 25.0 "C, which corresponds to 9 X lo4% if sodium chloride is assumed to be the ionic impurity. The 45% sucrose solution blank measured 3.0 p S cm-l at 25 "C. The potassium chloride not only was reagent grade but also was certified with an analysis of 100.0% (NIST, J. T. Baker). Procedure. Potassium chloride standard conductivity solutions were prepared as defined by ASTM procedure^.^ Acetic acid solutions, 0.010 00 M, were prepared by dilution of stock 1.000 M acetic acid (Aldrich, Catalog No. 31 859.0) and verified by titration of a 100.0-mLsample with standard 0.1OOO M NaOH to a pH 8.6 end point with a glass electrode and 10.00-mLburet. Alternatively, appropriate dilution and standardization from glacial acetic acid may be employed. Sucrose solution mixtures were prepared by weight/weight to 0.1 % relative error accuracy. RESULTS AND DISCUSSION Theory. The approximate relationship between the conductivity, K , and viscosity, 7,is known as the Walden product8 (7)NIST. Standard Reference Material, SRM 3190 (25 fiS cm-I solution, $230/500mL);National Institute of Standards and Technology: Gaithersburg, MD, 1991. (8) Nakahara, M.; Ibuki, K. J. Phys. Chem. 1986, 90,3026-3030. 0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 2, JANUARY 15, 1993
182
Table 1. Conductivity with and without Sucrose, at 25.0 OC (Values Based on Cell Constants Obtained i n Ultrapure Water As Described i n Reference 1)
Viscosity Variation of Aqueous Sucrose Solutions
(II
conductivitp (rtS cm-1) concn (M) potassium chloride
40
=
I
0.001Ooo 0.01Ooo 0.1Ooo concn (M) acetic acid 0.001Ooo 0.01Ooo 0.1Ooo
0
Flgwr 1.
20
40
60
[sucrose], % by wt Vlscoslty varlatlon of aqueous sucrose solutbns.sa-b
= constant (1) The inverse relationship between viscosity and the diffusion coefficientor conductivityis a property often used in physical chemical studies of aqueous solutions.gaJ0 The use of sucrose has proved to be a valuable device for substantially altering solution viscosity with a minimum of chemical interference. Figure 1 shows the viscosity variation in water due to the addition of sucrose9a~band illustrates both the increasing magnitude as well as the increasing rate of change of the viscosity at higher concentrations of sucrose. A 45% sucrose solution increases the viscosity of water 9-fold, while a 60% concentration increases the viscosity 58-fold. Although the Walden product is known to be an approximation,8similar decreases in conductivity are expected and found. The relations between cell constant ( K , cm-I), resistance (R, Q),resistivity ( p , Q cm), conductance (C, S)and conductivity ( K , S cm-1) is given by
50.00% sucrose
aqueous
ratio (aqueous/sucrose)
147.0 15.1 1410 146.0 13200 1390 conductivity (pS cm-') aqueous 48.3 164.6 520 conductivity
concn (M) acetic acid
aqueous
0.01Ooo
164.6
50.00% sucrose 4.26 15.7 56.6 (pS cm-l) 45.00% sucrose 23.16
9.74 9.66 9.50 ratio (aqueoualsucrose) 11.34 10.48 9.19 ratio (aqueous/sucrose)
7.11
KV
R / K = RCIK (2) Sucrose Standards. Solutionsof potassium chloridewere selected which corresponded to standard conductivity solutions described by ASTM.5 These solutions along with acetic acid of similar concentration were studied with and without additions of large amounts of sucrose. Table I shows that for a given concentration of either potassium chloride or acetic acid,the conductivityis decreased by a factor of approximately 10 with the addition of 50% sucrose (weight/weight),and a factor of 7 in the presence of 45% sucrose. Table I also demonstrates that with or without the addition of sucrose, changing the concentration of potassium chloride by a factor of 10 results in a conductivity change a little less than 10,but changing the concentration of acetic acid by a factor of 10, again with or without sucrose, changes the conductivity by slightly more than 3. Both of these results are consistent with the strong and weak electrolyte characteristics of potassium chloride and acetic acid, respectively, and demonstrate that sucrose is essentially electrochemically inert. A standard conductivity solution in the low-conductivity region is needed to obtain accurate cell constants for the several types of metal employed as electrodes in conductivity cella. Table I1 illustrates the variation in cell constants when calibration is made with standard solutions of varied resistance.l3 Minimum resistivity of several thousand o h m centimeters is apparently required to obtain cell constants p
(9) (a) Zhang, X.;Yang,H.; Bard, A. J. J. Am. Chern. SOC. 1987,109, 1916-1920. (b)Timmermane,J.InPhysico-ChemicalConstants ofBinory System; UnivereiU Libre: Brussels, Belgium, 1960;Vol. 4. (10)Licht, S.;Cammarata, V.;Wrighton, M. S. Science 1989, 243, 1176-1178.
a After subtraction of the conductivity of the water, or water plus sucrose, used to prepare the solutions.
Table 11. Measured Variation in Apparent Cell Constants (cm-l) for Conductivity Electrodes of Various Metals electrode
so1nCa
titanium rhodium platinum
0.1062 0.1106 0.1039
solnD4 0.1031 0.0995 0.1012
solnE* 0.1OOO
sucrosec
wateld
0.1OOO 0.0994 0.0986
0.0999
0.0993
ASTM solution C (0.001N KCl) and ASTM solution D (0.01N KCU.6 b Proposed solution (lo-' N KCl).3 Proposed standard, this paper, 45.00% sucrose + 0.01000 M acetic acid. d As measured in ultrapure water of resistivity 18.18 MO cm, 25 OC.1
which may be used in measurement of ultrapure water at the 18 MQ cm level. It is suggested that the solution, listed in Table I, which is composed of 0.01000 M acetic acid with 45.00% sucrose (weight/weight)is suitableas a standard conductivitysolution. The conductivityof this solution is 23.06 i 0.1pS cm-l(43 370 Q cm) at 25 OC and is suitable for determining cell constants of 0.1 or 0.01cm-' used for measurements of ultrapure water. It has proved to be fairly stable, increasing in conductivity, by about 0.2 pS cm-l, following 3 months of storage and showing no discoloration. Another sample of analytical grade Sucrose, from a source other than Aldrich, gave less stability and larger blanks and showed more rapid discoloration due to mold growth. Storage in air is not a problem since ita acidity inhibits absorption of COZ. Its conductivity is close to the lowest standard of 25 pS cm-l available from NIST? and the variation of viscosity with concentration is minimal (cf. Figure 1) so that minor errors in sucrose composition would have relatively little effect on the conductivity.
ACKNOWLEDGMENT The loan of instrumentation by Thornton Associates is appreciated. This paper has been presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy in New Orleans, March 9,1992.
RECEIVED August 10,1992. Accepted October 21,1992.
