pending on the specific gravity, volatility, and column length. Charges greater than 1 ml of 1,1,2-1,2,2-trichlorotrifluoroethane flood the 6-foot column to such an extent that the methanol is not separated from the halogenated hydrocarbon, Experimental evidence indicates that the deviation from a straight line of the water curve for a 5-pl charge is due to residual water in the syringe. This would not be significant with larger bore syringes. Interferences. If it is desired to analyze water in methanol by this method, obviously another internal standard will have to be used. Ethanol or a low molecular-weight hydrocarbon (C,or C,)should suffice. C1to Cq hydrocarbons may elute in the same region as either water or the internal standard, but this can be compensated for by adjusting the operating parameters. Derivation of Calculation. The derivation of the equation used to calculate weight per cent water using an internal standard that might contain water is described below.
Dividing numerator and denominator by D, 100A
-B
*
C
B
wt % HzO in sample =
100 - D
D or
-
B 100 -
D
.B
C
100 A ---
B
(9)
- -D ’ D
which reduces to
-c
loo(;) wt
x HzO in sample
100
=
(10)
--
1
D
Given : A = area of HzO X response factor, B = area of internal water in internal standard X response factor, C = wt standard, D = wt % internal standard in sample bottle, E = wt water in prepared solution (total HeO-water in sample water in internal standard), F = wt water in prepared solution contributed by water in sample only.
(as presented in Step 7 under Analytical Technique). If the wt int. std. D is less than 0.5 %, no detectable error is introduced by use of Equation 10:
+
E= F
=
(:
6)
D>
wt
- (1 C0 . 0) D
(3)
F x HzO in sample = -100 - D ~
If the wt % water in internal standard C is less than 0.1 % (which is normal) and the wt internal standard in sample bottle D is less than 0.2% (which is normal), an error of less than 2 ppm is introduced by reducing Equation 10 to:
(5)
+
wt % HzO in sample =
100
Multiplying numerator and denominator by 100, 100 -
~
D
wt % HzO in sample
=
6) D
ACKNOWLEDGMENT
100 F
The authors thank W. E. Dunkel for his critical appraisal of the work, J. V. Zmudzinski for his assistance in the precision and accuracy studies, and M. Sachter for his assistance with the algebraic derivations.
I
Substituting Equation 4 into Equation 6,
D
*
wt % HzO in sample =
(4)
=
100 -
By multiplying numerator and denominator by the wt internal standard D,Equation 10 becomes
Using a common denominator for fractions within brackets,
% H20 in sample
=
(5) - c D
F = D(:-&,)*
wt
Z HzO in sample
D,
Pulling out the common multiple D,
wt
100
(!!?!!F?) 100
-D
*
(7)
RECEIVED for review August 21, 1969. Accepted December 5, 1969.
Correction
A New Approach to the Solution of Electrochemical Problems Involving Diffusion In this article by Keith B. Oldham [ANAL.CHEM.,41, 1904 (1969)], there is an editorial error in the second sentence. The sentence should read as follows: “Here C(r,t)is the concentration of some electroactive species at a distance r from the 252
electrode at time t since commencement of the experiment, J(r,t) is the flux of that species and D is its diffusion coefficient.” Note that the word “since” (denoting time) has been substituted for the word “because.”
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
