REPORT FOR ANALYTICAL CHEMISTS
I FELT THIS BIG!
Atomic Weights Determined by Harvard Methods
(Taylor Comparators brought me back to normal) Have you ever tried to explain how things got out of whack because the comparator you selected wasn't right for the job? Making tests as often as we do, I should have known a comparator with fragile, individual color standards wouldn't last long. Somebody was bound to drop two or three of them. But, we solved the problem by buying Taylor Comparators. Taylor Comparators—with accurate, guaranteed non-fading color standards in a single, easy-to-use slide— give you fast, accurate, on-the-spot tests for p H , chlorine, silica, phosphate, chromâtes, nitrates, etc. You get dependable data for controlling chemical processing, water purification, waste treatment.
ALSO AVAILABLE Taylor Water Analyzer for colorimetric water analysis, Taylor Midget Iron Tester.
ALL TAYLOR COMPARATORS HAVE GUARANTEED N O N FADING COLOR STANDARDS SEE YOUR DEALER for T a y l o r S e t s or i m m e d i a t e replacement of Bupplies. FREE HANDBOOK, " M o d e r n p H a n d Chlorine C o n t r o l " . G i v e s t h e o r y a n d application o f p H control. I l l u s t r a t e s a n d describes f u l l T a y l o r line.
W. A. TAYLOR
AND CO.
7306 YORK RD. · BALTIMORE-4, MD. Circle No. 96 on Readers' Service Card
2 8 A · ANALYTICAL CHEMISTRY
Ag
Ce
Ga
Μη
Ra
Ti
AI
CI
Gd
Mo
Rb
Tl
As
Co
Ge
Ν
Sb
Tm
Β
Cr
Hf
Na
Se
U
Ba
Cs
Hg
Nb
Si
V
Be
Cu
Ho
Nd
Sm
W
Sn
Y
Bi
Dy
In
Ni
Br
Er
Κ
Ρ
Sr
Yb
C
Eu
La
Pb
Ta
Zn
Ca
F
Li
Pr
Te
Zr
Cd
Fe
Lu
Pt
Th
pears to be satisfactory for chloride analyses completed at room temper ature (R-end point) but has been a potential source of error in two other cases: chloride analyses in which the analytical solution was cooled to ice temperature (I-end point), and bromide analyses. T h a t t h e results of these last two types of analyses are not seri ously in error is probably due to two different reasons. I n the case of chloride analyses at I-end point there is reason to suppose t h a t the analyses were completed in such a way t h a t the analytical solutions contained colloidal AgCl which went back into solution in the neph elometric test samples, thereby raising the concentration of AgCl in these samples to a value not too dif ferent from t h a t of analytical solutions at the R-end point. I n the case of bromide analyses the seriousness of t h e error resulting from the acceptance of equal-opalescence as the true end point is min imized by the extreme sensitivity of the end point, which arises from the very low solubility of AgBr. To complete this discussion of the nephelometric end point, it is important to mention t h a t Johnson (1) has developed an alternative procedure, known as the " s t a n d a r d solution end point" which has def inite advantages over the equalopalescence end point. Unfortu nately, Johnson is the only investi gator to have made use of this alter native procedure in atomic weight analyses.
Anomalies in the Nephelometric Titration Step In the H a r v a r d titration method the analytical system appears to be simple, consisting of A g X precip itate in equilibrium with its ions. T h a t the system is actually more complex t h a n this is indicated by two kinds of irregularities or anom alies which we review below. T h e first of these anomalies is the "drifting end point" phenomenon which can be described as follows. In the analytical procedure, shortly after the precipitation of AgX, the relative concentrations of Ag+ and X~' in the supernatant solution arc determined nephelometrically and an a t t e m p t is made to bring the solution to the equal-opalescence end point by adding the estimated amount of the deficient ion. Several days later the nephelo metric test is repeated and another addition of Ag+ or X ~ is made, if indicated. This process of nephelo metric test and addition of ion is continued until the equal-opalcscence end point is reached or crossed. In the usual, normal anal yses the response of the analytical solution to the initial addition of Ag+ or X ~ is what one would ex pect. I n some cases, however, the observed response is irregular and m a y even be in the opposite direc tion from t h a t expected. I t is this phenomenon of irregular response to additions of the deficient ion, as observed by nephelometric tests, which is known as the drifting end point.
