INSTRUMENTATION
Ilpjz •Hi
instrument abstracts
ΓΙ!
Applied Physics
Corporation/Pasadena/California
At E a s t m a n K o d a k
Gary Model 14 Spectrophotometer measures absorbance to 7.1 without correction for stray light.
Run A represents the normal check of a Fabry-Perot interference niter similar to those used in densitometry of color films where the passband and shape of the niter curve is important up to densities of 7.0. This particular filter curve showed a nonsymmetrical peak at 639 mu. For a closer examination of this peak a second expanded curve of this wavelength region, Run B, was made in super-position on the first curve. In the Laboratories of the Eastman Kodak Company interest in absorbance values over seven —less than . 0 0 0 0 1 % transmission—is more than idle curiosity. Recently, Kodak physicists, using one of their Cary Model 14 Spectrophotometers, were pleased to find they could measure densities to 7.1 with out correction for stray light. In contrast, there are numerous instances where m o n t h s of hard work were wasted because unsuspected stray light of single rnonochromator instruments caused large e r r o r s s o m e t i m e s e v e n b e l o w 1.0 a b sorbance. Double monochromators cost m o r e to design and build. But they provide advan tages that can be had in n o other way. Besides low stray light, the double rnono chromator adds the dispersion of its sep arate sections and is a r r a n g e d t o cancel severe optical aberrations, giving increased resolution. In the Model 14, a silica prism and a 1 5 , 0 0 0 l i n e diffraction g r a t i n g add t h e higher ultraviolet dispersion and the low stray light of the prism to the excellent visible and infrared dispersion of the grat ing. Each complements the other to produce e x c e p t i o n a l p e r f o r m a n c e from I 8 6 0 to 26,500 Angstroms. A l l Cary i n s t r u m e n t s are t r u l y direct reading. Freedom from stray light provides one of the most dramatic examples of what this can m e a n to a user, but there are others. S o m e of t h e u n u s u a l f e a t u r e s of C a r y Recording Spectrophotometers which con tribute to accuracy by helping avoid correc tions are listed at right.
COUNTER-DIAL WAVELENGTH SCALES-Easily
read, no interpolation, corrections negli gible for most work. MULTIPOT-CORRECTED 1 0 0 % LINES-A11 Cary
S p e c t r o p h o t o m e t e r s h a v e M u l t i p o t s for c o m p e n s a t i n g s a m p l e a n d reference cell differences, and for compensating m i r r o r unbalances which inevitably occur in time. SPECIAL RECORDER FUNCTIONS AVAILABLE -
Log absorbance recording for qualitative — quantitative analyses. Kubelka—Munk func tion recording for paper and textile dye work. Expanded absorbance or transmission scales for weak a b s o r b e r s or d i f f e r e n t i a l photometry. ACCURATE PHOTOMETERS-Slide wires accu rate to within limits of recorder readability. P h o t o m e t e r accuracy r i g i d l y tested w i t h standard filters. H i g h power p e n motor and low friction p e n c a r r i a g e . C o n s t a n t con trolled pen d a m p i n g and fast response over entire absorbance range. HIGH ACCURACY ELECTRICAL ZEROING-Spur-
ious electrical pickup reduced by careful design and testing. PHOTOMETRIC PRECISION TO STATISTICAL
LIMITS—Statistically efficient p h o t o m e t e r systems, r e a c h i n g t h e t h e o r e t i c a l l i m i t s achievable with the best m o d e r n multiplier phototubes and semiconductor photocells. C o m p l e t e specifications are a v a i l a b l e o n all Cary R e c o r d i n g S p e c t r o p h o t o m e t e r s . Write to Applied Physics Corp., 362 West Colorado Street, Pasadena, California, for Bulletin AC-29.
For further information, circle number 78 A on Readers' Service Card, page 99 A 78 A
·
ANALYTICAL CHEMISTRY
s t a n t c u r r e n t electrode (see Figure 1). A m a n u s c r i p t describing t h i s work h a s been s u b m i t t e d for p u b l i c a t i o n . H e is able to m a k e a reliable p H m e a s u r e m e n t in t h e r a n g e of 5 t o 14 p H b y using a p l a t i n u m wire c o a t e d with m e r c u r y as t h e d e t e c t o r electrode. T h i s electrode is polarized anodically by a c o n s t a n t c u r r e n t of t h e order of 1 μ&. A calomel reference electrode completes t h e cell. Bricker uses a low-voltage b a t t e r y a n d a series re sistor of large v a l u e t o s u p p l y t h e constant current. The p H meter measures t h e voltage across t h e elec trodes. , Bricker explains t h e p H response of t h i s electrode b y p o s t u l a t i n g t h e anodic conversion of m e r c u r y to mer cury oxide. T h e p H of t h e solution controls f o r m a t i o n of t h e m e r c u r y oxide a n d v o l t a g e of t h e m e r c u r y elec t r o d e . T h i s electrode covers a p H r a n g e where performance of t h e glass electrode is inferior. Current Scanning Polarography. T h e m e t e r is being used t o m a k e m e a s u r e m e n t s b y a t e c h n i q u e de veloped a t P r i n c e t o n called p o t e n t i ometric p o l a r o g r a p h y with controlled c u r r e n t scanning, or m o r e simply, cur r e n t scanning p o l a r o g r a p h y . This t y p e of m e a s u r e m e n t is r e l a t e d t o o r d i n a r y p o l a r o g r a p h y such as car ried o u t w i t h t h e Leeds & N o r t h r u p E l e c t r o c h e m o g r a p h , which imposes a linearly changing v o l t a g e on a d r o p p i n g - m e r c u r y electrode a n d records the r e s u l t a n t c u r r e n t . T h e c u r r e n t level is t h e i n d e p e n d e n t \'ariable, a n d the r e s u l t a n t v o l t a g e is m e a s u r e d . C u r r e n t scanning p o l a r o g r a p h y can be carried o u t with a solid electrode or with t h e d r o p p i n g m e r c u r y elec t r o d e . T h e original description b y A d a m s , Reilley, a n d F u r m a n e m p h a sized its applicability t o m e a s u r e m e n t s with solid electrodes [ A N A L . C H E M . 25, 1160 (1953)]. T h i s m e t h o d
gives slightly b e t t e r results t h a n does conventional p o l a r o g r a p h y , a n d it provides a m e a n s for e v a l u a t i n g t h e performance of a n electrode in a coulometric g e n e r a t i o n process. T h e electrode sj^stem in c u r r e n t scanning p o l a r o g r a p h y usually c o m prises t w o solid electrodes, which carry t h e scanning c u r r e n t , a n d a reference electrode, which p e r m i t s a n e x a m i n a t i o n of t h e p o t e n t i a l of one of the solid electrodes. T w o p l a t i n u m wire electrodes in a stirred solution are c o m m o n l y used for t h e solid elec trodes. H o w e v e r , o t h e r solid elec t r o d e c o m b i n a t i o n s can be used. T h e scanning c u r r e n t c a n be s u p plied b y a n y circuit which provides a djus ta bility a n d control of t h e cur-
