REPORT FOR ANALYSTS
NEWEST OF
5
MICRO-MICROAMMETERS 412 Log Model indicates from
70'13 to TO''
ampere on a single six-decade scale
STABILITY, economy, and fast response are all combined in this versatile logarithmic instrument. Typical uses of the new Keithley 412 include reactor control, radiation monitoring, materials testing, and measurement of other widely varying micro currents from sources of one volt or more.
Figure 1. Comparison of pH numbers for ace tate buffer solu tions
KEITHLEY MODEL 412 able commercially, determinations of t h e e.m.f. of t h e glass-calomel assembly a n d of p H in soft solids, slurries, non aqueous media, a n d t h e like, a r e com monplace. T h e n u m b e r s obtained a r e useful for their reproducibility b u t o b viously bear hardly t h e remotest rela tion to hydrogen ion concentration. T h e propriety of calling t h e m " p H v a l u e s " is certainly questionable, al though t h e r e seems little d o u b t t h a t this practice will continue. Interpretation of pH Measurements T h e analytical chemist needs t h e answers to two i m p o r t a n t questions. T h e first is—can t h e measured p H ever be given a fundamental interpretation, a n d , if so, u n d e r w h a t conditions?— a n d t h e second—exactly h o w should t h e interpretation, when allowable, be made? T h e m e a s u r e m e n t of p H is a measure m e n t of a difference of e.m.f. If t h e l a t t e r is t o reflect t h e t r u e change of acidity, only one of t h e three possible sources of potential difference—namely, t h e glass electrode-solution interface— can change when t h e s t a n d a r d solution is replaced b y t h e " u n k n o w n . " T h e constancy of t h e liquid-junction p o tential is t h e kej- to t h e first question. F o r t u n a t e l y , t h e bridge of s a t u r a t e d potassium chloride solution is r a t h e r effective in bringing a b o u t this desired constancy. Xevertheless, t h e differ ence of potential across t h e liquidliquid interface is caused b y unbalanced diffusion of ions, a n d it can h a r d l y b e expected t h a t all u n k n o w n s will m a t c h
t h e s t a n d a r d s closely with respect t o ionic concentrations a n d mobilities. I n particular, t h e highly mobile hydrogen a n d hydroxyl ions, if present in t h e s t a n d a r d a n d u n k n o w n solutions a t dif ferent concentrations, have exagger ated effects. T h e same is t r u e of a n y ion present in high concentration or a n y molecular species in a m o u n t suf ficient t o alter t h e essentially aqueous character of t h e m e d i u m . T h e answer to t h e first question is, then, a qualified y e s : experimental p H values can be justifiably given a n inter p r e t a t i o n if (1) b o t h s t a n d a r d a n d u n k n o w n a r e dilute aqueous solutions ( < 0.2M) of simple ions, a n d (2) t h e p H is greater t h a n 2 a n d less t h a n 11, or m a t c h e s closely t h a t of t h e s t a n d a r d . T h e answer t o t h e second question facing t h e m o d e r n analyst—namely, how t o interpret experimental p H values when a n i n t e r p r e t a t i o n is allowable—is n o t readily found. I t requires first of all a n examination of t h e meaning of t h e p H s values assigned t o t h e s t a n d a r d solutions, for it is evident from t h e operational definition t h a t p H derives its character largely from p H s . One m u s t deal here with t h e great f u n d a m e n t a l problem of p H measure m e n t s : Although t h e p H measurement m a y yield w h a t is called a difference of hydrogen ion activity, aH, neither the l a t t e r n o r t h e single ionic a c t i v i t y co efficient, fa, h a s exact t h e r m o d y n a m i c significance. Hence, aH can neither be d e t e r m i n e d uniquely n o r applied exactly t o t h e chemical equilibria one deals with in calculating end points, ionic concentrations, dissociation constants,
LOG
MICRO-M1CROAMMETER
FEATURES include a single range of six decades from 10~13 to 10"7 ampere, accuracy of 0.2 decade, zero drift within 0.5 decade in eight hours, and response time of less than 2 seconds to 9 0 % of currents larger than 10 12 ampere with 5000 mmf across the input. IT'S SIMPLE to set up and use. The sole operating control is the on-off switch. It has only three calibration potentiometers, and reads out on a six-inch illuminated meter. C O N N E C T O R S furnished include a 216-volt tap for polarizing ion chambers and a single-ended 6-volt output that drives both 50-millivolt and 5-milliampere recorders. The instrument is furnished for bench or rack mounting. NEW CATALOG Β contains detailed data on the 412 and all other Keithley Instruments. A request on your company letterhead will bring your copy promptly.
K E I T H L E Y INSTRUMENTS,
INC.
12415Euclid Ave., Cleveland 6, Ohio Circle No. 17 A on Readers' Service Card, page 77 A V O L 2 9 , N O . 5, MAY 1 9 5 7
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17 A
