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With in­ creased span error, through aging or u s e , a m i n i m u m acceptable error should b e selected b a s e d on the application. A suggested span error value of ± 0 . 0 6 p H p e r p H from standardization may b e selected as a point of which rejuvenation or electrode r e p l a c e m e n t may b e n e c e s s a r y . 18 N B S buffer. 4). 9, and d o e s not improve with rejuvenation, the glass bulb may b e scratched. E x a m i n e the bulb u n d e r a m i c r o s c o p e to d e t e r m i n e if this is the case.

003 p H error/°C error at 1 p H unit from standardization can b e u s e d to predict a magnitude of error. 6 illustrates the change in slope (mV/pH unit) with respect to p H . W h e n a m e t e r is set at p H 7 and has zero millivolts on its input, the t e m p e r a t u r e c o m p e n s a t o r has n o effect on the reading. It is only as the reading potential b e c o m e s greater or less than z e r o millivolts that the t e m p e r a t u r e c o m p e n s a t o r h a s an effect. T h e r e ­ fore, as the e x t r e m e s of the p H scale are a p p r o a c h e d , the greater the effect the t e m p e r a t u r e c o m p e n s a t o r has on the reading.

T h e p r o c e s s of rejuvenation may depend on the electrode u s e . F o r e x a m p l e , if the electrode h a s b e e n used in an organic com­ p o u n d which m a y h a v e coated t h e b u l b , then t h e solvent that would dissolve the c o m p o u n d would b e the first rinse. This should be followed by a rinse which would r e m o v e the previous rinsing solution b u t b e m o r e polar, for e x a m p l e , an alcohol. 3). If this cycling fails to rejuvenate the glass e l e c t r o d e , a n e w glass surface can b e generated by etching its surface.