INSTRUMENTATION BY
RALPH H.
MÜLLER
Photometric Autotitrators Are Useful for Repeated Identical Analyses
Figure 1 . Photometric titrator, Type PMT-1 of Radiometer A / S , Denmark
Τ F W E MAY once again use our per-"- formance criterion of analytical in struments—i.e., that index which expresses the a m o u n t of useful infor mation which can be obtained per unit time and cost, then simple automated photometric systems deserve continued attention and development. Typical a r e : (1) autotitrators which perform batch titrations with automatic indica tion or registration of the end point; (2) incremental titrations in which plots of absorbance ν s volume of t i t r a n t can afford an analytical answer, b u t may also answer fundamental questions of a stoichiometric nature. These use ful things can be done with elegant spectrophotometers, the cost of which cannot be justified in terms of the meager information which is obtained. Similarly, methods which require plottings, the calculation of intersection points, and the corrections in absorb ance for volume increase are pleasant, academic exercises in physical chem istry and will not interest the practical analyst who wants a fast result a n d one which is in accord with an analyst's notions about precision a n d accuracy. One of the most useful tools for a p praising an a u t o t i t r a t o r is a simple stop watch, the device so dear to the hearts of the time-motion experts in manage ment. Comparisons with an old bench hand who has been titrating for 20 years can be shattering. We once clocked one of those whose average was 11 seconds. Circle No. 36 on Readers' Service Card
Figure 2. Automatic Type ABU-1
T h e subject is well documented. J. B. Headridge in "Photometric Titra tions," Pergamon Press, London-New York, 1958, gives an excellent treat ment of the subject in eight chapters devoted to introduction and theory, a p p a r a t u s (commercial instruments), acid-base titrations, oxidation-reduc tion systems, complexometric titrations, precipitation reactions, coulometric ti trations with photometric end points, and miscellaneous applications. T h e thorough and stimulating t r e a t m e n t of "Automatic T i t r a t i o n s " b y J. P . Phillips, Academic Press, New Y o r k London, 1959, covers titrators of all classes with one chapter devoted to photometric autotitrators. His t r e a t ment throughout is most sensible and he stresses the point t h a t perfection has by no means been attained if the fac tors of cost, speed, high precision, and reliability are k e p t in mind. T h e vast possibilities, from the chemical point of view, are to be found in "Die komplexometrische Titration," by Gerold Schwarzenbach and H e r m a n n
buret
unit,
Flaschka, Ferdinand E n k e Verlag, Stuttgart, 1965. ( I t is to be hoped t h a t the reported loss of manuscript of the English translation is not a perma nent disaster; these eminent chemists deserve the wider audience which the English version could provide.) As Phillips points out (p. 117) . . . "At the present time, photometric ti trators are considered second only t o potentiometric in popularity and gen erality of application even though only a few instruments are yet commercially available." I t is interesting and im p o r t a n t to see continuing developments in this direction. A recent offering is shown in Figure 1, the photometric titrator, T y p e P M T 1 of Radiometer A / S , Copenhagen Ν V, D e n m a r k . T h r o u g h t h e courtesy of T h e London Company, 811 Sharon Drive, Westlake, Ohio 44145, we p r e sent some details of this instrument. Operations are indicated by a ninedigit code which corresponds to the settings of a nine-digit switch on the titrator. T h e code contains informaVOL. 4 0 , NO. 7, JUNE 1968
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91
A
INSTRUMENTATION
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THERMOLYNE CORPORATION 2555 KERPER BLVD. DUBUQUE, IOWA 52003 U.S.A. " A s u b s i d i a r y o f Ritter Pfaudler C o r p o r a t i o n " Circle No. 76 on Readers' Service Card
92 A
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ANALYTICAL CHEMISTRY
tion on the desired end point, delay of shut off, control band, direction of titration, and degree of automation. A hand operated lift conveys the sample to the measuring position and auto matically starts the stirring. Detector and filters are in the base which sup ports the titration vessel. For detec tion, four photo resistors (presumably CdS cells) are connected in a bridge circuit and a filter combination red/ green or orange/blue is used. Illumi nation is from the top, downward toward the detectors. This arrange ment is said to make operation inde pendent of the shape of the glass vessel or beaker used for the titration, presence of dyestuff, haze of the sample or mechanical impurities, such as glass splinters, dirt, etc. Upon pressing the start button, the titrator automatically checks the zero and readjusts it if haze or inherent color makes this necessary; introduces the indicator, starts the titration and stops it when the desired end point has been reached. Each step is indicated by indicator lamps which show how far the program has ad vanced. The amount of titrant is dis played digitally on the attached buret unit, the type ABU-1, shown in Figure 2. This unit refills itself with reagent from a titrant reservoir at the turn of a switch. The use of filter pairs red/green or orange/blue with the photo resistors is a simple adaptation of the dichroic beam splitter which enhances the signal resulting from the color change of the indicator at the end point. Presum ably, other combinations could be found for a variety of indicators, the selection being based upon spectrophotometrie data on each form of the indicator. Introduction of the indicator into the sample is automatic, but may also be done by hand. A built-in valve is auto matically operated from the titrator and allows a predetermined quantity of indicator to be added (0.5, 1.0, 1.5, and 2.0 ml). The indicator reservoir (above console on the left, Figure 1.) can be charged with a different indi cator or more indicator reservoirs can be kept on hand for quicker change over. In performance, typical standard deviations of 0.1 to 0.2 % are reported. The manufacturers have been careful to point out that careful checks have to be made initially to establish the best conditions for operation with respect to the highest permissible rate of titrant delivery which is consistent with an accurate termination at the end point. This is reasonable and well warranted because automatic titrations are of im portance only when a large number of identical analyses are to be performed.
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