436 Analyst, July, 1968, Vol. 93, pp. 436440 A Semi-automatic Timed End-point Karl Fischer Titrator BY B. COPE (“Shell” Research Limited, Carrington Plastics Laboratory, Urmston, Manchester) The construction, operation and evaluation are described of a low-cost instrument for providing semi-automatic analysis of the water content of liquids and gases by means of a Karl Fischer titration. THE Karl Fischer titration is usually carried out manually by adding small volumes of titrant at regular intervals to the titrand until a given current is flowing between the indicating electrodes. If the end-point current is held for a given time, then the titration is complete. However, if the current falls below the given value during this period, then more titrant is added until a permanent end-point is reached.The instrument described does this automatically. DESCRIPTION- The instrument operates by means of a standard Karl Fischer dead-stop indicator circuit,l with a moving-coil relay replacing the micro ammeter. The moving-coil relay operates as a switch, in conjunction with a second relay, to de-activate (at the end-point), and to activate (when water is present) an automatic titrant dispenser, which dispenses a given volume of titrant into the titrand each time it is activated. End-point detector circuit D.C. power unit Burette unit End-point timer and alarm unit t +- MS 2 I + neutral 4 240 V neutral Reaction cell Fig. 1. Semi-automatic Karl Fischer water-content titrimeter circuit diagram In the end-point condition, the given interval of time allowed for the end-point is automatically measured, and an audible alarm energised when this condition is fulfilled.An impulse counter fitted to the dispenser mechanism counts the number of additions of titrant dispensed, from which the amount of water present in the titrand can be calculated. A circuit diagram is given in Fig. 1, the location of the components is shown in Figs. 2 and 3 and the components are listed in the Appendix. 0 SAC and the author.Fig. 2. Location of components, rear view [To face poge 436Fig. 3. Location of components, front view To face page 4371COPE: A SEMI-AUTOMATIC TIMED END-POINT KARL FISCHER TITRATOR 437 END-POINT DETECTOR CIRCUIT- A standard Karl Fischer “dead-stop” circuit is used, except that the micro ammeter normally used to detect the end-point current is replaced by a moving-coil relay, fitted with a “make” contact at 90pA.This relay, RLYl (Fig. l), provides a means of switching the reagent dispenser on and off via the second relay, RLY2, by breaking the electrical supply to the motor-driven syringe. THE D.C. POWER UNIT- 12-volt d.c. for the operation of the relay, RLY2, and the “Bleeptone” alarm, A. The d.c. power unit consists of a transformer, TR1, and rectifier, REC1, to provide THE BURETTE UNIT- The burette unit is made from a Fison’s automatic dispenser with several modifications. ELECTRICAL MODIFICATIONS- The control unit provided with the dispenser has been built into the burette unit as shown in Fig. 1. The microswitch, MS2, has been modified to provide an impulse source for the burette counter (Fig.1). DISPENSER-VALVE MODIFICATIONS- The dispenser valve, as supplied by the manufacturer, is made from polytetrafluoro- ethylene (PTFE) and stainless steel. However, as Karl Fischer reagent attacks the metal portion of the valve, this has been replaced by a PTFE section. PTFE inlet and outlet lines have been fitted to the valve, and full details are shown in Fig. 4. The unit is fitted with l-ml tuberculin syringe, with Luer fitting, and set to deliver about 0.05ml per stroke. n J - d ,PTFE block .Adjusting screw , ,e PTFE nozzle - t PTFE tubing I mm bore 2mm 0.d. Fig. 4. Burette valve construction438 COPE: A SEMI-AUTOMATIC TIMED END-POINT KARL FISCHER TITRATOR [A%&?ySt, VOI. 93 END-POINT TIMER AND ALARM UNIT- The end-point timer and alarm are connected as shown in Fig.1. The timer is auto- matically re-set to the given end-point time each time the burette is activated. If the burette is not activated and the timer allowed to traverse its cycle, then the “Bleeptone” alarm is activated signalling the completion of the titration. OPERATION- the mode of operation of the instrument. The following procedure for the determination of water in methanol is given to illustrate APPARATUS- .Karl Fischer semi-automatic titrimeter. Titration cell (Fig. 5 ) . Syringe, 10 pl. REAGENTS- Karl Fischer reagent (1 ml 2: 5 mg of water). Water. Reagent inlet B I9 joint 6” B 19 joint B 24 joint Nitrogen c-s & outlet Stirrer Plat i nu m electrodes Fig. 6. Karl Fischer titration cell PROCEDURE- Introduce 100 ml of dry methanol into the titration cell, switch on the stirrer and set the variable resistance, R, of the titrimeter at 125 ohms and the end-point timer at 2 minutes.Switch on cell current, S, and burette power supply, S,. Allow the titrator to operate until the “Bleeptone” alarm sounds.July, 19681 CALIBRATION- Set the burette counter at zero and add 5 pl of water to the titrand, by means of the syringe. Allow the titrator to operate until the “Bleeptone” alarm sounds, and note the burette counter reading. SAMPLE ANALYSIS- the titration cell, the burette counter reading (C, counts). COPE : A SEMI-AUTOMATIC TIMED END-POINT KARL FISCHER TITRATOR 439 5 pl of water = 5 mg of water = C counts. Set the burette counter at zero and transfer by pipette a volume of sample (V ml) into Allow the titrator to operate until the “Bleeptone” alarm sounds and note c, x 5 x 100 c x V x 1000‘ Percentage w/v of water in sample = EVALUATION- The instrument was evaluated with the above technique.Volumes of water in the range 1 to lop1 were introduced into the cell, under steady The conditions, and the number of injections of Karl Fischer reagent dispensed recorded. results are shown in Table I. TABLE I EVALUATION OF SEMI-AUTOMATIC KARL FISCHER TITRIMETER Volume of water added, 1 2 3 4 5 6 7 8 10 Pl Number of injections dispensed 6, 5, 6, 6, 6, 5 10, 10, 10 16, 15, 16, 15 21, 21 27, 27, 27, 27, 27, 26, 27, 28, 27, 27 32, 33 37, 37 44, 43, 43 54, 54 The standard deviation of the number of injections is 0.23 injection, i.e., k0.46 injection at 95 per cent. probability.As less than 1 injection is not practicable, the error of a single determination must be 0 or 1 injection. This corresponds to better-than 99.9 per cent. confidence limits. For the Karl Fischer reagent used in this evaluation (1 ml II 5 mg of water), 5-4 injections are equivalent to 1 pl of water and the coefficient of variation (99.9 per cent. + probability), based on an error of + 1 count, is given by- 100 5.4 x w where Wmg is the amount of water present in the titrand. titrand under the conditions quoted. a dilute Karl Fischer reagent should be used. For a 5 per cent. coefficient of variation, at least 4 mg of water should be present in the To determine smaller amounts of water at this accuracy, PERFORMANCE- A determination of the type illustrated takes 5 minutes, during which time the operator is available to carry out other work, such as preparation of samples.The instrument can also be used for the determination of moisture in gases. For example, in the method of Reid and Turner,2 for the determination of water in plastics, the water present in the polymer is vaporised into a nitrogen stream and swept into a Karl Fischer cell. Whereas previously the titration of the water was carried out manually, it may now be carried out automatically. The instrument has been in use in the laboratories of this Company for more than a year, and has given satisfactory service during this period. The cost of components for this instrument is about fl100 and the over-all cost fl200.440 COPE: A SEMI-AUTOMATIC TIMED END-POINT KARL FISCHER TITRATOR [Analyst, Vol.93 Appendix COMPONENTS LIST (Instrument assembly by I.C.A.M. Ltd., Northop, Mold, Flintshire) Item s1 .. .. .. s 2 .. .. .. TI and T2.. .. N1 .. .. .. TRl .. .. RECl .. * . RLY2 .. .. D1 .. .. .. PI .. .. RLYl .. .. PL1 and SOCl . . MS1 andMS2 . . v1 .. .. .. v 2 .. .. .. A .. .. .. Burette unit . . PTFE valve block Counter . . .. Timer . . .. Case .. .. Description Manufacturer Switch SPDT . . .. .. .. SwitchSPDT .. .. .. Insulated terminals . . .. Panel, neon clear, 240-volt . . Transformer, Hygrade, 240-volt 60 cycles 2 X 6*3-~0lt . . .. .. Rectifier Rec 20 . . .. . . Relay, type 1, 12-volt d.c., 120 ohms . . Diode 10 DE type, REC50A . . Potentiometer, Model A, 10 turns, 500 ohms and Duo-Dial Model RB .. Beckman, Glenrothes, Scotland S170 d.c. relay make at 90 PA, resistance 3300 ohms, Specification S170/1/457 . . 6-Pin plug and socket, Part No. P194 . . Microswitch, type HA1 . . . . . . Crouzet Ltd., Brentford, Middlesex .. .. :I . . Radiospares Limited, London, W. 1 Sangamo Weston, Enfield, Middlesex A. F. Bulgin, Barking, Essex - 1.Ei-Volt d.c. battery . . .. .. 240-Volt a.c. supply . . . . .. - Audible alarm, “Bleeptone”, 12-volt d.c. Fisons automatic dispenser includes It1 4700-ohm resistor and C1 0.4-pF con- denser . . .. .. . . . . Fisons Ltd., Loughborough, Leicester- I.C.A.M. Ltd., Northop, Mold, Flintshire Veeder-Root, Croydon, Surrey A. P. Besson Ltd., Hove, Sussex shire - Re-set vending counter, Part No. KK1441 Chronoset CF, 0-36 minutes direct-clutch model . . .. .. .. . . Technical Representations Ltd., Stock- port, Cheshire Type DA 40168 . . .. . . . . Bedco Ltd., Harpenden, Herts. REFERENCES 1. 2. Vogel, A. I., “A Text-book of Quantitative Inorganic Analysis, including Elementary Instrumental Reid, V. W., and Turner, L., Analyst, 1961, 86, 36. Received December 22nd, 1967 Analysis,’’ Third Edition, Longmans, Green & Co. Ltd., London, 1961, p. 945.