Sept., 19581 BROWN AND WEIR 491 A Completely Automatic Titration Unit for Process Use BY J. F. BROWN AND R. J. WEIR (Inaperial Chemical Industries Ltd., Billingham Division, Research Department, Billinglzam, Co. Durham) A completely automatic titration unit for continuous operation on a chemical plant is described. The unit performs all operations of the titration, Le., it takes the sample, delivers a measured volume into the beaker, adds a measured volume of diluent, fills the burette, titrates the sample to a pre-set pH end-point, reads the burette, records the reading on a strip-chart recorder, drains the beaker and repeats the sequence after a set time interval. The flow of liquids into the various vessels is controlled by solenoid- operated glass valves, and volumes of liquid are measured by using level electrodes.The sequence of operations is controlled by a Uniselector, the completion of any operation being used to move the Uniselector to its next position. The pH end-point unit stops the titration a t a pre-set end- point. A special circuit causes the reagent to be added in small increments as the end-point is approached. The level in the burette after the titration is measured by means of a photocell and lamp unit, which is driven up the burette until the meniscus is detected. The distance the unit has travelled, i.e., the level of reagent in the burette, is indicated on a strip-chart recorder. Laboratory tests on the unit give a standard deviation of k0.04 ml in a titre of 10 ml with N acid and N alkali. The unit has been successfully operated on the plant for periods of several months.A standard model has been designed and its commercial manufacture undertaken by Electronic Instruments Limited, Richmond, Surrey, under the name “Titromatic Analyser.” FROM a survey made in 1955 of process analytical instrumentation requirements, it was apparent that many problems could be solved satisfactorily by using a completely automatic titrator. Although several semi-automatic titrators had been developed for laboratory use,1~2J~4 no completely automatic unit of high accuracy suitable for plant use was available. The instrument described was developed to fill this gap ; it operates completely automatically, and records the results of titrations on a strip-chart recorder. It was tested in a prototype form under plant conditions, running for 24 hours each day for several months on the following duty- To sample a 14 to 20 per cent.solution of ammonia in water. Sample volume 1.30 ml, diluted with 100 ml of water before titration. To titrate with N sulphuric acid to an end-point of pH 4. Titre 10 to 15 ml. Titration frequency 7 or 8 per hour. It was found that results were accurate to kO.1 per cent. for the range of ammonia concentrations encountered. Details in the description refer to the prototype model, but the commercial version, the Titromatic Analyser (see Fig. 1) marketed by Electronic Instruments Limited of Rich- mond, Surrey, is very similar. GENERAL DESCRIPTION A successful automatic titrator must be able t o replace the whole routine analytical procedure and produce as accurate an answer.Hence, the unit must perform all the operations of the titration, i.e., take the sample, deliver a measured volume into a beaker, dilute the sample as required, titrate, stop the titration at the desired end-point, read the burette, record the reading, drain the beaker and repeat the sequence after a set time interval. The accuracy and reproducibility of the results must be at least equal to those of the normal process-laboratory method, i.e., better than 1 per cent. of the amount present. The instru- ment must be capable of operating continuously and reliably under plant conditions with the minimum of maintenance. It is also desirable that the unit should operate as far as possible on principles that are readily understood and followed by the plant personnel in charge.Hence, all-glass burettes, pipettes and valves are used and the level in the burette is “observed” by a photocell. Volumes are measured between two liquid levels in a vessel, these levels492 BROWN AND WEIR : A COMPLETELY AUTOMATIC [Vol. 83 being defined either by a suitable overflow or, more usually, by two electrodes. The sequence of operations follows the normal laboratory practice, and a series of indicator lights permits the course of the titration to be followed. The unit consists of three parts (see Fig. IZ), as follows- (a) Titrator unit. ( b ) Control unit. (c) Recorder. The titrator unit houses the various parts with which the titration is carried out, i.e., the It includes an The control burette, pipette, beaker, electrodes and associated electrical equipment.electrical distribution unit, which links the titration unit to the control unit. b Sample h c Valve t Sample Burette- n ( i i= 1 Valve 5 1 3 c =J Diluent pipette \ galve 3” Valve ’ b 3 Valve Valve 3ii””’ Valve Fig. 3. Diagram of the automatic titrator unit comprises two parts, the sequencing unit, which controls all the operations in the titration, and the end-point detector. Finally, the result of the titration is presented on a strip-chart recorder. The titrator is shown diagrammatically in Fig. 3.492 BROWN AND WEIR : A COMPLETELY AUTOMATIC [Vol. 83 being defined either by a suitable overflow or, more usually, by two electrodes. The sequence of operations follows the normal laboratory practice, and a series of indicator lights permits the course of the titration to be followed.The unit consists of three parts (see Fig. IZ), as follows- (a) Titrator unit. ( b ) Control unit. (c) Recorder. The titrator unit houses the various parts with which the titration is carried out, i.e., the It includes an The control burette, pipette, beaker, electrodes and associated electrical equipment. electrical distribution unit, which links the titration unit to the control unit. b Sample h c Valve t Sample Burette- n ( i i= 1 Valve 5 1 3 c =J Diluent pipette \ galve 3” Valve ’ b 3 Valve Valve 3ii””’ Valve Fig. 3. Diagram of the automatic titrator unit comprises two parts, the sequencing unit, which controls all the operations in the titration, and the end-point detector.Finally, the result of the titration is presented on a strip-chart recorder. The titrator is shown diagrammatically in Fig. 3.492 BROWN AND WEIR : A COMPLETELY AUTOMATIC [Vol. 83 being defined either by a suitable overflow or, more usually, by two electrodes. The sequence of operations follows the normal laboratory practice, and a series of indicator lights permits the course of the titration to be followed. The unit consists of three parts (see Fig. IZ), as follows- (a) Titrator unit. ( b ) Control unit. (c) Recorder. The titrator unit houses the various parts with which the titration is carried out, i.e., the It includes an The control burette, pipette, beaker, electrodes and associated electrical equipment. electrical distribution unit, which links the titration unit to the control unit.b Sample h c Valve t Sample Burette- n ( i i= 1 Valve 5 1 3 c =J Diluent pipette \ galve 3” Valve ’ b 3 Valve Valve 3ii””’ Valve Fig. 3. Diagram of the automatic titrator unit comprises two parts, the sequencing unit, which controls all the operations in the titration, and the end-point detector. Finally, the result of the titration is presented on a strip-chart recorder. The titrator is shown diagrammatically in Fig. 3.Sept., 19581 TITRATIOS UNIT FOR PROCESS USE 493 SOLENOID VALVES- The flow of all liquids in the titrator is controlled by solenoid-operated valves similar to those used on an automatic Fischer titration unit previously de~cribed.~ Two types of valve are used, a straight pattern and a side-arm pattern.One straight valve and one side- arm valve together form a unit, the straight valve performing the filling operation and the side-arm valve the emptying operation. The design is such that, by locating the side-arm near the valve seat, the “dead” volume is reduced to a minimum (see Fig. 4). , . I pliii i n c h e s 4 I I I I j I T’ 5 + inch Fig. 4. Solenoid valve with side-arm Common earth contact ‘1 Sample “make“ Sample I ‘ break *’ contact \I 07 If Fig. 5 . Liquid level contacts Each valve after manufacture is subjected to a life test of several hundred operations to ensure that no leaking or sticking occurs. This design has proved to be most satisfactory, and it has been found that the valves tend to grind themselves in with continued operation to give a very good seat.LEVEL-MEASURING ELECTRODES- The method used to deliver accurate volumes of sample and diluent is by measuring between two levels. The simplest method is to have an overflow at the upper level and com- pletely drain the vessel, but complete draining proved to be unreliable and difficult to incor- porate in the automatic system. The apparatus therefore has an overflow upper level in the sample pipette so that flushing of this pipette is possible, but all other fixed levels are defined by the use of two level electrodes (see Fig. 5 ) . The circuit between these electrodes is either made or broken by the liquid level corre- sponding either to filling or emptying operations. The electrodes are of platinum wire sealed into glass tubes with about f inch of wire exposed.The glass tubes are supported in a detachable polythene cone-shaped plug, which fits into a corresponding glass-cone joint on the measuring vessel. Each pair of electrodes is fed through a variable series resistor from a low-voltage a.c. supply, the resistor being adjusted to allow for the variation in different liquids, etc. When the electrodes are uncovered, there is some leakage current, which, in the Leakage rates of less than 0.5 ml per day are normally achieved.494 BROWN AND WEIR: A COMPLETELY AUTOMATIC [Vol. 83 design described above, is equivalent to a resistance of 10,000 to 100,000 ohms, but, when they are both in contact with liquid, the resistance falls to 100 to 10,000 ohms, depending on the disposition of the electrodes and the composition of the liquid.This resistance change is ample to operate an electrical control circuit actuating the solenoid valves through which the liquid flows. The speed of response of the electrical circuit and the valves is such that, for moderate flows of the order of 3 litres per hour into or out of the measuring vessel, the level can be defined by the electrodes to becter than 0.01 ml. By using slower rates of flow, greater accuracy can be obtained. DILUENT PIPETTE- Owing to the small volume of sample used, it is necessary to dilute it to about 100 ml with water before the titration is carried out. The pipette is shown in Fig. 5. Both upper and lower levels of this pipette are defined with level electrodes, and the pipette delivers 120ml of water from a constant-head tank, but could be used to deliver a second reagent instead of water if a back titration were necessary.SAMPLE PIPETTE- For the titration of approximately 10N ammonia solution, a sample volume of about 1.5 ml was required, which was to be measured with a reproducibility of better than 1 per cent. and to be representative of the sample stream with a minimum of sample lag. In order to reduce sample lag and ensure complete flushing of the pipette and sample lines with fresh sample, an overflow is used instead of electrodes to define the upper liquid level. Its method of operation is as follows. The sample flows through the pipette via the solenoid valve and out at the overflow for a definite time interval. This time interval is set to give the required number of titrations per hour, but it must be at least long enough to ensure adequate flushing of the sampling lines.At the end of this time interval, the solenoid valve shuts and the excess of liquid in the pipette drains off to give a fixed upper level. The lower level is de- fined by two level electrodes. The volume delivered by this pipette was found to be about 1.30 ml; the exact volume could be varied by altering the position of the level electrodes. Tests on this pipette system indicated that the volume delivered had a standard deviation of better than 10.005 ml. Other pipettes of larger volumes have been designed and can easily be fitted to the titrator as required. BURETTE- The burette used is an NPL grade A Pyrex-glass burette of 25-ml capacity fitted with a B7 cone joint, in place of the normal tap, for attaching to the solenoid valve.The upper level to which the burette is filled is determined by two level electrodes of the usual type. The reagent is fed to the burette from a 10-litre vessel fitted with a constant-feed device of the Marriotti type. Burettes of other capacities, or precision-bore tubing, can replace the existing burette to permit other titrations to be carried out. BEAKER AND ELECTRODE SYSTEM- The beaker is made of polythene and has a glass siphon tube let in through the side, the lower end almost touching the bottom of the beaker. The draining of the beaker through the siphon tube and solenoid valve is controlled by two level electrodes of the usual type. By careful positioning of the level electrodes, stirrer and siphon tube, the amount of liquid remaining in the beaker after drainage is small, and, since it has been titrated to a neutral end-point, no further flushing is required.The delivery tips of the sample pipette, the diluent pipette and the burette feed into the beaker and are located so that they are all immersed during the titration. The burette tip is located near the stirrer paddle to ensure good mixing during titration. The stirrer is of the normal glass paddle type and is driven by a modified Plessey motor, type CP88208/1, at approximately 500 r.p.m. These are a calomel electrode (Electronic Instruments Ltd., type R J23), a glass electrode (Electronic Instruments Ltd., type GG23) and a platinum-wire electrode, which earths the solution.This earthing electrode is necessary for the correct operation of the pH end-point detector in the presence of stray a.c. currents from the various level electrodes. Also located in the beaker are the three electrodes needed for pH measurement.Sept., 19583 TITRATION UNIT FOR PROCESS USE 495 pH END-POINT DETECTOR- This unit is essentially a pH meter that produces an electrical signal when the sample being titrated reaches a pre-selected pH value. The unit controls the addition of reagent from the burette via a solenoid valve. A special anticipation circuit is so arranged that, near the end-point, the reagent is added in small increments with a suitable time interval between additions to ensure that adequate mixing has occurred and the solution and electrodes are in equilibrium.This is achieved in the following way. A voltage equivalent to the desired “end-point pH voltage” is set on the instrument by means of a calibrated potentiometer. Superimposed on this “end-point pH voltage” is an “anticipatory voltage,” derived from a resistance - capacity network; the “anticipatory voltage” varies during titration. The total of the “end-point pH voltage” and the “anticipatory voltage” is termed the “trigger voltage” and corresponds to a pH voltage slightly in advance of the desired pH end-point. The “trigger voltage” is compared with the indicated pH voltage from the electrodes, and the difference is fed to an amplifier, which controls, through a relay, the solenoid valve on the burette. Provided that the indicated pH voltage is greater than the trigger voltage for longer than 0.4 second, the relay and burette solenoid valve will be energised and allow reagent to flow into the beaker.When the indicated pH voltage, falls below the trigger voltage, the relay is de-energised and the burette solenoid valve is closed, thus shutting off the flow of reagent to the beaker. A second pair of contacts on the relay now allows the “anticipatory voltage” to decay, hence reducing the total “trigger voltage.” At some point the total trigger voltage will fall below the indicated pH voltage, and after 0.4 second will again energise the relay. The time delay of 0.4 second is introduced to ensure that conditions in the beaker have reached equilibrium before adding further reagent. The “anticipatory voltage” continues to decay during this time delay, but returns towards its original value while the relay is energised.Before the original value is reached, the reagent added to the beaker will have caused the indicated pH voltage to fall once more below the “trigger voltage” and the relay will be de-energised, shutting off the solenoid valve, and the cycle of operations will be repeated (see Fig. 6). This pattern of anticipatory voltage decay, time delay and anticipatory voltage growth will continue until, finally, the solution pH will fall slightly below the set end-point pH voltage, the anticipatory voltage will decay to zero and the titration will be completed. The completion of the titration is indicated by a pulse produced when the solution pH potential has remained below the end-point pH potential for 30 seconds, this pulse being fed into the sequencing unit.The pH end-point detector therefore contains three main parts; a pH amplifier incor- porating valves V,, V,, V, and V,, a trigger circuit incorporating valves V, and V, and a time delay incorporating valve V,, (see Fig. 7 ) .496 BROWX ASD WEIR: A COMPLETELY ACTOXMATIC [Vol. 83 As the burette is filled initially to a constant upper level (defined by the level electrodes), the volume of reagent used in the titration is accurately related to the final meniscus level. For determining this meniscus, a unit incorporating a lamp and a photocell is used. The BURETTE RJ3ADER- > > m E! > -+ 0 4 4 > 2 I t n l I I lamp throws a narrow horizontal beam of light through the liquid in the burette, which focuses the light on the photocell as a narrow vertical beam.A motor driving a lead screw raises the lamp - photocell unit up the burette from a fixed starting position. When it reaches the meniscus, the light beam is interrupted and the resulting change in photocellSept., 19581 TITRATION UNIT FOR PROCESS USE 497 current is made to give a pulse to the control unit and t o stop the motor. The number of revolutions of the lead screw necessary to raise the unit from its fixed starting position to the meniscus is used as a measure of the burette level; the lead screw is geared to a variable resistor, which is connected in circuit with a recorder. The position of the meniscus and hence the titre is indicated on a suitably calibrated recorder chart.Tests with the unit on a normal 50-ml burette indicated that it was capable of reading the level equivalent to a volume of about 0.01 ml. The burette reader consists of a head containing a lamp and a photocell (LP, and PC) with a valve amplifier and trigger circuit (V, and V6) on the sequencing unit chassis (Fig. 8). The head is driven up (or down) the burette by a 6-volt a.c. servo motor and receives its power from relay RLB (up) or RLC (down). SEQUENCING CONTROL UNIT- The approach that has been adopted in this instrument is to use the completion of any individual operation to start the next operation. All the sequencing operations are carried out by a 4-bank 25-position Uniselector, which is stepped forward by pulses from a 50-volt ax.supply and the thyratron, V, (see Fig. 8). Under normal conditions the thyratron is held non-conducting, and, when an operation is completed, the “operation completed” signal is arranged to bring the grid more positive than the cathode. This makes the thyratron conduct and the anode current passed operates the Uniselector electromagnet, thus moving the contact arms to the next position. The Uniselector has 4 banks of 25 contacts: No. 1 bank is used to switch power to an interval timer and to operate relays controlling the burette-reader motor; No. 2 bank switches power to the solenoid valves and Nos. 3 and 4 banks switch the “operation completed” signals from the level electrodes and other sensing devices to the thyratron grid. A typical sequence for a normal acid - alkali titration is described below and can be followed by reference to Fig.3 and Table I. SEQUENCE OF OPERATIONS OF THE AUTOMATIC TITRATOR FOR A NORMAL ACID - ALKALI TITRATION OPERATION 1. UNISELECTOR POSITION 1- Operation 1 represents the beginning of a cycle. The previous cycle will have left the beaker full of neutralised solution, the burette partly empty and the burette reader and recorder showing the result of the titration. During operation 1, the burette reader remains stationary, hence the follower potentio- meter and recorder remain at the position corresponding to the last titration. The active function of position 1 is to open the solenoid valve that allows the sample pipette to be filled. Power goes to this valve from the Uniselector bank Xo.2 and holds it open until the “operation completed” signal appropriate to this position is sent to the thyratron. For this particular filling operation, the signal can be derived from contacts at the top of the pipette or alternatively from an interval timer, a selector switch being provided. With the small volume used, flushing of the long sample line is required and the interval timer is used. The interval-timer circuit consists of valve V,, condenser C, and resistors R,, to R,, (see Fig. S), the resistors being chosen so that the timer can be set for intervals of 1, 2, 5, 10, 15, 20 or 30 minutes. When the pulse from this circuit at the end of the pre-set time interval is received by the Uniselector, it moves to position 2 . An electrode at the top of the pipette can be used instead of the interval timer to produce the “operation completed” signal if maximum speed is required.This consists essentially of a wire that is touched by the earthed solution in the pipette as it rises to the required level. When the wire is earthed by the solution, the thyratron conducts, thus causing the Uniselector to move to position 2. OPERATION 2 . UNISELECTOR POSITION 2- The following operations now occur- (i) Valve 1 closes, stopping the supply of sample to pipette A, which drains to a (ii) The motor of the burette-reading unit, B, is switched on and the unit begins to (iii) Valve 2 opens and the contents of the beaker empty through the siphon until the constant upper level. return to its zero starting position. The recorder pen moves with it.falling liquid level breaks the circuit between level electrodes 8.498 BROWN AND WEIR: A COMPLETELY AUTOhlATIC [Vol. 83 The main essential of these electrodes is a wire, which is uncovered by an earthed solution when the pre-determined level is reached. When the wire is uncovered by the solution, the thyratron conducts and the Uniselector moves to position 3. OPERATION 3. UNISELECTOR POSITION 3- into pipette C. when an “operation complete” signal is derived similar to that described on position 1. causes the Uniselector to move to the next position. UNISELECTOR POSITIONS 4 TO 12- Other solenoid valves controlling flows are operated on positions 4 to 12, the electronic system being identical to that used on the previously described positions 2 and 3.The odd numbers are filling operations terminated by a rising level making contact with an electrode, and the even numbers are emptying operations terminated by a falling level breaking contact with an electrode. Valve 2 shuts and valve 3 opens, allowing the diluent (water) to flow from the reservoir The level rises until it completes the circuit between level electrodes 9, This In the example being considered, the operations continue as follows. Valve 3 shuts and valve 4 opens, allowing the diluent to flow into the beaker until the This actuates This corresponds to a fill The appropriate contacts have been set to give an immediate OPERATION 4. UNISELECTOR POSITION 4- level in the pipette falls, breaking the circuit between level electrodes 10.the sequencing circuit and the Uniselector moves to position 5. operation that is not required. “operation complete” signal, and the Uniselectclr therefore moves a t once to position 6. OPERATION 5 . UNISELECTOR POSITION 6- until the level in the pipette falls, breaking the circuit between level electrodes 11. actuates the sequencing circuit and moves the TJniselector to the next position. OPERATION 6. UNISELECTOR POSITION 7- burette until the liquid completes the circuit between level electrodes 12. the sequencing circuit and moves the Uniselector to position 8. wired so that the Uniselector will move straight through to position 13. design, positions 13 to 22 are not used; the Uniselector therefore moves to position 23. Valve 4 shuts and valve 5 opens, allowing the sample in pipette A to flow into the beaker This Valve 5 shuts and valve 6 opens, allowing reagent to flow from the reservoir into the This actuates No more filling or emptying operations are required and therefore contacts 8 to 12 are However, in the OPERATION 7.UNISELECTOR POSITION 23- Valve 6 has now shut. On position 23 the relay, RLC, is energised unless the burette The Unisdector therefore waits when necessary on The “operation complete” signal reader has reached the lower limit. this position for the burette reader to reach its lower limit. is given by a “break” contact on RLC and the Uniselector moves to position 24. OPERATION 8. UNISELECTOR POSITIOK 24- Valve 7 is now put under the control of the pH end-point detector. Under normal conditions, valve 7 will now open and allow the reagent to flow into the sample. When the pH of the solution reaches the “triggering pH,” valve 7 will be momentarily shut off and then actuated intermittently so that small additions of reagent are made until the pH of the solution remains a t the set end-point for 30 seconds, when a pulse is sent to the sequencing circuit, which moves the Uniselector to position 25.OPERATION 9. UNISELECTOR POSITION 25- The motor of the burette-reading unit is now energised and t he unit rises up the burette until it reaches the meniscus; it then stops, and its position is indicated on the recorder. As it stops, the unit sends a pulse to the sequencing circuit, which moves the Uniselector to position 1 to start the next cycle. This complete cycle takes about 5 minutes j&us the interval set on the timing circuit, during which the sample is flushing through the sample pipette.Valve 7 is now shut and is no longer under the control of the pH meter.Sept., 19.581 TITRATION UNIT FOR PROCESS USE 499 The complete sequence of operations and the corresponding Uniselector positions are shown in Table I. TABLE I SEQUENCE OF OPERATIONS Uniselector position 1 2 3 4 5 6 c 8 9 10 11 12 13 to 22 23 24 25 Operation Fill sample Empty beaker Fill diluent Empty diluent Fill (not used) Empty sample Fill burette Empty (not used) Fill (not used) Empty (not used) Fill (not used) Empty (not used) Motor Uniselector Wait for burette motor down Titrate Read burette Operation No. 1 2 3 4 5 6 - - - 7 8 9 Burette reader Motor down Motor down Motor down Motor down Motor down Motor down Motor down Motor down Motor down Motor down Motor down Motor down Motor down Motor up - - CONSTRUCTION OF THE AUTOMATIC TITRATOR As mentioned on p.492, the complete titrator consists of three units. The titration unit is mounted in a metal cabinet of the type made for electronic equip- ment, but with a Perspex front. Inside the cabinet a framework supports the various units. There are three main units; the sample unit, the diluent unit and the burette unit, each having two solenoid valves with the associated glassware and level electrodes. Inlet and exit lines for the liquids are mounted on a board at the bottom of one side of the unit. There are lines for sample, diluent and reagent and two drain lines, one from the unit and one from a polythene tray in the base of the cabinet in case of overflow.In addition, there is an air inlet so that the cabinet can be purged with air. Sample and diluent pass through ceramic- disc filters before passing to the appropriate valves. The burette-reading unit is mounted behind the burette. At the back of the cabinet is the distribution box, which contains the adjustable resistors associated with the level electrodes and the subsidiary relays for the motor of the burette- reading unit. An indicator panel is mounted at the front, which shows, by means of neon lights, the operation in progress. There is also a switch that can be used to control the solenoid valves manually when setting up and a press button for stepping the Uniselector on one position at a time.The titration unit is connected through 15 yards of multicore cable to the control unit. This consists of the sequencing and pH units, which are mounted in standard 19-inch G.P.O. rack-type chassis. The control unit is connected to the recorder through 150 feet of cable. Both the control unit and the recorder can be purged with air for plant use. OTHER APPLICATIONS OF THE AUTOMATIC TITRATOR The apparatus described carries out a straightforward titration with a measured sample, a measured diluent and a pH end-point. I t is designed so that modification for other duties is a simple matter, and, in order to illustrate this point, the modification necessary for a particular titration is described below. The example chosen is a titration in which the pH of the diluent varies and must, therefore, be neutralised before the normal operations start.The extra operations that must be brought in are “fill burette” and “titrate,” and they must come after the “empty diluent” operation, but before the “fill burette” for the true titration. Table I shows that operation 4 is normally the “empty diluent vessel” operation, and therefore Uniselector position 5 may be used for the extra “fill burette” operation. It was previously unused, so this calls for no other change. The new “titrate” operation then has500 BROWN AND WEIR: A COMPLETELY AUTOMATIC [Vol. 83 to be fitted into position 6, and since this was previously used, some changes must be intro- duced. The changes are quite simple; each subsequent operation is moved up two places, Le., position 6 goes to position 8 and position 7 to position 9. Both these positions were unused in the original arrangement.ADJUSTMENTS OF THE AUTOMATIC TITRATOR Switches are provided so that the apparatus can be changed from automatic to manual operation for testing and fault tracing and also when setting up contacts and adjusting control resistors. The only adjustments needed in the electronic equipment are to the resistors, of which there is one for each level-electrode assembly. Adjustment is necessary to allow for the different conductivities of the liquids being controlled and for variations in leakage resis- tance. The method used is to set the level b,y hand to a point just before the “operation completed” signal should be given and to note the range of adjustment over which a false signal appears.The level is then altered by hand until it reaches the point at which the signal should be produced, and then the range of adjustment is again tested. The part of the range over which the signal fails to appear is also unsuitable. The part of the range not rejected by either test is satisfactory and the adjustment is best set in the middle of it. This part of the range must be marked as unsuitable. REX LTS After preliminary trials, the apparatus was tested continuously on constant samples in the laboratory for more than 1 week. The apparatus performed eight titrations per hour, a total of over 1000 titrations. Since, over long periods of the run, the sample composition was constant, it was possible to measure the relxoducibility of the complete instrument from the recorder chart.Two periods of steady conditions were taken and the titration figures measured on the recorder chart. The recorder had been calibrated in terms of millilitres of reagent added. The results from the two runs can be summarised as follows- Run No. 1: 11.45 a.m. to 5.00 p.m., November 22nd, 1955. 44 __ Number of titrations . . -- Standard deviation . . . . -- -- rt0.20 3 k0.043 ml Coefficient of variation . , == h0.35 per cent. Run No. 2: 12.30 p.m. to 5.15 p.m., November 24th, 1955. Number of titrations . . _- -- Mean recorder reading . . -_ -- 56.0 3 12.03 ml Standard deviation . . . . __ -- 1 0 . 2 2 = f0.040 ml Coefficient of variation . , == 1 0 . 3 8 per cent.Mean recorder reading . . -_ -- 52.7 = 11.32 ml 44 The response time of the instrument was also studied. With no flushing of the sample lines, a change in sample strength was not registered for about 50 titrations. This is because the sample lines have a volume of about 100ml and the sample pipette takes only about 2 ml per sample. When, before taking a sampk, the sample pipette is flushed for 1 minute at a flow rate of about 100 ml per minute, a change in sample strength was recorded in the second following titration and a steady reading at the new value was obtained after four titrations. With a 2-minute flushing period, a 75 per cent. change was recorded in the first following titration and a steady reading was obtained after three titrations. For continuous running on the plant, continuous flushing of the sample lines was incorporated and a further 1 minute’s flushing of the actual sample pipette.Under these conditions, 90 per cent. of the change was indicated in two readings and a steady reading was obtained after the third titration. Use of the instrument on various duties on the plant has shown that, with routine main- tenance of reagent, filters, recorders, etc., periods of several months of reliable operation can be achieved (see Fig. 10). In fact, instrumental runs were in most instances terminated by plant shut-downs rather than instrumental failures. The electronic and mechanical parts have proved to be most satisfactory, and only minor alterations in design and lay-out were necessary to convert the prototype instrument into the model now being marketed by Electronic Instruments Limited.Sept., 19581 TITRATIOX UNIT FOR PROCESS USE Fig. 10.Recorder chart 50 = 14.5 per cent. of ammonia 60 = 17.4 per cent. of ammonia Calibration- APPENDIX LIST OF COMPONENTS FOR pH END-POINT DETECTOR (Fig. 7) 1.8-megohm high-stability resistor. 27,000-ohm resistor. 1-meghom resistor. 10-ohm f-watt resistor. 22,000-ohm resistor. 1-meghom high-stability resistor. 2.2-megohm resistor. 2.2-megohm high-stability resistor. 39,000-ohm wire-wound resistor. 1800-ohm wire-wound resistor. 47,000-ohm wire-wound resistor. 100,000-ohm helical potentiometer (10 turn). 220,000-ohm potentiometer. 10-megohm resistor. 560,000-ohm resistor. 1.8-megohm resistor. 501502 BROWN AND WEIR: A C,OMPLETELY AUTOMATIC = 33,000-ohm &watt.resistor. = 1000-ohm resistor. = 33,000-ohm 7-watt. resistor. = 22,000-ohm 10-watt resistor. = 33-megohm resistor. = 470,000-ohm resistor. = 22-megohm resistor. = 0.005-pF condenser, 3000-volt working. = 0.001-pF condenser, 750-volt working. = 0.25-pF condenser, 350-volt working. = I-pF condenser, 3ljO-volt working. = 0.05-pF condenser, 350-volt working. = 32-pF condenser, 450-volt working. = 2-pF condenser, 3ii0-volt working. = 8-pF condenser, 450-volt working. = 0.02-pF condenser. 400-volt working. = 20-pF condenser, 1.50-volt working. = hfE1400 valve. = EF37A valve. = ECC83 valve. = ECC81 valve. = EZ80 valve. = 108C1 valve. = CC7L valve. = N8/15 metal rectifier. = Plessey CZ49112j6 12-pin fixed plug. = Belling - Lee L764 socket.LIST OF COMPONENTS FOR SEOUENC[NG UNIT AND DISTRIBUTION UNIT (figs. 8 and 9) = 100,000-ohm pre-set potentiometer. = 5600-ohm resistor. = 2.2-megohm pre.-set potentiometer. = 6800-ohm resistor. = 220-ohm resistor. = 1800-ohm resistor. = 470-ohm helical potentiometer (10 turn). = 33-megohm resistor. = 10,000-ohm resistor. = 47,000-ohm resistor. = 47-megohm resistor. = 15-megohm resistor. = 22-megohm resistor. = 33,000-ohm resistor. = 220,000-ohm res:stor. = 1-megohm resistor. = 10-ohm resistor. = 560,000-ohm resistor. = 4700-ohm pre-set potentiometer. = 22,000-ohm resistor. = 120,000-ohm resistor. = 180,000-ohm resistor. = 3.3-megohm resi:jtor. = 27,000-ohm resistor. = 4700-ohm resistcr. = 82,000-ohm resistor. = 100,000-ohm resistor. = 1500-ohm resistor. = 10-megohm resistor.= 330-ohm resistor. = 100-ohm resistor. = 15,000-ohm resistor. = 1000-pF Condenser. = 8-pF condenser, 450-volt working. = 20-pF condenser, 150-volt working. = 0.25-pF condenser. = 32-pF condenser, 450-volt working. = 1-pF condenser, 150-volt working. = 200-pF condenser, 20-volt working. = 2-pF condenser. = 500-pF condenser, 50-volt working. = EN32 valve. [Vol. 83PL5 PL I PL2 P L7 M L K J H G B A D C F E 3 i 1 i I-, ri sw I 2A p-- X U Z V T Y W UVJT Y t t z J A B C D E F G H J K L M P O Q T ; R “ Break ” contacts 0 ake ” contacts I I ! ! I I J 1 I J c8T 2 Fig. 8. Circuit diagram of sequencing unit (for values of components, see Appendix. 1’. 502)LP5- I7 0 * To recorder or controller (1 * Level contacts Note circuit is completed by liquid between contacts Contact resistance when closed: 100-10.000 Q Contact resistance when open: 10-I00 k i2 (depends on liquid) \ 12 \ 3 pH electrodes motor limit switches - calomel t I I I I I I I I I I I I I ;lass I I I I I I I I I I I I I I I I I I I I I I R27 IT t I I I I I I I f I I I I I I 1 , 1 I I $ I I I I I I I I I I I I I I I I I I i II; 4 Set contact I Q manual s u w 7 6 8 M N S R O P S PL8 t X U Z V T Y W A B C D E F C H J K L M N P O Q T V Y Y PL9 ? 4 A B C D E F G H 3 PL6 i I - Fig. 9. Circuit diagram of distribution unit (for values of components, see Appendix, p. 502)Sept., 19581 TITRATION CNIT FOR PROCESS USE PL.% = Z900T valve. = ECC81 valve. = KT2 valve. = 9OC1 valve. = 6.3-volt lamp. = &volt KO. 2 follow lamp. = 24-volt KO. 2 lamp. = 60-volt No. 2 lamp. = QS/5 metal rectifier. = 4 x RMO metal rectifier. = S-440-3-1W metal rectifier. = Plessey C249229j1 25-pin fixed socket. = Plessey CZ49229/0 25-pin fixed socket. = Plessey C249459/5 12-pin fixed socket. = Plessey CZ59498 fixed socket. = Plessey C259496 fixed plug. = Plessey CZ49061/1 25-pin fixed plug. = Plessey CZ49061j5 25-pin fixed plug. = Belling - Lee L764 plug. = Follow motor, Plessey type 2521/4. = Stirrer motor, Plessey type CP89208jl. = Photo-electric cell, type GS47X. REFERENCES 503 1. 2. 3. 4. 5. Haslam, J.. and Squirrell, D. C. JI., Analyst, 1954, 79, 689. Bett, N., lTock, W., and Morris, G., Ibid., 1954, 79, 607. Hawes, R. C., Strickler, A, and Petterson, T. H., Elec. N j g . , 1951, 47, 76 and 212. Malmstadt, H. V., and Fett, E. R., Anal. Chew., 1954, 26, 1348. Brown, J. F., and Volume, W. F., Analyst, 1956, 81, 308. Received March 28th, 1958