generally , in automatic titration procedure based on 
 flow injection analysis process , the volume of the titrant 
 solution delivered has been determined considering both flow rate 
 and time interval elapsed while the titration was run 
 [ 13 ] . in principle , it has been admitted that the flow rate was constant and the time interval was a very 
 well - controllable variable [ 4 , 5 ] . in these cases , peristaltic pump has been the device widely used to propel titrant solution 
 [ 68 ] . because pumping pulsation , due to 
 peristaltic pump , could impair the precision of the delivered 
 solution volume , a synchronization strategy has been implemented 
 [ 4 , 9 ] . since peristaltic pump maintain a constant pulsation pattern , small volume of titrant solution , presenting good 
 precision , could be delivered . nevertheless , to accomplish this 
 requirement , it is necessary to resort facilities comprising 
 electronic and software [ 4 , 9 , 10 ] . furthermore , the aging of the pumping tube could affect flow rate , 
 thus this effect could become an error source . in this work , we intend to develop a device with 
 ability to deliver small aliquots of titrant solution , determining 
 also its volume with good precision and accuracy independent of 
 both time and flow rate . the usefulness of the device will be 
 demonstrated , coupling then to an automatic titration module based 
 on multicommutation process [ 11 , 
 12 ] . an 89.20.9 moll hcl solution was prepared by 
 dilution from a 1 moll   hcl solution . this solution was standardized 
 by titration using an 88.70.8 moll naoh solution , which was 
 previously standardized by titration using a potassium biphthalate 
 solution and a 0.01% ( w / v ) phenolphthalein solution as indicator . the equipment set up comprised a pentium iii 
 microcomputer furnished with an electronic interface card 
 ( pcl-711s , advantech ) , an ipc-4 ismatec peristaltic pump with 
 tygon pumping tubes , six three - way solenoid valves ( 161t031 , nresearch ) , one solenoid pinch valve normally closed ( 161p011 , nresearch ) , a homemade 12 v regulated power supply 
 to feed the solenoid valves , a homemade electronic interface   to provide the potential difference and current intensity required to drive the solenoid 
 valves , and a homemade photometric detector . the control unit comprised one photodiode ( hamamatsu , s1337 - 1010br ) , one phototransistor ( til78 ) wired as an 
 optic switch , two infrared led ( =930 nm ) , two operational amplifiers ( op07 ) , one 
 comparator device ( lm339 ) , and one transistor ( bc547 ) . the assays 
 comprising the volume meter calibration and photometric titration 
 were performed employing a software wrote in quick basic 4.5 . , it is shown a front view of the volume 
 meter , where we can see that the device is coupled to the titrant 
 solution reservoir ( tr ) through the solenoid valves v1 and v2 . the v2 was assembled to permit the container loading ( tc ) with titrant 
 solution when valve v1 was switched on . when the container was 
 filled with titrant solution , the floating device ( b ) interrupted the infrared radiation beam of 
 the optic switch ( opt ) . when the electronic interface showed in 
 figure 2 sensed the interruption of the radiation beam , it 
 generated the control signal to switch off valve v2 in order to 
 stop the stream of the titrant solution . the optic switch ( opt ) comprising an infrared 
 radiation emitting diode ( =930 nm ) and a phototransistor ( til78 ) , which was attached 
 in a metallic piece ( 550.4 cm ) with a central lead hole to permit the inserting 
 of the solution container ( tc ) , thus allowing that the container 
 was positioned between the phototransistor ( pht ) and the infrared 
 led . a hole with a diameter of 1.0 mm was drilled at the orthogonal 
 position to the lead hole in order to permit that the infrared 
 beam attained the phototransistor ( pht ) after it crosses the 
 titrant solution container ( tc ) . as it is depicted in figure 1 , an infrared 
 radiation emitting diode ( led1 ) was installed at the bottom of the 
 titrant solution container ( tc ) , thus the radiation beam emitted 
 by this led was directed towards the photodiode ( pd ) through the 
 liquid column maintained inside of the container . the liquid 
 column behaves as a wave guide and it was observed that its length 
 affected the intensity of the infrared radiation beam that reaches 
 the photodiode ( pd ) . the volume meter was developed exploiting 
 this effect . the filling of titrant solution container ( tc ) was controlled by mean of the electronic interface showed in figure 2 . prior to load the container with titration solution , 
 the system was adjusted following the steps summarized in 
 table 1 . maintaining the titrant solution container empty , the 
 signal generated by the optic switch ( opt ) was 3.8 v that was 
 applied to the no inverting input of the comparator device ( lm339 , 
 pin 5 ) . reference signal applied to the comparator input ( pin 4 ) 
 was adjusted to 3.5 v turning the variable resistor ( 20 k ) wired to it . 
 under this condition , the output of the comparator device ( pin 2 ) 
 was at a high - state condition and the power - driving device ( pw ) 
 was maintained deactivate , while the control enabling line ( ens ) 
 was at low level ( 0 v ) . when the software was run , the 
 microcomputer sent a control signal ( ttl high level ) through 
 pcl711 interface card to enable the control line ( ens , figure 2 ) . under this condition , the input line ( d0 ) of the power - driving device ( pw ) 
 was enabled to switch on valve v1 , thus the titrant solution flowed from its vessel ( tr , figure 1 ) by the gravity acceleration action to fill the titrant container ( tc ) 
 with titrant solution . while the liquid column increased into the 
 container , the floating device ( b ) was displaced toward the top . when it crossed the light beam of the optic switch ( opt ) , the 
 signal sent to the comparator input ( pin 5 ) fall lower than 3.0 v. 
 under this condition , the output signal of the comparator device 
 ( pin 2 ) was drove to the low - state condition ( 0 v ) , thus disabling 
 the line input ( d0 ) of the power - driving device ( pw ) , which 
 switched off valve v1 in order to stop the stream of titrant 
 solution . the light emitting diode ( led1 ) installed in the 
 container bottom ( tc , figure 1 ) was switched off by turning the 
 variable resistor wired to the base of the transistor ( tr1 , figure 3 ) . the signal output ( so ) of the operational amplifier ( oa2 ) was adjusted to 0 v by turning the variable resistor ( 20 k ) wired to the 
 no inverting input of this operational amplifier . afterwards , the 
 emission intensity of the led1 was raised up to the output signal 
 ( so ) attained 2000 mv . this was done turning the variable resistor 
 coupled to the base of the transistor ( tr1 ) . the flow diagram of the titration system is shown in 
 figure 4 . in this configuration , all valves are switched off , 
 thus all solutions propelled by the peristaltic pump ( pp ) are 
 directed towards their storing vessels . when valve v2 was switched 
 on , the titrant solution flowed through it . aiming to find the 
 relationship between the solution volumes delivered from the 
 container ( tc ) and the output signal ( so ) generated by photodiode 
 ( figure 3 ) , a set of assays the microcomputer 
 sent , through the output port of the pcl711s interface card , a 
 control signal to switch on valve v2 . while solution flowed from 
 the container ( tc ) , the microcomputer read the variation of the 
 signal ( so ) through the analog input of the pcl711s interface 
 card . when signal variation attained a preset value ( vs ) , valve v2 
 was switched off in order to stop the solution stream , and the 
 delivered volume of solution was measured . assays were performed 
 programming potentials difference ranging from 50 up to 800 mv . 
 after calibration stage , the valve v2 was maintained switched on 
 during a time interval of 2 minutes to empty the titrant solution container . after calibration , a set of titration assays was 
 carried out to demonstrate the usefulness of the proposed volume 
 meter , which was implemented using the flow system showed in 
 figure 4 , which was designed based on multicommutation process 
 [ 1012 ] . titration chamber ( tch ) and photometer detector ( not showed ) were similar to those 
 employed in previous work . as it is depicted in table 1 , in the standby 
 condition all valves are switched off , thus all solutions are 
 pumped back to their storing vessels ( figure 4 ) . when the 
 software was started to carry out a titration run , the microcomputer sent a control signal ( ttl high level ) through the enabling line ( ens , figure 2 ) to enable the power - driving 
 device ( pw , figure 2 ) . under this condition , valve v1 
 was switched on to fill the container ( tc ) with titrant solution . afterwards , 
 valves v5 and v6 were switched on during a time interval of 
 20 seconds ( step 3 ) to fill the flow lines up to the titration 
 chamber ( tch ) . afterwards , valve v7 was switched on during a time 
 interval of 15 seconds ( step 4 ) to empty the titration chamber . the 
 washing of the titration chamber with carrier solution comprised 
 the steps 5 and 6 ( table 1 ) , which were repeated twice to 
 assure good cleaning . following the working sequence showed in table 1 
 ( steps 7 , 8) , valves v5 and v6 were sequentially switched on during 
 preset time intervals to insert into the titration chamber 
 aliquots of sample solution ( s ) and of indicator solution ( in ) , 
 respectively . the time interval to maintain valve v6 switched on 
 was settled at 2.0 seconds , while time for valve v5 was varied from 
 3.0 up to 8.0 seconds . afterwards , valves v3 and v4 were switched on 
 during a time interval of 8.0 to insert 800 l of the carrier solution into the titration chamber . as it is shown in table 1 ( step 10 ) , 
                 valve v2 was switched on to permit that the titrant solution flowed continuously by the gravity acceleration action toward the titration chamber ( tch ) . the 
 monitoring of the signal generated by the photometer was performed 
 by the microcomputer through the analog input of the pcl711s 
 interface card . when a quick transition of the generated signal 
 occurred , the microcomputer sent a control signal through the 
 enabling line ( ens , figure 2 ) to switch off valve v2 in order to stop the stream of the titration solution . afterwards , the microcomputer read 
 the signal ( s0 ) generated by the photodiode ( pd ) of the volume 
 meter ( figure 3 ) that was saved as an ascii file . the volume of 
 the titrant solution used to find the end of titration was 
 calculated using this measurement . when the signal ( s0 ) surpassed a 
 preset threshold value , which was related to a delivered solution 
 volume of the titrant solution about 85 l , the 
 microcomputer halted the stream of the titrant solution and saved 
 the signal value related with the solution volume that was used at 
 that moment . afterwards , the volume meter - filling step was carried 
 out again and after that the titration run was reestablished in 
 order to find the end of titration . the volume meter working principle was based on the 
 attenuation of the radiation beam intensity , which was affected by 
 the height of the liquid column inserted between the 
 electromagnetic radiation source ( led1 , figure 1 ) and the 
 photodiode ( pd ) . the value of the potential difference generated 
 by the photodiode decreased when the gap between the liquid column 
 top and the photodiode ( pd ) increased . previous assays performed 
 using leds with wavelength of 470 nm ( blue ) , 650 nm ( red ) , and 930 nm 
 ( infrared ) showed that better linear range could be obtained with the infrared led . in this sense , the results commented below were obtained employing this component . intending to find the relationship between the volume 
 of the solution delivered and the potential difference generated 
 by the photodiode ( pd ) , a set of assays was carried out yielding 
 the results showed in table 2 . processing these data , we found 
 the following linear relationship : e(mv ) = 25.7 7.4 v(l ) , ( r=0.999 ) . aiming to confirm the linear relationship , 
 experiments were carried out at several days . the assays were done 
 settling different values of potential difference and no 
 significant difference was observed . every day prior to begin the first titration run , the 
 calibration stage was accomplished in order to find the parameters 
 related to the linear relationship , which were incorporated in the 
 control software to permit that the volume of titrant solution was 
 calculated when titration run was ended . intending to prove the 
 ability of the device to deliver a preset solution volume , a set 
 of experiments was performed using distilled water and results are 
 showed in table 3 . processing these data , a linear relationship 
 presenting a slope of 0.999 and an intercept tending to 0.0 was achieved . these results indicated that the system was able to determine the 
 solution volume processing the potential difference related to 
 liquid column height . furthermore , the precision of the delivered 
 volumes was better than 1.0% ( n=10 ) which was confirmed repeating the experiments 
 maintaining the preset values . aiming to demonstrate the usefulness of the proposed 
 volume meter device , photometric titration of hydrochloric acid 
 with sodium hydroxide solution was performed yielding the results 
 showed in table 4 . the volumes of the hcl solution aliquots 
 were varied from 43 up to 115 l . as we can 
 see the concentration of the hcl solution found is closed to 
 expected values . in the assays labeled as 5 and 6 the volumes of the 
 hcl solution aliquots used were 100 and 115 l , 
 respectively , thus the solution container ( tc , figure 1 ) was 
 filled twice . analyzing the results , we can see , that in both 
 cases accuracy and precision were similar to the other ones showed 
 in table 4 . the performance of the proposed volume meter shows 
 that it can be used to deliver small volume with good precision 
 independent of the flow rate and the time interval . the 
 instrumental arrangement is very simple and easy to be replicated 
 in laboratory with a few little electronic facilities . the volume meter was successfully applied in the 
 determination of the titrant solution volume in photometric 
 titration procedure of hydrochloric acid with sodium hydroxide 
 solution . albeit , the operational work range was between 5 and 105 l , the 
 appropriated combination of software and hardware were able to 
 refill the titrant solution container while the titration run was 
 in course . the volume meter device presented a very good 
 long - term stability , which was ascertained by working several days 
 and no significant variation concerning to linear response range , 
 precision , and accuracy was observed .