GB2145828A - Circuit arrangement for coulometric titration - Google Patents
Circuit arrangement for coulometric titration Download PDFInfo
- Publication number
- GB2145828A GB2145828A GB08421006A GB8421006A GB2145828A GB 2145828 A GB2145828 A GB 2145828A GB 08421006 A GB08421006 A GB 08421006A GB 8421006 A GB8421006 A GB 8421006A GB 2145828 A GB2145828 A GB 2145828A
- Authority
- GB
- United Kingdom
- Prior art keywords
- circuit
- indicator
- differential amplifier
- titration
- amplifier
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 238000005443 coulometric titration Methods 0.000 title claims abstract description 9
- 238000004448 titration Methods 0.000 claims abstract description 30
- 230000015572 biosynthetic process Effects 0.000 claims abstract description 3
- 230000003287 optical effect Effects 0.000 claims description 3
- 239000000243 solution Substances 0.000 description 10
- 239000003637 basic solution Substances 0.000 description 5
- 238000006243 chemical reaction Methods 0.000 description 5
- 238000005259 measurement Methods 0.000 description 4
- 150000002500 ions Chemical class 0.000 description 3
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 2
- 238000010276 construction Methods 0.000 description 2
- JGJLWPGRMCADHB-UHFFFAOYSA-N hypobromite Chemical compound Br[O-] JGJLWPGRMCADHB-UHFFFAOYSA-N 0.000 description 2
- 239000012528 membrane Substances 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- BUGBHKTXTAQXES-UHFFFAOYSA-N Selenium Chemical compound [Se] BUGBHKTXTAQXES-UHFFFAOYSA-N 0.000 description 1
- 229910021529 ammonia Inorganic materials 0.000 description 1
- 230000003321 amplification Effects 0.000 description 1
- 230000033228 biological regulation Effects 0.000 description 1
- 238000004891 communication Methods 0.000 description 1
- 238000003869 coulometry Methods 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 239000003814 drug Substances 0.000 description 1
- 230000005611 electricity Effects 0.000 description 1
- 238000005868 electrolysis reaction Methods 0.000 description 1
- 239000008151 electrolyte solution Substances 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 230000010354 integration Effects 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- 238000000034 method Methods 0.000 description 1
- 238000003199 nucleic acid amplification method Methods 0.000 description 1
- 238000005192 partition Methods 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 230000007363 regulatory process Effects 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052711 selenium Inorganic materials 0.000 description 1
- 239000011669 selenium Substances 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 238000000954 titration curve Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/416—Systems
- G01N27/42—Measuring deposition or liberation of materials from an electrolyte; Coulometry, i.e. measuring coulomb-equivalent of material in an electrolyte
- G01N27/423—Coulometry
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/416—Systems
- G01N27/42—Measuring deposition or liberation of materials from an electrolyte; Coulometry, i.e. measuring coulomb-equivalent of material in an electrolyte
- G01N27/44—Measuring deposition or liberation of materials from an electrolyte; Coulometry, i.e. measuring coulomb-equivalent of material in an electrolyte using electrolysis to generate a reagent, e.g. for titration
Landscapes
- Chemical & Material Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Health & Medical Sciences (AREA)
- Physics & Mathematics (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Molecular Biology (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Non-Biological Materials By The Use Of Chemical Means (AREA)
- Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)
Abstract
A circuit arrangement for coulometric titration comprises a measuring cell (1) with a working electrode (9) auxiliary electrode (10) and indicating electrode means (7,8) an indicator circuit (11) fed by the indicator electrode means (7,8), an electrical decoupling stage (13), a generator circuit (12) and a measured value forming device (14). A differential amplifier for the indicator signal and a power amplifier for the generator circuit are united in a single amplifier (19) and the working electrode (9) is connected directly to the output of the differential amplifier (19), the auxiliary electrode (10) being connected to earth via a precision resistor (32) and the negative operating voltage input of the differential amplifier being connected directly to earth. A start- stop discriminator (22) is provided for controlling measure-value formation in A/D integrating converter 28 and is connected at its input to a point in the circuit which carries a voltage proportional to the indicator signal and to a desired- value setting device (26,27) for the end point of the titration. <IMAGE>
Description
SPECIFICATION
Circuit arrangement for coulometric titration
The invention relates to a circuit arrangement for coulometric titration. Coulometric titration is a high-precision volumetric process which has found wide application in the automation of analytical serial testing in research, medicine, industry and agriculture.
All known circuit arrangements for coulometric titration are fundamentally composed of an electrochemical measuring cell containing the titrating and indicator electrodes, and indicator circuit, electrical decoupler stage; a generator circuit and a measured-value forming device. The end point of the titration is detected electrochemically by the indicator electrodes. For this purpose, the indicator circuit is equipped with a comparator, generally a differential amplifier, to the reference input of which, a threshold voltage assigned to the end point of the titration is applied. The output of the differential amplifier is connected to the input stager of the generator circuit via a galvanic separating stage. A signal proportional to the indicator voltage is amplified in the generator circuit.
The amplified signal controls a power stage to the output of which, the titrating electrodes are connected via a resistor. The voltage drop caused by the titrating current in this resistor is integrated in time in the measured-value forming device.
Improvements to this basic construction relate, in particular, to the control of the generator or titrating current. They are all directed to an analogue variation in the titrating current proportional to the course of the indicator signal. Thus a logarithmically controlled reduction of the titrating current in order to improve the titration speed is described in DE
OS 2 656 936. According to the DD-PS 1 22 258 and 139 765, a control of the electrolysis current in direct proportion to the indicator signal is provided while in the DE-OS 1 773 574, in addition to this, a direct adjustment of the current is proposed instead of the adjustment of the current via the voltage.
The known titration devices, which regulate the generator current depending on the indicator signal, basically switch off the generator current when a specific threshold value is reached or even permit negative generator currents as a result of their circuit construction. For this reason, no continuously acting regulation of the generator current about the end point of the titration can be carried out with these devices.
Particularly when a number of titrations are carried out on the same basic solution, that is to say without changing the basic solution, disturbing substances can cause reactions which use up volumetric solution in the measuring vessel in the intervals between titrations. The consumption of volumetric solution is seen as a slow drop in the indicator signal.
If a renewed titration is then effected in the same basic solution, then an amount of current which reverses this disturbing reaction must first be applied. Only after this can a contribution be made to the titration- of the sample. The first-mentioned current component is very noticeable as a measuring error, depending on the duration of the intervals between titrations. One example of these disturbing reactions is the reacton of selenium during the hypobromite titration of ammonia.
In the DD-PS 122 258, 139 765 and the
DE-AS 2 921 651, and after-titration is recommended as a way out. The after-titrations fulfil the purpose of avoiding gross errors and long-term drift. The source of error as a result of the disturbing reaction is not completely eliminated, however, because the result of the measurement depends on the amount of the start of the new titration in the interval between two after-titrations. The precision of the measurement result is considerably impaired as a result.
Object of the invention:
The present invention seeks to reduce the measuring error during multiple titrations with the same basic solution.
According to the invention, there is provided a circuit arrangement for coulometric titration comprising a measuring cell with a working electrode, and auxiliary electrode and indicator electrode means, and indicator circuit fed by the indicator electrode means, and electrical coupling stage, a generator circuit and a measured-value forming device, wherein a differential amplifier for the indicator circuit and power amplifier for the generator circuit are united in one amplifier, the working electrode is connected directly to the output of the differential amplifier. the auxiliary electrode is connected to earth potential via a precision resistor, and the input of the differential amplifier for the negative operating voltage is connected to earth potential and a start-stop discriminator is provided for measured-value formation and is connected, at the input side, to a point in the circuit which carries a voltage proportional to the indicator signal, and to a desired-value setting device for setting the end point of the titration.
The invention will now be described in greater detail, by way of example, with reference to the drawings, in which:
Figure 1 shows the basic circuit diagram of a circuit arrangement according to the invention, and
Figure 2 shows the courses of titrations with and without compensation for the influence of disturbing ions, according to the invention.
The measuring vessel or cell 1 is divided, by a partition 4 with membranes into two compartments 2,3 which are filled with a basic electrolytic solution 6. The polarizable indicator electrodes 7, 8 and the working electrode 9 of the pair of titrating electrodes are disposed in the compartment 3. The associated auxiliary electrode 10 is accommodated in the compartment 2. The indicator electrodes 7,8 are connected to the input of the indicator circuit 11. The titrating electrodes 9, 10 are connected to the output of the generator circuit 1 2. An optical or photocoupler 1 3 is introduced, as an electrical decoupling stage, between the indicator circuit 11 and the generator circuit 1 2. The output of the generator circuit 1 2 is also connected to the input of the measured-value former 14.
An operating voltage UA is applied to the indicator electrode 7 via a resistor 1 5. The indicator electrode 7 is directly connected to the input of a first amplifier 16, the output of which is coupled to a first contact of a switch 1 8 and a negating amplifier 1 7. The amplifier 1 7 has the amplification factor 1. It permits the measurement of the course of reverse titrations. The output of the amplifier 1 7 is connected to a second contact of the switch 18, the third contact of which is in communication with the input of the photo-coupler 1 3.
The amplifiers 16, 1 7 and the switch 18 form the indicator circuit 11.
The output of the photo-coupler 1 3 is connected to the input of a differential amplifier 1 9 and to the input of a start-stop discriminator 22. The reference input of the differential amplifier 1 9 is connected to a desired-value setting device in the form of a voltage divider comprising resistors 20, 21. The output of the differential amplifier 1 9 is connected directly to the work electrode 9 while the auxiliary electrode 10 is connected to earth via a precision resistor 32. The auxiliary electrode 10 is also connected to the measured-value input of the measured-value forming device 14.At the supply side, the differential amplifier 1 9 is connected by one input to a positive operating voltage UA and by its input 34 for the negative operating voltage, to earth. The earth potential represents the voltage reference point 33 for the auxiliary electrode 10.
The amplifier 19, the desired-value setting device and the titrating electrodes 9, 10 for the generator circuit 1 2. The start-stop discriminator 22 comprises a comparator 23, a following bistable trigger circuit 24 with delay line 25 in the reset line and a desired-value setting device for the end point of the titration. The desired-value setting device comprises resistors 26, 27 in the manner of a voltage divider. Like the desired-value setting device of the differential amplifier 19, it is connected to an operating voltage Ua The output of the bistable trigger stage 24 is connected to the control input of the measured-value forming device 14.
The principal item of the measured-value forming device 14 is an A/D converter 28 which is connected to an indicating device 29 by its output. Furthermore, the converter 28 has inputs for the connection of an ignorevalue preselector 30.
An agitator 31 is disposed in the compartment 3.
The operation of the above described circuit arrangement will now be considered:
Titration is started with the addition of the sample 5,. to be determined. to the compartment 3. The volumetric solution present in the basic solution immediately reacts with the sample 5 as a result of which the indicator current Jl, and with it the indicator signal, drop practically to zero. The decline of the indicator signal is expressed at the input of the differential amplifier 1 9 as a rise in the differential voltage which causes a generator current jq Through this, new volumetric solution is produced electrochemically and immediately reacts with the sample. The membrane 4 prevents volumetric solution from passing into the compartment 2.As the point of equivalence is approached, the concentration of the volumetric solution effective at the indicator electrodes 7, 8 again increases. With this, an indicator signal again results which reduces the differential voltage and hence the generator current J . At the point of equivalence, the differential voltage and the generator current J are zero: the titration is ended.
This state would be permanent if disturbing ions did not react with volumetric solution.
The loss of volumetric solution is compensated in the circuit arrangement, however. by a regulating process. The deadline in the indicator current J, leads to a differential voltage at the input of the differential amplifier 1 9 and consequently to a generator current
Jg. This again produces fresh volumetric solution so that the indicator current J increases again. Indicator current J and generator current Jg swing slightly about constant values.
Thus indicator current J and generator current
Jg are practically constant independently of the moment of a renewed titration; the same measuring conditions are always present. A generator current J in the other direction as a result of exceeding the threshold voltage preset by the desired-value setter 20, 21 is not possible because the voltage reference point 33 and the input 34 are connected to earth, that is to say they have the same potential.
Such a thing would disturb the equilibrium of the measuring vessel 1.
The output voltage of the photo-coupler 1 3 is compared, in the comparator 23, with the voltage of the desired-value setting device for setting the point of equivalence, which is applied via the resistor 26. In the event of equality, the output signal of the comparator 23 changes the bistable trigger stage 24 over into the other stable state, via the delay line 25, and the bistable trigger stage in turn interrupts the A/D conversion and the integration in the A/D converter 28, through a stop signal. On a renewed titration, the output voltage of the photo-coupler 1 3 again becomes higher than the voltage across the resistor 26. The comparator 23 now sets the bistable trigger stage 24 into the other state which is acknowledged with a start signal to the A/D converter 28. Disturbing ions also react with the volumetric solution during the titration. The ignore value formed as a result is fed, by means of the ignore-value preselector 30, into the A/D converter 28 and there automatically subtracted. The result of the coulometric measurement of the sample is indicated in digital form in the indication device 29.
Figure 2 shows titration curves in the form of indicator current J, and generator current
Jg -time graphs for the following three cases:
a) Titration without compensation of the disturbing influences,
b) Titration with after-titration,
c) Titration with a circuit arrangement according to the invention.
The hatched areas represent the particular amounts of electricity consumed and hence the amounts of substance analyzed.
Claims (4)
1. A circuit arrangement for coulometric titration comprising a measuring cell with a working electrode, an auxiliary electrode and indicator electrode means an indicator circuit fed by the indicator electrode means, an electrical decoupling stage, a generator circuit and a measured-value forming device, wherein a differential amplifier for the indicator circuit and power amplifier for the generator circuit are united in one amplifier, the working electrode is connected directly to the output of the differential amplifier, the auxiliary electrode is connected to earth potential via a precision resistor, and the input of the differential amplifier for the negative operating voltage is connected to earth potential and a start-stop discriminator is provided for measured-value formation and is connected, at the input side, to a point in the circuit which carries a voltage proportional to the indicator signal, and to a desired-value setting device for setting the end point of the titration.
2. A circuit arrangement as claimed in
Claim 1, wherein the differential amplifier is connected to an indicator-signal amplifier via an optical or photo coupler.
3. A circuit arrangement as claimed in
Claim 1 or 2, wherein the start-stop discriminator comprises a comparator and a bistable trigger circuit with a delay device in the reset line, one input of the comparator being connected to the output of the optical or photo coupler and the output of the bistable trigger stage being connected to a control input of an
A/D converter.
4. A circuit arrangement for coulometric titration substantially as described herein with reference to the drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DD83254382A DD217316A1 (en) | 1983-08-31 | 1983-08-31 | CIRCUIT ARRANGEMENT FOR COULOMETRIC TITRATION |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8421006D0 GB8421006D0 (en) | 1984-09-19 |
| GB2145828A true GB2145828A (en) | 1985-04-03 |
| GB2145828B GB2145828B (en) | 1987-04-15 |
Family
ID=5550092
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08421006A Expired GB2145828B (en) | 1983-08-31 | 1984-08-17 | Circuit arrangement for coulometric titration |
Country Status (7)
| Country | Link |
|---|---|
| CH (1) | CH665289A5 (en) |
| DD (1) | DD217316A1 (en) |
| DE (1) | DE3421975A1 (en) |
| FR (1) | FR2551549A1 (en) |
| GB (1) | GB2145828B (en) |
| IT (1) | IT1221752B (en) |
| SE (1) | SE457190B (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| ES2168224A1 (en) * | 2000-10-20 | 2002-06-01 | Univ Valencia Politecnica | Electro-optic procedure of analysis of electro-chemical processes in real time and device for putting it into practice |
| CN105223255A (en) * | 2015-09-08 | 2016-01-06 | 王小云 | A kind of intelligent chlorine residue fast detector |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE329023B (en) * | 1967-06-06 | 1970-09-28 | Jungner Instrument Ab | |
| DD139765B2 (en) * | 1975-11-13 | 1984-11-07 | Georg Kaltenborn | DEVICE FOR THE AMPEROMETRIC DEAD-STOP TITRATION |
| DD122285B1 (en) * | 1975-11-13 | 1980-12-24 | Georg Kaltenborn | Apparatus for amperometric dead-stop titration |
-
1983
- 1983-08-31 DD DD83254382A patent/DD217316A1/en not_active IP Right Cessation
-
1984
- 1984-06-14 DE DE3421975A patent/DE3421975A1/en not_active Ceased
- 1984-07-11 IT IT21833/84A patent/IT1221752B/en active
- 1984-07-13 CH CH3410/84A patent/CH665289A5/en not_active IP Right Cessation
- 1984-08-16 FR FR8412862A patent/FR2551549A1/en not_active Withdrawn
- 1984-08-17 GB GB08421006A patent/GB2145828B/en not_active Expired
- 1984-08-30 SE SE8404316A patent/SE457190B/en not_active IP Right Cessation
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| ES2168224A1 (en) * | 2000-10-20 | 2002-06-01 | Univ Valencia Politecnica | Electro-optic procedure of analysis of electro-chemical processes in real time and device for putting it into practice |
| CN105223255A (en) * | 2015-09-08 | 2016-01-06 | 王小云 | A kind of intelligent chlorine residue fast detector |
| CN105223255B (en) * | 2015-09-08 | 2019-04-12 | 王小云 | A kind of intelligence chlorine residue fast detector |
Also Published As
| Publication number | Publication date |
|---|---|
| DE3421975A1 (en) | 1985-03-21 |
| CH665289A5 (en) | 1988-04-29 |
| IT8421833A0 (en) | 1984-07-11 |
| SE457190B (en) | 1988-12-05 |
| IT1221752B (en) | 1990-07-12 |
| SE8404316L (en) | 1985-03-01 |
| FR2551549A1 (en) | 1985-03-08 |
| GB8421006D0 (en) | 1984-09-19 |
| SE8404316D0 (en) | 1984-08-30 |
| DD217316A1 (en) | 1985-01-09 |
| GB2145828B (en) | 1987-04-15 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |