DD267331A1 - METHOD OF CONTROLLING THE ADDITION OF ACCESSION TO AUTOMATIC TITRUTIONS - Google Patents

Abstract

The procedure for controlling titrant addition in automatic titrations is applicable to automatic potentiometric titration in discrete volume increments when dosing the titrant with motorized or fixed-volume dispensers. According to the invention, an adaptation of the titrant addition to the steepness of the titration curve is achieved by a new algorithm which does not require an upper limit for the respective volume increments and allows a preview of the amount of titrant to be consumed up to the equivalence point. This makes it possible to carry out titrations in which the titrant consumption can differ by several orders of magnitude up to the equivalence point, without rebuilding the apparatus and changing the control parameters.

Description

VSn = Vrj + i-2 £ follows,

where Vg is the total amount of titrant consumed at each time as a multiple of the minimum volume increment realized in volume and titrant concentration by the dosing system.

7. The method according to claim 1, characterized in that from the wercen VSn formed in the course of the titration, the mean VSm is formed and after the presence of a certain number of VSn values, preferably of four values, the value farthest from the mean, without taking into account of the last added value and a new mean is formed.

8. The method according to claim 1, characterized in that the size of the / u metering Volumeninkrements AVx is formed in the following manner:

1 ^ = Vx AVHn wcnriAVn ^ 2 /VSj.- Vg / AVx = -i / VGjj-Vg / v / cnn AVp _n Z \ / VS ,, - Vg /

9. Method according to claim 1, characterized in that the size of the volume increments to be dosed is kept constant once AVx <A * Vsm, where A is a value between 0.001 and 0.1, preferably between 0.01 and 0, 05, is set.

For this 1 page drawing

Field of application of the invention

The invention is applicable to automatic potentiometric titration in discrete volume increments when metering the Tiiriormlttels by motor burette or fixed-volume dispenser.

It should also be applied to coulometric titrations with potentiometric measurement if the titration system allows discontinuous production of the titrant.

Characteristic of the known state of the art

It is well known to those skilled in the art that the introduction of computer-controlled dynamic titration techniques has provided a great methodological advantage in terms of accuracy and rapidity in performing automated potentiometric titrations. An integral part of titration methods of this type is a method of controlling titrant addition to discrete volume increments such that, in the vicinity of the equivalence point, the relatively smallest volume increment of titrant is metered. The conventional methods of controlling titrant addition, however, have significant deficiencies.

Some of these methods require detailed knowledge of the course of the titration curve (DE-PS 2320193) or are applicable to this knowledge only for relatively few types of titration (Shinichito Yamaguchi and Takashi Kasujama, Z. AnI Chemie 295 [1979], 256-259). These prerequisites required for the applicability of these methods substantially limit their applicability and require extensive and time-consuming preparatory work in cases where they are applicable.

Generally applicable methods (DE-PS 2617346 and JC Smit and HC Smit, Anal. Chim Acta 143 (1982) 45-77) have the significant disadvantage that for their application, the specification of an upper limit of the size of the volume increments of titrant is required This upper limit of the size of the volume increments depends on the titrant consumption up to the equivalence point, which is to be determined only by the titration, ie the size of the titrant consumption to be expected must be known in advance from a certain extent In this case, the applicability of these methods is limited or requires preliminary work, which unnecessarily increases the time required for the titration and disturbs the work flow

Volumeninkrements is formed from only relatively few preceding measurement points. This has the consequence that there is no forecast of the expected equivalence point and the size of the volume increments is strongly influenced by relatively small measurement errors. These shortcomings indicate that there is no possibility to move to equal-dose volume increments when approaching the equivalence point, when the size of the volume increments falls below a predetermined threshold or better depending on the location of the equivalence point, since such a reduction in the volume increment size occurs at different titrations Form of their titration curve also far from the equivalence point on or may be due to measurement errors. For these reasons, there is no way of determining the equivalence point by interpolation methods which require equal volume increments near the equivalence point in the above methods. For these reasons, in the above-mentioned method of Smit and Smit, the equivalence point is determined by a curve fitting ancestor. This method requires relatively many measuring points in the vicinity of the equivalence point, which significantly prolongs the titration time, since in this part of the titration curve the response time of the measuring electrodes is particularly long. In addition, the chosen equivalence point determination method is not suitable for accurately determining heterovalent titrations.

In the method according to DE-PS 2617346, the equivalence point is determined by the interpolation to the zero value of the second difference, although it is known (FL Hahn, ".pH and potentiometric titrations", Akademische Verlagsgesellschaft Frankfurt (M.) 1964, S. Wolf, Z Analyt. Chem. 250 [1070] 13; S.Ebel and S. Kalb, Z. Anal. Chem. 278 [1976] 109-114) that this interpolation method has the comparatively lowest accuracy and for the determination of the equivalence point in heterovalent titrations is unsuitable.

The need to specify an upper limit on the volume increments of titrant and the inability to use interpolation methods that require equal volume increments are significant shortcomings of conventional methods that could not be overcome by the proposed improvement. Ebel and Reger (Z. Anal. Chem. 312 [1982] 346-351) propose improvements that prevent the equivalence point from being exceeded due to a measurement error in too large a volume increment. According to EP PS 0188667 it is proposed and claimed that the lower limit of the volume increments is not given as an absolute value, but is calculated in the course of the titration from the respective titrant consumption. In both cases, the elimination of the abovementioned deficiencies is neither desired nor achieved.

Object of the invention

The aim of the invention is to develop a fast, accurate method for controlling the discontinuous addition of titrant in automatic titrations which can be used in a variety of ways for different applications of titrations with potentiometric measurement.

Explanation of the essence of the invention

The object of the invention is to provide a method for automatically controlling the titrant addition, which extends and facilitates the applicability of the economically favorable dynamic potentiometric titration by the following properties:

Neither knowledge of the Ve ^one run of the titration curve, special constants to be titrated substances on the electrode slope still targets an upper limit of the volume increments are to be required on the applicability of the method. The method should make it possible to carry out titrations one after the other, without changing over and without changing control parameters, in which the consumption of titrant differs by several orders of magnitude. The method should allow, without requiring calibration of the measuring chain, to proceed in the vicinity of the equivalence point for titration in constant volume increments in order to enable the applicability of advantageous interpolation methods.

This object has been achieved according to the invention by a method comprising the following steps, which are carried out under the control of a digital computer under control:

1. Start the titration

The start of the titration is carried out by the formation of the size of the first Volumeninkrements titration AVx in such a way that AVx is the size of the volume of metering and concentration of the titrant smallest by the metering to be realized volume increments.

2. Dosing of the titrant

The metering of the titrant is carried out by a metering system, which is formed from one or preferably several motor burettes and / or one or more Fostvolumendosierern. The dosage is carried out in the course of the titration in each case after the determination of the value AVx by the computer. The dosage is organized by the computer so that the dosing (motor burettes or fixed volume dispensers), which may contain titrant of different concentrations are selected for the dosage so that the absolute volume of the titrant mc * is very low and dose as many uosierorgane quasi simultaneously can. The calculator takes into account the smallest volume increments to be implemented by the individual metering devices, the concentration of the titrant in the individual metering devices and the accuracy of the metering of individual metering devices.

After each dosage, the calculator stores the size of the actually metered volume increment of titrant (AVn) in multiples of the smallest increment of the volume in terms of dosing volume and titrant concentration. By summing up these volume increments, the calculator builds and stores the total amount of spent titrant (Vg) throughout the titration. Next is. It is necessary for the implementation of the method according to the invention that the size of the previous Volumeninkrements is stored.

3. Measurement of the potential difference

The measurement of the potential difference takes place in a known manner. From the measured potential difference, the size of the following Volumeninkrements is formed in the further course of the process in conjunction with the associated size of the Volumeninkrements. However, this can only be done sensibly if the potential differences have a value that clearly differs from the random scattering, the "noise." If this is not the case, instead of the measured values, a value (AEs) should be used which stands out as the It must be chosen according to the resolution of the measuring system used.

4. Formation de ^r size of the second volume increment of titrant.

This stage is preceded by the dosage stage and the potential difference measurement stage. The size of the following volume increment is formed by the following expression.

10 ^F (10

The size F indicates the desired potential step size, which is between 0.05 and 0.3. It has proven to be appropriate

proved to choose F = 0.2.

By reducing the measuring point density is increased and vice versa. The quantity "Sa" in expression (1) stands for the electrode slope, also called electrode slope or Nernst factor. This size need not be known since, as will be explained below, this size will be according to the invention Method is corrected during the titration. It is sufficient if at the beginning of the titration

Sa = - is.

Here zi is the charge of the Meßions, which can be assumed to be known. In order to take account of an uncertainty regarding the electrode slope, which exists at the beginning of the titration, it has become known as

it is expedient to limit the first volume increment formed by expression (1) to half its value:

AVx = y AV _B

The formation of the second AVx value is followed by the steps of metering and measuring the potential difference and then the step of forming further volume increments.

S. Formation of further volume increments

This stage begins with the electrode slope correction. This correction is not necessary if the measured values AEn or AEn-1 have been replaced by Es for one or both stored potential values. The correction is further unnecessary if

Vn-I

results.

Even if for the expressions

P π ι L · and

^d 10¹"

P = (1 - i 10

Pj = »P, the correction for the electrode slope is not required. In order to carry out the correction, it has proved necessary to define an upper (So) and a lower limit (Su) for the electrode slope. The choice of the following limits has proved to be expedient:

Su-0.5-H

With the specification of these limits for the electrode slope, a correction is not required if P ₂ <P and Sa = Su resp

P ₂ > P and Sa = So

In all other cases, the correction is performed according to the following procedures. After the following equation 2 becomes

new value (S _n ) for the electrode slope formed.

lg

io * ^{En / Sat}

As long as S _n <So (at P <P ₂ ) or S _n > Su, (at P> P ₂ ) and / Sa - S _n /> 1, the calculation of a respective new value for the electrode division becomes Sa = S _n continued. If the termination condition is reached, Sa = S _f , formed. Another abort condition is when at P> P ₂ the S _N value according to equation (2) is greater than Sa b? n. at P <P ₂ S _N value of less than Sa. In these cases, the original value for Sa remains. This is followed by the formation of AV ₈ according to equation (1). The calculation of the provisional size for the following volume increment (AVp) is followed by the determination of an estimated value (VSn) for the titrant consumption up to the equivalence point by the computer according to the following τι expression:

VSn = Vg +

^P 2

In the course of the titration, the mean value (VS _M ) is formed from the values thus formed for VSn. As a result of this method, it is achieved that with VS _M an estimated value for the consumption of titrant until the equivalence point, which gradually improves in the course of the titration, is present.

It also makes it possible to apply the method to those titrations whose titration curve deviates from the typical S-shaped curve such that a relatively steep section merges into an almost horizontal section, which is again followed by a steep section. Such a course is typical for various redox titrations. A similar course is found in the titration of weak acids and weak bases. in the latter case averaging from all VS "values leads to an unnecessary extension of the titration time. In the titration of weak acids and weak bases, it has proved expedient to begin averaging only when

 = 0.1.

^VS n

A further improvement of the estimated value VS _M is achieved by the computer, as soon as the number of VS values reaches a certain value (preferably 4 values), discarding the VS value which is furthest away from the mean value and forming a new mean value. However, the last added VS value (VS _n ) may not be discarded. This discard and discard may be tied to the condition that a VS value is discarded only if its deviation from the mean is greater than the standard deviation.

After the formation of the mean value VSm, the difference between VS _M and the total consumption Vg is formed. This difference indicates how much titrant is expected to be needed until the equivalence point is reached. The computer now compares whether the size of the predicted volume increment (AVb), the size of which can be corrupted by a measurement error, is compatible with the distance from the equivalence point.

If AV ₈ > y (VSm - Vg), then AVx = y (VS _M - Vg)

In all other cases AVx results from AV ₈ .

The prediction of titrant consumption to the equivalence point also allows the transition to metering constant volume increments, if necessary. By specifying a size A, the size of the volume increments is kept constant as soon as AVx s A VS _M is required. A is set as a number between 0.001 and 0.1, preferably 0.01-0.05. The specification of the size A is expediently to be combined with the specification of an upper limit (B) for the value of A VS _M.

The formation of the size of the volume increment to be dosed (AVx) is followed by the steps of dosing, potential measurement and the formation of a further volume increment. This cycle repeats until the completion of the titration, which can be done after detecting the crossing of the equivalence point by a suitable method. The confirmation of the tiration can also be linked to the condition that, after the equivalence point has been exceeded, a predetermined number of measurement points should be present after the equivalence point.

If more than one equivalence point is to be determined, it suffices according to the method of the invention that, after determining the equivalence point, its value and the value for the total consumption are stored and the titration is restarted. It is in the sense of the method possible to design the computer program so that when determining one or more further equivalence points each new values for F (for controlling the Meßpunktdichte) and for A and B specify. The titration can then be finally terminated after determining a predetermined number of equivalence points or after a predetermined titrant consumption in terms of volume and concentration or after a predetermined titrant volume. It is also possible to stop the titration after exceeding a certain potential value. In addition to these titration termination options, which are also applicable to conventional titrant addition control methods, it is possible in the method according to the invention to terminate the titration when the estimated consumption exceeds a predetermined value until the equivalence point.

Exemplary embodiments

The invention will be explained in more detail below with reference to the sanctifying schematic diagram based on application examples. The solution to be titrated is located in the titration vessel 1 and is moved during the titration by means of the stirrer 2. In the solution to be titrated emerges an existing measuring and reference electrode electrode 3, which is connected to the measuring device 4. The measured values are taken over time or drift controlled by the computer 6. The computer is further connected to the dosing system 7. The dosing system consists of one or more doser organs, which may consist of motor burettes and / or fixed-volume dosing devices. Via hose connections 8, the titrant passes into the solution to be titrated, wherein the computer takes over the control and monitoring according to the process steps of the present invention. Before the start of a titration or a series of titrations, the computer receives the required information about the dosing system and the parameters for controlling the titrant addition by means of the keyboard 5.

example 1 In this example, concerning the titration of an acid, these are the following information. dosing ·

metering Type of dosing volume resolution smallest to be dosed titrant volume increment concentration DO1 motor brush ΙμΙ 1 μΙ 0.1N DO2 Engine burette ΙμΙ 10μΙ 0.5 N DO3 burette ΙμΙ 10μΙ 0.5 N

The smallest volume increment to be metered can result from the construction of the metering device, but it can also be chosen to be smaller to minimize the metering error.

Control parameters:

zi: (charge of the measurement): 1

F: (potential step size): 0.2 B: 0.2 ml; 0.5N solution Number of equivalence points: 1 Furthermore, it was specified that, in accordance with the achievable measuring accuracy for measured values below 1 mV, the value AEs - 1 mV

is to use.

The following table shows how the control of the titrant addition takes place according to the method according to the invention. Table I

η Vg W ΔΕ AV ₀₀ , EV ₀₀₂ AV ₀₀₃ 1 0 0 - - - - 2 1 0 <1 1 - - 3 5 2 2 4 - - 4 23 15 13 18 - -

Continuation Table 1

η Vg Ü ΔΕ AV «,, AV ₀₀₂ AV ₀₀₃

5 26 18 3 3 6 53 26 8th 7 7 38 35 9 5 8th 41 42 7 3 9 44 52 10 3 10 46 62 10 2 11 48 80 18 2 12 50 360 280 2 13 52 630 270 2 14 54 649 18 2 15 56 657 8th 2

In the table mean:

n: the number of the measuring point

Vg: the respective total amount of dosed titrant In μΙΟ, Ι Ν solution E: the respectively measured potential in mV

ΔΕ: the potential difference in mV

AVooi ... AVooj: the volume increment In μΙ dosed by each dosing device Example 2 In Example 2, an acid was also titrated. The same dosing system as in Example 1 was used. Also with the Control parameters were not changed. Table 2 shows the course of the titrant addition. Table 2

η Vg e ΔΕ AV ₀₀₁ AV ₀₀₂ AV ₀₀₃ 1 0 100 2 1 100 0 1 - - 3 5 100 0 4 4 40 100 0 35 - - 5 360 100 0 32 32 6 3175 101 1 - 281 282 7 30665 109 8th - 2749 2749 8th 58345 119 10 276S 2768 9 82330 131 12 - 2398 2399 10 99005 144 13 1667 1668 11 109490 157 13 - 1048 1049 12 116065 171 14 - 657 658 13 119335 182 11 - 327 327 14 121730 196 14 - 239 240 15 122775 206 10 - 104 105 16 123520 216 10 - 74 75 17 124265 234 18 - 74 75 18 125010 557 323 - 74 75 19 125755 664 107 - 74 75 20 126500 692 28 74 75

The two examples clearly show how the method for controlling the addition of titrant according to the present invention succeeds in carrying out titrations whose titrant consumption is several orders of magnitude apart up to the equivalence point without rebuilding the apparatus and without changing the control parameters. By using two burettes with Titriei medium of the same concentration could be compared to the use of only one of these burettes the time required for the Titriermitteldosierung be reduced by almost half. In both application examples, the titration was controlled for a determination of the equivalence point through interpolation. Other methods of determining the equivalence point may require that a larger measuring point density be required. This can be achieved by reducing the value F. In addition, it can be achieved by appropriate specifications that a certain number of measurement points obtained after the equivalence point or the Volumeninkremente according to the equivalence point corresponding to the curve slope again larger.

Claims (5)

A method for controlling the addition of titrant in automated titrations with potentiometric measurement, in which repeatedly added to a reaction mixture certain amounts of titrant whose size is adapted to the slope of the titration curve, characterized in that the titrant dosage in volume elements by a metering consisting from a odar preferably several dosing (motor burettes and / or fixed-volume dispensers), which may contain different concentration TRrier is realized, the dosing be selected taking into account the concentration of the titrant and the accuracy of the dosage for the dosage from the computer that the absolute volume of titrant is as low as possible and can dose as many dosing as quasi simultaneously. 2. The method according to claim 1, characterized in that the size of the first to be metered Volumeninkrements of titrant (AVx ₁ ) is equal to the size of the smallest in terms of metering volume and titrant concentration to be realized by the metering Volumeninkrements. 3. The method according to claim 1, characterized in that for the size of further Volumeninkremente a preliminary value AV _8n according to the following equation where F is the predetermined potential step size which can be between 0.05 and 0.3, preferably 0.2, AVn is the magnitude of the previously metered volume increment, ΔΕη is the potential difference in mV due to the dosage of AVn, and Sa is a measure of the electrode slope.
4. The method according to claim 1, characterized in that for the calculation of the first value CQ according to Equation 1AV ₈₂ Sa = - ^ -, where zi is the charge of the measurement ion and that by the Dosing system to be realized volume increment AVx ₂ = 4-AV ₈₂ is. 5. The method according to claim 1, characterized in that the calculation of further AV _B values precedes a correction of the electrode slope, if δ Vn-1 ^ δ Vn and - Ι ^Δ En j. f λ l ν ^{Δ v} βn 10 F 10 ^{AU / 5a} AVn wherein a new value SN for the electrode slope is formed according to the following equation lg ( ) ^ V _{n Δ} V _n . 10 * ^{E / Sa} and with Sa = SN this calculation is repeated until one of the termination conditions SN> So and SN <Sa at P <P ₂
SN <Su and SN> Sa at P> P ₂
and / SN - Sa / <1 with Pz = ιV (*> ⁴ ^ so - i, i. | 2, su = ° ' ⁵ · fr is reached. 6. The method according to claim 1, characterized in that the determination of the provisional size for the following Volumeninkrement AV _Bn the E ^ r.lttlung an estimated value VSn for the titrant consumption to the equivalence point according to the following expression