Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – Including titration
Reexamination Certificate
2000-04-18
2001-11-13
Warden, Sr., Robert J. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
Including titration
C205S788000, C204S405000, C422S077000, C436S042000
Reexamination Certificate
active
06315888
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a constant-current polarization voltage detecting method used suitably in a Karl Fischer's moisture content measuring method and a Karl Fischer's moisture content measuring apparatus using such detecting method.
2. Discussion of the Background
For a detecting means to measure a moisture content for which a Karl Fischer's (hereinbelow, referred to as “KF”) volumetric titration method of this kind is used, a so-called constant-current polarization voltage detecting method has often been employed in recent years. Namely, in the constant-current polarization voltage detecting method, a minute current is fed to dual platinum electrodes, as a detecting electrode, to measure a voltage across the dual platinum electrodes. In this case, although a minute current to be fed can either be a direct current or an alternating current, a pulse-like current has recently been used.
On the other hand, when a titration solvent composed mainly of chloroform as solvent was used for the constant-current polarization voltage detecting method, there was in fact difficulty in conducting the measurement of a large amount of sample in a moisture content measuring apparatus by a conventional technique. This was because the monitoring of a polarization voltage could not properly be effected since the liquid resistance of a chloroform solvent is high, and the liquid resistance is further increased by the incorporation of a large amount of sample having no-polarization properties in the titration solvent.
The moisture content measurement using the above-mentioned KF volumetric titration method is such one utilizing KF titration reactions as follows:
I
2
+SO
2
+H
2
O+3BASE→2BASE.HI+BASE.SO
3
BASE.SO
3
+CH
3
OH→BASE.HSO
4
CH
4
wherein BASE: an amine compound
Namely, in the titration reactions in the moisture content measurement by the KF volumetric titration method, since the reaction with water accelerates selectively, a wide application has conventionally been made for the measurement of moisture content. In this case, in a KF volumetric titration method, measurement is carried out by using an iodine-containing solution as titrant. In a coulometric titration method, iodine is generated by anode oxidation of iodide ions. Thus, the above-mentioned reactions are effected. In either method of detection, the end point of titration is recognized when an excessive amount of iodine is detected on the detecting electrode.
In such detecting method for detecting an excessive iodine, the above-mentioned constant-current polarization voltage detecting method is generally used, wherein according to the volumetric titration method, the titration is finished at the time when a state that an amount of iodine is excessive (a state a potential which is lower than that at the end point) continues 30 seconds, and according to the coulometric titration method, the titration is finished at the time when a potential is beyond the potential at the end point at which a state of a slight iodine being excessive can be detected.
Generally, there is no problem in a conventional titration method even when solvent containing much methanol is used as a titration solvent, and the titration is conducted by using a KF reagent. Even though any sample is incorporated, it is possible to reach the end point if the KF reagent is dropped excessively, and the moisture content measurement can be conducted regularly.
However, in a case of measuring oils, the titration is usually conducted in a titration solvent containing chloroform as a main solvent because the oils do not dissolve in methanol. However, since oils generally have a small moisture content, a large amount of sample is to be poured into the titration solvent. In this case, the presence of a much amount of chloroform increases the liquid resistance of the titration solvent. The incorporation of a much amount of sample further increases the liquid resistance. Under such condition, when a predetermined minute current of pulse form is applied, an apparent polarization voltage is assumed to be the sum of the true polarization voltage and a polarization voltage resulted from the liquid resistance. Accordingly, even though the titration in fact reaches the end point of titration, there is found a phenomenon as if it does not reach the end point of titration.
SUMMARY OF THE INVENTION
The present invention is to solve the problems in the conventional technique, and aims at providing a constant-current polarization voltage detecting method for measuring a moisture content by a KF titration reaction wherein the detection or the monitoring of a polarization voltage under a regular condition can be conducted, and a Karl Fischer's moisture content measuring apparatus using such method.
In order to achieve the above-mentioned object, the inventors of this application have made various studies, to improve the above-mentioned problems, on the constant-current polarization voltage detecting method for measuring a moisture content in the KF titration reaction, as the result of which they have found an important fact. Namely, influence to a polarization voltage due to solvent, when a current is applied in a pulse form, is usually large immediately after the application.
Description will be made in the following as to the result of the studies.
FIG. 1
is a diagram showing a relation of time to polarization voltage in Karl Fischer's moisture content measurement using the constant-current polarization voltage detecting method. In this case, determination is made so that a cycle of the application of a pulsating minute current is 500 ms and the pulse width of the current actually applied is 50 ms.
As shown in
FIG. 1
, when an ordinarily used sample is measured with a methanol type solvent, a polarization voltage measured on a detecting electrode increases gradually under the condition of the moisture content being excessive, and it decreases when the applied current is stopped.
When a KF reagent containing iodine is dropped to titrate a moisture content, an increase of polarization voltage tends to gradually decrease to thereby cause the KF reagent becoming excessive, with the result that the polarization voltage becomes very low. In this case, in a commercially available moisture content measuring apparatus using a volumetric method, an end point of analysis is determined when a polarization voltage, which has been detected as it is, takes a predetermined value (i.e., a value at the time when the KF reagent becomes excessive. This expression is applicable hereinbelow). Or, the end point is so determined that a waveform of a polarization voltage in a time is detected; an integration value of the polarization voltage in a one cycle (the surface area of a hatched portion in
FIG. 1
) is detected, and the integration value reaches a predetermined value at which the end point is determined.
On the other hand, in the measurement of a moisture content in oils, since oils are insoluble to methanol, solvent containing chloroform as the main solvent is used. As described above, however, since the solvent containing chloroform as the main solvent has a high liquid resistance, a detected voltage is the sum of a voltage due to the liquid resistance and a true polarization voltage. Further, when a sample is added to the solvent, the influence of the liquid resistance becomes remarkable. As clear from
FIG. 1
, it is understood that the polarization voltage shows an abnormal pattern just after the application of a pulsating current. Although the dropping of the KF reagent to titrate a moisture content reduces somewhat the polarization voltage, a value lower than a certain value can not be expected. As a result, although the moisture content in the sample has been able to be titrated by the KF reagent, the dropping of the KF reagent is continued because a polarization voltage does not decrease to a voltage at the end point (or an integration value of polarization voltage).
In co
Kato Hiromasa
Nakatani Masafumi
Mitsubishi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Olsen Kaj K.
Warden, Sr. Robert J.
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