Method of measuring a silver or halogen ion concentration and an apparatus for the same

Abstract: A method of measuring a silver or halogen ion concentration wherein, in a sensor system which detects as a potential a silver or halogen ion concentration in a gelatin aqueous solution containing silver halide crystals, a reference electrode which functions as a reference of a potential measurement is inserted into a heat insulating bath without being directly inserted into the measured liquid, said bath being accurately controlled to have a constant temperature and electrically insulated, the measured liquid and said reference electrode are electrically connected with each other by a salt bridge, only one end portion of an indicator electrode is immersed into the measured liquid, said reference electrode and another end portion of said indicator electrode are connected with a potentiometer, and a potential is measured.

Description: The invention relates to a method of measuring a silver or halogen ion concentration of an emulsion during or after the formation of a silver halide photographic emulsion, and to an apparatus for use in said method. As a prior art technique, US-A-3,031,304 discloses a method of producing a fine particle emulsion which has a mean particle diameter of 0.06 µm. In the specification, a convenient method is disclosed in which particles are formed in a pAg range of about 2 to 6 by using a method of simultaneously mixing a silver nitrate solution and a potassium bromide solution which are reaction liquids, and four pumps are used for injecting the reaction liquids so as to automatically control the pAg. Specifically, the silver nitrate solution and the potassium bromide solution are separately provided with a pump so as to be injected in a substantially stoichiometrically equivalent amount. A potentiometer circuit has a limit switch which, when a silver ion concentration of an emulsion in a precipitation solution is raised to pAg of 5 or more, functions so as to decrease the amount of potassium bromide pumped to be injected, by 1% by means of a third pump. When pAg reaches 5 or more, the third pump is stopped. When pAg is lowered to a predetermined level, usually 4.3 or less, potassium bromide to be injected is added by the third pump. The fourth potassium bromide injecting pump is used for a manual addition. In accordance with the reading of the potentiometer or a recorder, the operator can adequately adjust the addition of potassium bromide. Furthermore, US-A-3,821,002 discloses a control apparatus and a method of producing a silver halide emulsion. In the apparatus and method, pAg in a precipitation bath is made constant or changed, and the flow rates of a silver nitrate solution and a halogen salt solution to be added are changed in accordance with a program so that the required accuracy of pAg is maintained. Furthermore, Photogr. Korresp. 101, 37 (1965) teaches relationships of crystal diameters of silver halide and the number of particles which are obtained by maintaining the temperature, and adjusting valves for adding a silver nitrate solution and a halogen salt solution, by an electrical control, thereby controlling pAg and pH. Japanese Patent Unexamined Publication No. SHO 61-65305 discloses an optimum control method in which a defect of the conventional PID control is eliminated and a computer control is done in accordance with a mathematical model. Japanese Patent Unexamined Publication No. HEI 5-181504 discloses an adaptive control method having a feedforward element in which a sequential plant model in the control of a physical quantity of a system is estimated, the control is conducted on the basis of the plant model, a variation quantity at an elapse of a dead time with respect to a variation externally applied to the system is predicted by using a variation pattern of a physical quantity which causes the external variation, and the external variation at an elapse of a dead time is previously canceled. US-A-4,933,870 discloses a method of producing a silver halide emulsion which employs an apparatus and method of converting an output signal of a nonlinear ion sensor into a linear signal. US-A-5,248,577 discloses an apparatus and method of producing a silver halide emulsion in which the density of halogen ions and flow rates of added halogen salt and silver nitrate solutions are periodically measured, the measured data are accumulated, an internal calculation is conducted by an equation estimated on the basis of the accumulated data, and the flow rates of the added halogen salt and silver nitrate solutions are controlled. sufficiently cope with the control at a steady state as far as However, relationships between a potential E Ag corresponding to the silver ion activity and ion concentrations of silver nitrate and halogen salt (e.g., potassium bromide) in a liquid containing silver halide crystals are non-linear and abruptly changed at the equivalence point as shown in Fig. 1. In the E Ag range of -50 mV to +150 mV where precipitation of a silver halide emulsion is often conducted while controlling the silver ion concentration, a very small change in concentration of silver ions or halogen ions causes the potential to be abruptly changed. Even when, in the uncontrolled state at the start of precipitation, the control is to be conducted at the preset target E Ag potential, pAg in a conventional precipitation bath is largely changed in an initial period of precipitation and hardly converged into the target value, with the result that several minutes must be elapsed before pAg is stabilized. Furthermore, the potential locus of E Ag obtained until the controlled state is attained cannot be reproduced. As the scale of a precipitation bath is increased, the control is further unstabilized, and hence it is difficult to stably produce a silver halide emulsion of constant quality. Also, conventionally, in order to obtain desired photographic characteristics, it is essential to control the silver or halogen ion concentration during or after the formation of a silver halide photographic emulsion, and a technique is widely employed in which reference and indicator electrodes for the above-mentioned control are directly inserted into a precipitation vessel in which a halogen salt aqueous solution reacts with a silver nitrate aqueous solution and which contains a gelatin aqueous solution. The relationships between a silver or halogen ion concentration and an electrode potential is described in "The Theory of the Photographic Process, Third edition or Fourth edition (Macmillan Publishing Co., Inc.)". Silver and halogen ion concentrations are respectively defined by equations (1) and (2) : pAg = -log[Ag + ] pX = -log[X - ] where [Ag + ] indicates the silver ion activity, and [X - ] indicates the halogen ion (Br - , Cl - , or I - ) activity. The electrode potentials E Ag and E X in relation to the silver or halogen ion activity in silver halide crystals are expressed as follows: E Ag = E° Ag - 2.30259 × (RT/F) × pAg where E° Ag indicates the standard potential for a silver half cell, R indicates the gas constant, F indicates the Faraday constant, and T indicates an absolute temperature. In a silver halide emulsion which is practically used, halogen halide is often in excess, and hence a silver indicator electrode is covered by a silver halide layer and saturated with silver halide salt. Therefore, the silver ion and halogen ion activities on the surface of the electrode have the relationship of equation (4) below: [Ag + ][X - ] = Ksp where Ksp indicates the solubility product of silver halide. In other words, a silver/silver halide electrode is essentially equivalent to a silver electrode in which the silver ion activity is governed by the halogen ion activity in a solution. Therefore, Ex is expressed by equation (5) below, but an indicator electrode in a silver halide emulsion indicates the same potential because the emulsion solution is in equilibrium with silver halide crystals. Ex = E° AgX + 2.30259 × (RT/F) × pX E Ag = Ex (it is assumed that Ex = E) The electrode potential E can be measured by forming a cell system in combination with a potential E R of a reference electrode which produces the reference potential, and detecting a potential difference. The relationships between E and pAg and PX can be expressed by the following equations: E = E° Ag - E R - 2.30259 × (RT/F) × pAg E = E° AgX - E R - 2.30259 × (RT/F) × pX Therefore, the states of pAg and PX of a silver halide photographic emulsion can be grasped by measuring the potential E of the indicator electrode. When the reference electrode which functions as the reference of a potential measurement is inserted into a measured liquid, however, the temperature variation of the measured liquid causes a long period to be elapsed before a constant potential is obtained. Therefore, it is impossible to continuously measure instantaneous variations of an ion concentration, gelatin and silver halide particles adhere to the liquid junction of the reference electrode to clog the liquid junction, whereby an asymmetry potential is produced so that it is difficult to obtain a constant potential which functions as the reference. (See "Photographic Emulsion Chemistry 1966" by G. F. Duffin, p. 14, FOCAL PRESS LIMITED.) When a silver ion activity of a system such as a gelatin aqueous solution containing the silver halide crystals is measured with using a conventional silver metal rod as an indicator electrode, the reproducibility of the measured potential in repeated measurements is not always satisfactory. Furthermore, silver halide crystals obtained from the system vary in size distribution, shape, photographic characteristics, etc. Japanese Patent Unexamined Publication No. SHO 60-213858 discloses a method in which, as a countermeasure for stabilizing a conventional electrode for detecting a silver ion concentration in order to obtain stabilization of the indicator electrode, an alloy electrode made of silver and a metal of one or more kind which is nobler than silver, or of metals of two or more kinds which are nobler than silver is used. In the method disclosed in Japanese Patent Examined Publication No. SHO 60-213858, however, a measured liquid penetrates into a small gap between the metal silver and its alloy which function as the indicator electrode, and a holding cover for the electrode, and adheres to the electrode and the cover. When liquids of different kinds are to be measured, therefore, it is impossible to obtain an accurate value. US-A-4,094,684 and US-A-3,801,326 disclose methods for measuring a silver or halogen ion concentration using a silver electrode and a reference electrode, said reference electrode being immersed in a thermostabilized bath and being connected to a silver halide emulsion via a salt bridge. Figure 2 of US-A-5,071,537 shows an apparatus for measuring an electric potential, said apparatus comprising thermostabilized cells A and B each containing buffer solutions, a reference electrode inserted into cell A, a SSCE inserted into cell B, a salt bridge connecting cells A and B, and a potentiometer connecting the reference electrode and the SSCE, both buffer solutions in cells A and B being circulated by way of magnetic stirrers. The object of the present invention is to provide a method and apparatus for stably and accurately measuring a very little electric potential corresponding to a silver or halogen ion concentration in a gelatin aqueous solution containing silver halide crystals without influences due to outer noises. The present invention provides a method of measuring a silver or halogen ion concentration in a gelatin aqueous solution containing silver halide crystals, said method comprising the step of measuring the potential of said silver or halogen ions by using a sensor system which detects as a potential the silver or halogen ion concentration in the gelatin aqueous solution containing silver halide grains, said sensor system comprising (1) a reference electrode which functions as a reference of the potential measurement and being inserted into a heat-insulating bath without being directly inserted into said gelatin aqueous solution, said bath being accurately controlled to have a constant temperature and being electrically insulated, and said gelatin aqueous solution and said reference electrode being electrically connected with each other by a salt bridge, (2) an indicator electrode only one end portion of which being immersed into said gelatin aqueous solution, and (3) a potentiometer, said reference electrode and another end portion of said indicator electrode being connected with said potentiometer. The present invention further provides an apparatus for measuring a silver or halogen ion concentration in a gelatin aqueous solution containing silver halide crystals, said apparatus comprising (1) a reference electrode which is disposed in a heat-insulating bath having a constant temperature and being electrically insulated, only one end portion of said reference electrode being electrically connected with said gelatin aqueous solution by a salt bridge, (2) an indicator electrode only one end portion of which being immersed into said gelatin aqueous solution, said indicator electrode being a silver metal rod having a purity of 99.9% or more, a platinum plating or an insulating material coating being applied onto a portion of said indicator electrode, said portion contacting a holder unit, and a surface of said portion contacting said gelatin aqueous solution being plated with AgBr or Ag 2 S in a thickness of 0,1 µm or less, and (3) a potentiometer which is electrically connected with said reference electrode and another end portion of said indicator electrode via a silver wire. Preferably, a ceramic having micropores is used in a portion of said salt bridge, said portion contacting the gelatin aqueous solution containing silver halide grains, and a potassium nitrate solution is used as an inner liquid of said salt bridge. In the invention, the salt bridge between the gelatin aqueous solution containing silver halide crystals (hereinafter, referred to as "measured liquid") and the reference electrode means that a flexible plastic hose is used, a KNO 3 solution is used as an inner liquid in the hose, and the concentration of the solution is from 0.01 to 5 Mol/l, preferably from 0.8 to 1.2 Mol/l. In the invention, the immersion of only one end portion of the indicator electrode into the measured liquid means that only the tip end of one end portion of the indicator electrode is immersed and the body portion of the silver rod is not immersed into the measured liquid. The measurement of the potential is performed by measuring the potential difference between the reference electrode and the indicator electrode by means of a potentiometer. In the invention, the heat insulating bath which has a constant temperature and is electrically insulated means that the heat insulating bath is made of vinyl chloride or acrylic resin or provided with an insulation property and the inner liquid (the same as the salt bridge of inner liquid) of the vessel having an insulation property of 100 MΩ or higher is maintained within ±0.5 °C by a thermostatic chamber or the like, whereby the stability of the reference potential depending on the temperature is maintained. In the invention, the use of ceramic having micropores in a portion of the salt bridge which makes contact with the gelatin aqueous solution containing silver halide crystals means that one end of the salt bridge is blocked by porous from ceramic having a porosity from 2 to 40%, preferably from 5 to 15% so that the potassium nitrate solution which is the inner liquid passes through the ceramic plug to flow out from the heat insulating bath into the gelatin aqueous solution containing silver halide crystals in a flow rate from 0.01 to 10 ml/min., preferably from 0.1 to 1 ml/min. at a head pressure of 9.8 KPa. In the invention, a silver metal rod of a purity of 99.9% or higher is used as the indicator electrode, and platinum plating or an insulating material coating is applied onto a portion of the silver metal rod which makes contact with the holder unit. As the insulating material, Teflon or ceramic is used. The silver metal rod is inserted through the holder unit and supported thereby via, for example, an O-ring. The surface of the one end portion which makes contact with the gelatin aqueous solution containing silver halide crystals is plated by AgBr or Ag 2 S in a thickness of 0.1 µm or less. This allows the accuracy of the potential of the indicator electrode to be maintained. Fig. 1 is a graph illustrating relationships between concentrations of KBr and AgNO 3 versus the E Ag potential; Fig. 2 is a diagram showing the arrangement of the apparatus for measuring a silver or halogen ion concentration according to an embodiment of the invention; Fig. 3 is a diagram showing the arrangement of the apparatus for measuring a silver or halogen ion concentration according to another embodiment of the invention; and Fig. 4 is a diagram showing the arrangement of the apparatus for measuring a silver or halogen ion concentration according to a further embodiment of the invention. (Example 1) In a sensor system which detects as a potential a silver or halogen ion concentration in a gelatin aqueous solution 101 containing silver halide crystals, a method and apparatus for measuring the silver or halogen ion concentration are configured as shown in Fig.2. A reference electrode 102 which functions as the reference of the potential measurement is'inserted into a heat insulating bath 103 without being directly inserted into the measured liquid 101. The temperature of the heat insulating bath is accurately controlled within ±0.5 °C by a thermostatic chamber so as to have a constant temperature, and is made of vinyl chloride or acrylic resin or provided with an insulation property such as a Teflon coating. The measured liquid 101 and the reference electrode 102 are electrically connected with each other by a salt bridge 104. Only one end portion of an indicator electrode 105 is immersed into the measured liquid 101. The reference electrode 102 and the other end portion of the indicator electrode 105 are connected with a potentiometer 106 via an electrically shielded silver wire 107, and the potential difference is measured. A saturated calomel electrode was used as the reference electrode 102, and ceramic having a porosity of 5 to 15% was used as the ceramic 108 having micropores. The ceramic 108 having micropores is used in the portion of the salt bridge 104 which makes contact with the gelatin aqueous solution 101 containing silver halide. A potassium nitrate solution of 0.5 to 1.2 Mol/l is used as the inner liquid of the salt bridge 104. A silver metal rod 109 of a purity of 99.9% or higher was used as the indicator electrode 105. The portion 111 of the indicator electrode 105 which makes contact with a holder unit 10 was plated by Pt or applied with an insulative Teflon coat or a ceramic coat, and supported by the holder unit 110 via two 0-rings 114. The surface of the portion 112 which makes contact with the gelatin aqueous solution 101 containing silver halide is plated by AgBr or Ag 2 S in a thickness of 0.1 µm or less. (Example-2) In the embodiment described above, the present method and apparatus for measuring a silver or halogen ion concentration are used in a precipitation vessel 113 for a silver halide emulsion. Fig. 3 shows another embodiment of the measuring apparatus in which a gelatin aqueous solution containing silver halide is sampled. A reference electrode 102 is immersed into a heat insulating bath 103 for a potassium nitrate solution, and electrically connected with a measured liquid 101 which is a gelatin aqueous solution containing silver halide, via a salt bridge 104 which has at its both ends ceramic 108 having micropores. An indicator electrode 105 has a configuration in which the body portion made of a silver metal rod is electrically insulated and an end portion making contact with the liquid is plated by AgBr or Ag 2 S. The indicator electrode 105 is immersed into the measured liquid 101. The reference electrode 102 and the other end portion of the indicator electrode 105 are electrically connected with a potentiometer 106 via by a shielded silver wire 107. The measured liquid is maintained to a constant temperature by a thermostatic chamber. (Example-3) In Example-1, the reference electrode 102 and the indicator electrode 105 are modified so as to be separated from a single holder unit. An n number of the indicator electrodes 105 are disposed at arbitrary positions of the precipitation vessel 113 for a silver halide emulsion, and the connections between the indicator electrodes and the potentiometer 106 are switched by a connection switching device 115. In this configuration shown in Fig. 4, it is possible to measure the distribution of the silver or halogen ion concentration in the precipitation vessel. According to the present method and apparatus for measuring a silver or halogen ion concentration, the temperature variation of a reference electrode and the generation of an asymmetry potential in the liquid junction of the reference electrode are prevented from occurring, and hence a constant reference potential can be obtained. Furthermore, the portion of an indicator electrode which makes contact with the measured liquid is prevented from being affected by adherence of foreign substances, so that a correct measurement is enabled. Therefore, the invention can attain effects such as the followings: (1) The electrode potential in relation to the silver or halogen ion activity in various silver halide emulsions can be measured immediately or within 1 s or shorter (in the prior art, when the indicator electrode is immersed into another measured liquid, a period of about 5 to 50 min. must be elapsed until a constant potential is obtained). (2) In the prior art, the silver potential of a precipitation vessel during the formation of silver halide crystals is often deviated by a degree of about 50 mV or more. In contrast, according to the invention, the potential can be detected with accuracy of ±1 mV and excellent reproducibility, with the result that a silver halide photographic emulsion can be produced with excellent reproducibility. (3) In addition, the life of the reference electrode can be prolonged. When the interior of a tank is subjected to an automatic high temperature cleaning process, also the sensor unit can be cleaned simultaneously. Consequently, the preparation of the next product of another kind can be conducted in a perfectly automatic manner.

Claims: A method of measuring a silver or halogen ion concentration in a gelatin aqueous solution containing silver halide crystals, said method comprising the step of measuring the potential of said silver or halogen ions by using a sensor system which detects as a potential the silver or halogen ion concentration in the gelatin aqueous solution containing silver halide grains, said sensor system comprising (1) a reference electrode which functions as a reference of the potential measurement and being inserted into a heat-insulating bath without being directly inserted into said gelatin aqueous solution, said bath being accurately controlled to have a constant temperature and being electrically insulated, and said gelatin aqueous solution and said reference electrode being electrically connected with each other by a salt bridge, (2) an indicator electrode only one end portion of which being immersed into said gelatin aqueous solution, and (3) a potentiometer, said reference electrode and another end portion of said indicator electrode being connected with said potentiometer. An apparatus for measuring a silver or halogen ion concentration in a gelatin aqueous solution containing silver halide crystals, said apparatus comprising (1) a reference electrode which is disposed in a heat-insulating bath having a constant temperature and being electrically insulated, only one end portion of said reference electrode being electrically connected with said gelatin aqueous solution by a salt bridge, (2) an indicator electrode, only one end portion of which being immersed into said gelatin aqueous solution, said indicator electrode being a silver metal rod having a purity of 99.9% or more, a platinum plating or an insulating material coating being applied onto a portion of said indicator electrode, said portion contacting a holder unit, and a surface of said portion contacting said gelatin aqueous solution being plated with AgBr or Ag 2 S in a thickness of 0,1 µm or less, and (3) a potentiometer which is electrically connected with said reference electrode and another end portion of said indicator electrode via a silver wire. The apparatus of claim 2, wherein a ceramic having micropores is used in a portion of said salt bridge, said portion contacting the gelatin aqueous solution containing silver halide grains, and a potassium nitrate solution is used as an inner liquid of said salt bridge.

Inventors: SAITOH HIROKAZU, TADA SUGIHIKO, SAITOH, HIROKAZU, TADA, SUGIHIKO
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1995
Main classification: G01N27
IPC codes: G03C1