Abstract:
Apparatus for calibrating a pH/ISE meter includes a solution chamber for receiving a standard pH/ISE electrode adjustably suspended in the chamber. Inlets are provided into the chamber so that a wash solution and buffers may be introduced into the chamber by gravity, the introduction of these liquids being controlled by valves controlled by a controller. The chamber also has a drain outlet which is opened and closed by a similar valve under the control of the controller. The controller controls the valves in a pre-determined sequence to flush and fill the chamber first with a wash solution, and then with buffers so that the electrode is immersed in each buffer long enough to enable an associated pH/ISE meter to obtain a stable reading of the pH/ISE value of the buffer. After the final buffer has been emptied from the chamber, the controller controls the valves to flush and fill the chamber with a solution. The controller may also include provisions for introducing into the chamber a specimen solution whose pH/ISE value is to be determined and measuring the value of that solution. Alternatively, the electrode in the chamber may be removed and placed in a separate sample chamber to be read by the calibrated pH/ISE meter. A calibration method is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is a continuation-in-part of Ser. No. 09/875,752, filed Jun. 06, 2001, now, which is a continuation of Ser. No. 09/262,416, filed on Mar. 04, 1999, now abandoned. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to pH/ISE meters. It relates more particularly to a method and apparatus for calibrating a pH/ISE meter.  
           [0003]    A pH/ISE meter is an instrument for measuring the pH and/or ISE value of a solution in order to determine the hydrogen-ion concentration of that solution on a pH scale from 0 to 14 and/or an ISE scale as listed in ThermoOrion Product Catalog 240176-001, cited in the Annual Book of ASTM Standards, Water and Environmental Technology, American Society for Testing and Materials 1992. A typical pH/ISE meter includes an electrode or probe in the form of small sealed tube filled with a reference fluid. A conductor extends into one end of the tube and contacts the fluid specimen, a second contact or conductor on the outside of the tube is grounded, both conductors being connected to a pH/ISE meter. In use, the probe is immersed in a specimen solution whose pH/ISE value is to be measured. Due to the characteristics of the fluid inside and outside of the probe, a voltage is produced which is applied to the pH/ISE meter which thereupon determines the pH/ISE value of the specimen solution and then displays that value.  
           [0004]    Before measuring the pH/ISE value of a specimen solution, it is common practice to calibrate the meter and the meter&#39;s pH/ISE probe using standard buffer or calibration solutions with known pH/ISE values. Currently, the industry uses three standard pH calibration solutions with pH values of 4.00, 7.00 and 10.00 and specific ion calibration solutions, e.g. chloride, fluoride and sodium, based upon a specific ion selective electrode (ISE). Invariably all pH/ISE meters and their probes are calibrated each time they are used and the technician usually performs the calibration with one or two of the above three standard solutions, the particular solution depending on the estimated pH/ISE value of the unknown solution or sample. This is usually referred to as a one or two point calibration. There are applications where more than a two-point calibration is needed, and in some applications, a five-point calibration is routinely performed. For example, assuming the user wants to calibrate the pH/ISE meter the two-point method (using the pH 4 and 7 standards, for example) the user carries out the following steps manually:  
           [0005]    1. clean the pH/ISE probe with a wash solution such as de-ionized water and dry the probe;  
           [0006]    2. place one of the two standard buffer solutions, usually the one with the lower pH/ISE value, i.e., pH 4.00, in a clean vial and immerse the electrode in that solution;  
           [0007]    3. after a stable pH/ISE reading is obtained, set the pH/ISE meter to the value of that standard solution, i.e., pH=4.00, as the reported value regardless of the actual meter reading;  
           [0008]    4. clean and dry the electrodes as described in Step 1;  
           [0009]    5. repeat Steps 2 and 3 using the second standard solution, i.e. pH—7.00;  
           [0010]    6. repeat the cleaning step to have the electrode ready for measuring the pH/ISE value of the specimen solution.  
           [0011]    The procedure for a one point calibration is similar to the above except only one standard solution, e.g., pH=4.00, is used.  
           [0012]    It is apparent that the above manual procedure requires the use of several different clean vials or the repeated re-washing of the same vial and repeated washing of the pH/ISE electrode prior to taking each reading. This calibration process is tedious, time consuming, damages electrodes due to the calibration manipulation, is prone to human error, cannot be replicated easily, encourages less frequent calibrations and increases the likelihood of sample reading errors.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is an object of the present invention to provide a method for automatically calibrating a pH/ISE meter of the portable, handheld, benchtop variety, and its electrode before the meter is used to measure the pH/ISE value of a given solution.  
           [0014]    Another object of the invention is to provide such a method which is universal to all pH meters and pH electrodes and all ISE meters and ISE electrodes.  
           [0015]    Yet another object of the invention is to provide an automatic pH/ISE meter calibration method which can be performed quickly and easily by relatively unskilled personnel simply by their placing the pH/ISE electrode in a solution chamber and pushing a single button.  
           [0016]    A further object of the invention is to provide apparatus for carrying out the above method to automatically calibrate pH/ISE meters.  
           [0017]    Another object of the invention is to provide calibration apparatus of this type which dispenses buffers and samples by gravity using low voltage solenoid valves only so that the apparatus presents no danger in a laboratory.  
           [0018]    A further object is to provide in calibration apparatus a safe storage location and environment for a pH/ISE electrode, so as to prolong the useful life of the electrode.  
           [0019]    Still another object of the invention is to provide such apparatus having an electrode holder and chamber to facilitate the safe and easy placement of all standard electrodes in and out of buffers and fluid samples.  
           [0020]    A further object of the invention is to provide apparatus of this type which allows for calibration documentation for good manufacturing practice (GMP), good laboratory practice (GLP), standard operating procedure (SOP) and which minimizes the manipulation of the pH/ISE electrode during the calibration procedure.  
           [0021]    The invention accordingly comprises the several steps and the relation of one or more of such steps in respect to each of the others, and the apparatus embodying the features of construction, combination of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed description, and the scope of the invention will be indicated in the claims.  
           [0022]    Briefly, in accordance with our method, a wash solution and the various standard buffers are routed in a predetermined sequence to a special solution chamber in which the electrode of the pH/ISE meter being calibrated may be placed. A combination of gravity and special dedicated fluid routing valves is used to control the flows of the various solutions to and from the solution chamber. A programmable controller interfaced with the pH/ISE meter controls the various valves to carry out the sequence of steps required to perform the automatic pH/ISE calibration procedure to be described in detail hereinafter.  
           [0023]    The apparatus that carries out our calibration method is specifically designed for laboratory use and operates on low voltage power, i.e. 24 volts DC. One embodiment of the apparatus is separate from the pH/ISE meter and may be electrically connected thereto by a standard RS-232 connector. A second apparatus embodiment incorporates the pH/ISE meter right into the calibration apparatus and may display pH/ISE values which are temperature corrected according to the measured buffer and sample temperatures. Both apparatus embodiments can perform a standard calibration automatically with the push of a single button.  
           [0024]    After a pH/ISE meter and its electrode have been calibrated in accordance with our method, the pH/ISE value of a specimen solution may be obtained in any one of three ways. As we shall see, the calibration apparatus may include provisions for delivering a specimen solution from a built-in sample container in a controlled manner to the solution chamber whereupon the pH/ISE value of the sample may be displayed by the pH/ISE meter. On the other hand, if the apparatus does not include a separate sample container and a dedicated flow path to the solution chamber for the sample, the sample can be manually poured into the solution chamber containing the electrode. As a third choice, after calibration the electrode can be removed manually from the solution chamber and placed in a separate vial containing the sample solution and the pH/ISE value for the sample read by the pH/ISE meter.  
           [0025]    Our calibration method is not limited as to the number of buffers or calibration solutions that may be used during a given calibration procedure. Preferably, however, the apparatus for carrying out our method should have containers and dedicated flow paths from the containers to the solution chamber for at least three pH buffers, e.g. pH 4, pH 7 and pH 10, or three ISE buffers, e.g. chloride, fluoride, and sodium, as well as for a cleaning or wash solution such as de-ionized water.  
           [0026]    As we shall see, our calibration apparatus is relatively easy to manufacture and simple to operate. Therefore it should find wide acceptance in laboratories required to perform pH/ISE measurements of large numbers of specimen solutions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in connection the accompanying drawings, in which:  
         [0028]    [0028]FIG. 1 is a left front perspective view from above of apparatus for automatically calibrating a pH/ISE meter;  
         [0029]    [0029]FIG. 2 is a similar view of a second apparatus embodiment;  
         [0030]    [0030]FIG. 3 is a right rear perspective view from below on a smaller scale of the FIGS. 1 and 2 apparatus embodiments;  
         [0031]    [0031]FIG. 4 is a left rear perspective view from below showing the FIGS. 1 and 2 apparatus with the bottom cover removed;  
         [0032]    [0032]FIG. 5 is a diagrammatic view of a calibration apparatus embodiment including provisions for automatically measuring a sample solution following the calibration procedure, and  
         [0033]    [0033]FIG. 6 is a timing diagram illustrating the operation of the FIG. 5 apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    [0034]FIGS. 1 and 3 show generally at  10  an embodiment of our calibration apparatus adapted to calibrate a separate conventional pH/ISE meter  12  and the meter&#39;s electrode  14  connected to the meter by a wire  16 . Preferably, meter  10  may also be connected electrically to meter  12  by a cable  18  terminated by a standard RS-232 connector. The measured pH/ ISE readings may be displayed by the meter&#39;s display panel  12   a  and the results of the calibration may be printed out by a conventional printer  22  connected by a similar cable  24  to apparatus  10 .  
         [0035]    The pH/ISE electrode  14 , which is a standard item of manufacture, is basically a closed, thin-wall glass tube or vial about 12 mm in diameter filled with a reference liquid having a selected pH/ISE value. The electrode includes a sensing bulb at the working end of the electrode, a wire with a built-in reference electrode, reference junctions and usually also one or more temperature sensors. When the working end of the electrode is immersed in a liquid, an electrical potential is developed across the electrode contacts which reflects the difference in the pH/ISE values of the liquids inside and outside the electrode  14 . That voltage is applied to the meter  12  which thereupon displays on its display panel  12   a  the pH/ISE value of the liquid in which the electrode  14  is immersed. The operation of pH/ISE meters is well known and therefore will not be described in more detail here; but see the above-referenced ThermoOrion catalog.  
         [0036]    As shown in FIGS. 1 and 3, apparatus  10  includes a housing  30  having a rear section  30   a  that supports a plurality of vented containers. The illustrated apparatus  10  has four such containers, i.e. a container  32  for containing a wash or cleaning solution such as de-ionized water, a container  34  for containing pH 4 buffer solution, a container  36  for pH 10 buffer and a larger container  38  for containing the most commonly used buffer, pH 7. Preferably, these containers seat in receptacles  40  formed in the top wall of housing section  30   a.    
         [0037]    Housing  30  also includes a forwardly projecting section  30   b  having formed in its top wall an open solution chamber  42  adapted to receive the working end of the electrode  14  of pH/ISE meter  12 . In order to conveniently move the electrode  14  in and out of solution chamber  42 , apparatus  10  includes a clip-like holder  44  having a pair of resilient arms  44   a  adapted to engage electrode  14  from opposite sides. The base of the holder  44  is formed as a slider  46  which is adapted to slide up and down along a vertical slide  48  mounted to the front of housing section  30   a  behind chamber  42  so that the electrode is in line with the chamber  42 .  
         [0038]    Housing section  30   b  also contains a control panel  52  having various control buttons  52   a  for directing a controller  54  in section  30   b  to perform a programmed calibration procedure using the pH or ISE buffers in containers  34 ,  36 , and  38 . For example, there may be a pH4, pH7, pH10, pH4/7, pH7/10, pH 4/7/10 buttons which may be de-pressed to cause the apparatus to perform the indicated one, two or three-point calibration. There may also be a WATER button to initiate a procedure that rinses electrode  14  to fill the chamber with wash solution, i.e. clean water. There may also be a DRAIN button to drain chamber  42  and a CANCEL/STOP button to stop a calibration sequence. Panel  52  may also include various LED status indicators  52   b  including POWER-ON, READY-TO-CALIBRATE and ERROR.  
         [0039]    As shown in FIG. 3, the rear wall of housing section  30   a  may support various control elements including an on/off power switch  62  and RS 232 connectors  64  for electrically connecting the meter cable  16  and printer cable  24  to apparatus  10 . There is also a drain outlet  66  by way of which liquids are drained from the solution chamber  42  to a waste container or sink S as shown in FIG. 5. The bottom of housing  30  is normally closed by a bottom panel  68  to protect the housing contents.  
         [0040]    [0040]FIG. 2 of the drawings shows generally at  10 ′ an enhanced embodiment of our calibration apparatus in which the pH/ISE meter  12  is incorporated right into the housing section  30   b . Accordingly, apparatus  10 ′ includes a display panel  77  which displays both the measured pH/ISE values and the results of the calibration procedure.  
         [0041]    Apparatus  10 ′ also differs from apparatus  10  in that its controller  54  will interrogate the temperature sensor(s) in electrode  14  during calibration and automatically account for buffer temperatures during calibration and factor that information into the reported pH/ISE values.  
         [0042]    Following a calibration sequence using either apparatus  10  or  10 ′, the electrode  14  may be removed from holder  44  and inserted into a vial containing a sample solution whose pH or ISE value is to be measured. Alternatively, the electrode  14  may be positioned in solution chamber  42  and the sample poured manually into that chamber for the measurement. Most preferably, the apparatus may include another separate container for the sample, and dedicated means for automatically routing the sample to chamber  42  following the calibration procedure as will be described later in connection with FIG.5. In any event, the controllers  54  in apparatus  10  and  10 ′ are programmed as to carry out the functions to be described as commanded by instructions entered into control panel  52  by the operator.  
         [0043]    As seen in FIGS. 4 and 5, the solution chamber  42  of apparatus  10  or  10 ′ is generally cylindrical having a flared side wall formed with a plurality of inlets  42   a  in the flare. The bottom of the electrode is closed except for a drain outlet  42   b.    
         [0044]    Each container receptacle  40  is connected to a different dedicated inlet  42   a  of solution chamber  42  by a dedicated fluid conduit or tube  72 , the fluid flow through each conduit being controlled by an in-line, solenoid valve  74 . In other words, each conduit includes a first tube section extending from the particular receptacle  40  to its valve and a second section extending from that valve to a different inlet  42   a  of solution chamber  42 . Thus in the illustrated apparatus, there is a dedicated valve  74   w  controlling the flow of wash solution under gravity from container  32  to chamber  42 , and separate dedicated valves  74   4 ,  74   10  and  74   7  controlling the gravity flow of buffer solutions from containers  34 ,  36 , and  38 , respectively, to chamber  42 .  
         [0045]    The drain outlet  42   b  of solution chamber  42  is connected by a fluid conduit  78  to one arm  80   a  of a Y-fitting  80 , the fluid flow through that conduit  78  being controlled by an in-line solenoid valve  82 . The leg  80   b  of fitting  80  is connected by a conduit  84  to the drain outlet  66  at the back of housing  30 .  
         [0046]    As shown in FIGS. 4 and 5, an overflow outlet  92  is provided in the side wall of solution chamber  42  at the flare thereof. Opening  92  communicates with an exterior depending tubular stem  92   a  which is connected by a conduit  94  to the other arm  80   c  of the Y-fitting  80 . As best seen in FIG. 5, the lower edge of the overflow opening  92  is lies just below the inlets  42   a  enabling the solution chamber  42  to be filled with liquid up to the lower edge of opening  92 , any excess liquid being conducted out of the chamber via stem  92   a  and conduit  94 .  
         [0047]    When the apparatus  10  or  10 ′ is in use, the drain connector  66  at the back of housing  30  is connected by a suitable length of hose  96  to a waste container or sink S located below connector  66 . Thus, when the drain valve  82  is opened, any liquid in the solution chamber  42  will drain out under gravity through outlet  42   b  to waste container or sink S. Likewise, any excess liquid in the chamber will pass out of the chamber through the overflow opening  92  directly to the waste container S.  
         [0048]    Preferably, as shown in FIG. 5, a sensor  98  is provided in the side wall of chamber  42  below the level of the overflow opening  92  therein and which senses when chamber  42  is filled with liquid, the sensor providing a FULL signal to controller  54  when that event occurs. A second sensor  102  may be provided in the wall of chamber  42  just above the drain outlet  42   b  thereof to signal the controller by an EMPTY signal that the chamber  42  is substantially empty of liquid.  
         [0049]    All of the solenoid valves  74  and  82  described above may be low voltage, 2-way valves, preferably of the type disclosed in our U.S. Pat. No. 6,050,543. They are operated under the control of controller  54  to cause the proper sequence of separate liquid flows from the various containers  32 ,  34 ,  36  and  38  solely under gravity to the solution chamber  42  and the drainage, under gravity, of those liquids from that chamber at the appropriate times in the calibration sequence.  
         [0050]    In order to use the present apparatus  10 , the electrode  14  of pH/ISE meter  12  is inserted into the solution chamber  42 . This may be done by engaging the electrode to holder  44  and sliding the holder along slide  48  so that the electrode is lowered into the chamber. Alternatively, the electrode may be attached to an independent holder and placed in the chamber.  
         [0051]    To start the calibration procedure, the operator places electrode  14  in chamber  42  and presses a START button  52   a  on control panel  52  which causes the controller  54  to initiate a wash cycle. The cycle can vary depending upon the desired needs, cycle time and sequence as programmed into the controller. In any event, controller  54  controls as follows:  
         [0052]    A. Wash  
                                       Step 1 - Pre-clean and Fill   controller 54 opens the valve 74 w             from the wash solution container 32           and closes the drain valve 82 so that           wash solution flows solely under           gravity into the solution chamber 42           bathing electrode 14 and then drains           therefrom to the vented waste con-           tainer S; after, e.g. 10 seconds, the           controller closes the drain valve 82           whereby chamber 42 fills with wash           solution; controller 54 closes that           valve 74 w  when sensor 98 detects that           chamber 42 is filled; any overflow in           chamber 42 may flow out through the           overflow opening 92 to the vented           waste container S.       Step 2 - Soak   drain valve 82 remains closed for a           selected time, e.g. 10 seconds, allow-           ing the electrode 14 to soak in the           wash solution;       Step 3 - Empty   drain valve 82 is opened so that the           wash solution in chamber 42 drains           under gravity to the vented waste           container 42; after a selected time,           e.g. 15 seconds, or when the sensor           102 detects that chamber 42 is empty,           the controller 54 closes the drain           valve 82.       Repeat Steps 1 through 3 (optional)       Step 4 - Wash and Fill   Controller 54 opens the valve 74 w             controlling the flow of first buffer           solution from container 34 to solution           chamber 42 and opens drain valve 82           so that the first buffer solution flows           under gravity into chamber 42 there-           by bathing the working end of elec-           trode 14 in that solution and imme-           diately drains from the chamber into           the vented waste container S; after           bathing the electrode in the first           buffer solution for a selected time,           e.g., 10 seconds, controller 54 closes           the drain valve 82 so that chamber 42           fills with first buffer solution until           sensor 98 signals the controller that           the chamber is full;       Step 5 - Soak   electrode 14 remains immersed in           chamber 42 for a selected time suffi-           cient to allow pH/ISE meter 12 to           obtain a stable reading of the pH/ISE           value of the first calibration solution           in chamber 42;       Step 6 - Calibration Measurement   controller 54 controls meter 12 to           take a pH/ISE reading of the first           buffer solution and sets the meter to a           first pH/ISE calibration point 4 and           that measurement is recorded in the           meter&#39;s memory;       Step 7 - Empty   drain valve 82 is opened so that the           first calibration solution drains from           chamber 42 to the vented waste con-           tainer S.                  
 
         [0053]    B. Calibration of First Buffer Solution, e.g. pH/ISE 4  
         [0054]    Washing Between Measurements (optional)—Repeat Steps 1 to 3  
         [0055]    C. Calibration of Second Buffer Solution, e.g. pH/ISE 7  
         [0056]    Same as Steps 4 to 7 except that the controller controls the valve  747  controlling the flow of pH/ISE  7  solution from container  38  so that that buffer solution flows to and from solution chamber  42 .  
         [0057]    D. Calibration Completion of Buffer Solutions  
         [0058]    The pH/ISE meter  12  will read an acceptable slope or give an appropriate reading indicating a proper calibration between the pH/ISE meter  12  and its electrode  14 .  
         [0059]    E. Final Washing  
         [0060]    Repeat Steps 1 to 3-pH/ISE electrode  14  and meter  12  are now ready for use.  
         [0061]    After E. FINAL WASHING, the controller  54  controls printer  22  so that the printer prints out final calibration data such as the date and time, each buffer&#39;s measured pH/ISE value, the calculated slope and coefficient, as well as the intercept. The printer also prints out a final result, i.e. calibration successful or not. If the calibration was successful, the apparatus  10  and pH/I SE meter  12  shown in FIG. 1, or the apparatus  10 ′ with the integrated pH meter  12  shown in FIG. 2, is used to carry out a pH/ISE measurement on an unknown liquid specimen. This may be accomplished in one of three ways. In accordance with one method, the pH/ISE electrode may be placed in a separate vial containing the unknown sample and the pH /ISE meter  12  operated to display the pH/ISE value of that sample. Alternatively, the sample may poured manually into the solution chamber  42  of apparatus  10  or  10 ′ and the electrode  14  placed in that chamber as described above with a measurement carried out using the separate pH/ISE meter  12  or the integrated meter in apparatus  10 ′. As a third possibility, the calibration apparatus can include provisions in that apparatus for automatically flowing the sample solution to and from the solution chamber  42 , after calibration, in the same manner as described above for the buffers.  
         [0062]    An arrangement such as this is illustrated in FIG. 5 wherein a sample solution container  110  is connected by a dedicated conduit  112  to a separate, dedicated inlet  42   a  of solution chamber  42 . Conduit  112  includes a dedicated in-line sample valve  74   s  which is controlled by controller  54  in the same manner as the valves  74  described above. After a successful calibration as described above, controller  54  carries out the above Steps 4 to 7 for the valve  74   s , and the drain valve  82  so that the specimen or sample solution is first flowed through the solution chamber  42  to wash the working end of the electrode  14  and then accumulated in that chamber so that a pH/ISE measurement may be taken for the sample solution, following which the sample solution is drained from the chamber and the chamber is filled with wash solution in accordance with STEPS 1 to 3 above.  
         [0063]    If the apparatus  10  or  10 ′ is not used for a selected time, e.g. 30 minutes, following completion of a calibration cycle or following the measurement of a sample, the controller  54  is programmed to drain the wash solution from chamber  42  by opening the drain valve  82  and then filling the chamber with a storage solution, which is typically the pH/ISE 7 buffer solution from container  38 , in order to protect the electrode  14 .  
         [0064]    [0064]FIG. 6 shows the sequence of events that occur during a two-point pH/ISE 4/7 calibration as described above. In this diagram, the optional, repeated washing steps between measurement steps are not performed.  
         [0065]    Of course, if a one-point calibration were being carried out on the specimen in the solution chamber  42 , only a single calibration or buffer solution, e.g. pH/ISE 4, would be gravity fed to chamber  42  during calibration procedure. Also, if the estimated pH/ISE value of the specimen solution were estimated to be higher then, e.g. pH/ISE 7, then a two-point calibration using the pH/ISE 7 and pH/ISE 10 buffers would be used, with controller  54  being programmed to operate the appropriate valves  74  and  82  to gravity feed the buffers from their respective containers at the appropriate times in the calibration cycle.  
         [0066]    Various valving arrangements may be used to conduct solutions to and drain them from the solution chamber  42 . The arrangement shown in FIG. 5 is particularly suitable for the two-way solenoid valves described in the above-mentioned patent.  
         [0067]    It will thus be seen that the objects set forth above, among those being apparent from the preceding description, are efficiently attained. It should also be understood that certain changes may be made in carrying out the above method and in the constructions set forth without departing from the scope of the invention. Therefore, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.  
         [0068]    It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein.