Abstract:
A titration apparatus includes a syringe and a stirring arrangement located within the syringe for mixing of its content. A single narrow bore for drawing and discharging of liquids and an arrangement for measuring volume of liquids are also provided. The stirring arrangement is located within the syringe.

Description:
This application is a Divisional Application of Ser. No. 09/215,350 filed Dec. 18, 1998, now U.S. Pat. No. 6,082,204. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to the quantitative chemical analysis of liquids by means of volumetric titration, more specifically it provides a device that is more compact, rugged, and easier to use than the presently available apparatus, while maintaining accuracy of measurement. 
     DESCRIPTION OF THE PRIOR ART 
     The analysis of fluids for a specific chemical constituent is often accomplished by a procedure known as titration, in which a standard solution is mixed in increments with a sample to which has been added a color-forming indicator so that a marked color change occurs at the point where the amount of standard solution just neutralizes all of the constituent present in the sample. At this endpoint, the amount of the unknown constituent in the sample may be ascertained from the amount of standard solution used. 
     The prior art basic apparatus used for titration has hardly changed since the beginning, and remains cumbersome and difficult to use. The unknown is delivered to a titration flask with a pipette, then standard is added by means of a burette until the endpoint is reached. Specifically, it suffers from the following disadvantages: 
     a) The apparatus is of multiple pieces. At a minimum, six pieces are required: a pipette, a burette, a burette stand, a burette clamp, a titration flask, and a funnel. 
     b) The apparatus is expensive. The minimum cost is about 150 dollars for the above assembly. 
     c) The apparatus is bulky and difficult to store. The pipette, burette, and stand are all of substantial length. 
     d) The burette, pipette, and titration flask are made of glass and are easily broken. 
     e) The insides of the pipette and burette must be kept scrupulously clean to avoid drainage errors. This may require the use of dangerous or toxic cleaning agents. 
     f) The burette must be rinses before use with the standard. This takes time and wastes standard. Also, standard remaining in the burette at the end of a series of titrations must be discarded. 
     g) Because liquids wet the insides of the pipette and burette, time must be allowed for drainage of these surfaces. 
     h) Burette measurements are made from the position of the meniscus. The meniscus is curved and is difficult to view. If viewed from an angle, a parallax error may be made 
     i) A small amount of unknown or standard may be splashed on the side of the titration flask. The titration must be paused to wash down this deposit, or a titration error will occur. 
     j) Any partial droplet on the tip of the burette is shown by burette reading as having been delivered, but has not been delivered to the titration flask. For best accuracy, it must be washed off into the titration flask. 
     k) The contents of the flask must be mixed by swirling. Therefore, two hands are required, one to control delivery from the burette, the other to swirl the flask. This may be tiring to the operator. 
     l) The operator must add precisely the right amount of standard to achieve the endpoint. One must proceed cautiously or too much standard will be added, overshooting the end point. A good deal of time may be consumed doing a titration because of fear of overshooting the end point. This is especially true for an inexperienced operator. If the end point is overshot, the operator must then repeat the titration, or live with a less than optimum result. 
     m) Because the apparatus is open to the atmosphere, it is not suitable for titration of moisture sensitive, air sensitive, or volatile substances. 
     n) The apparatus is most accurate when a substantial portion of the contents of the burette is used for a titration. Therefore, to attain the required accuracy, it may be necessary to repeat the titration using a different amount of unknown, either by using a different pipette or by quantitatively diluting the unknown; or it may be necessary to use a standard of greater or lesser strength. 
     o) It is sometimes advantageous or necessary to perform reverse titrations, where the standard is titrated with the unknown. It is very inconvenient to do a series of reverse titrations, as the burette must be drained and filled with each new unknown. 
     p) The apparatus is not suitable for filed use. A level work surface is required. 
     q) A computation of the amount of unknown must be made. 
     Improvements have been made by the invention upon the basic apparatus. The glassware may be replaced by plastic. The burette can be arranged so that it is automatically filled. The burette may be replaced by a dispenser with digital readout. Stirring may be done with a magnetic stirrer and stir bar. However, the basic manipulations remain the same, with the result that performing a titration remains a complex and time consuming matter. Simpler methods using drip counting have been described and are used, however, they are of limited accuracy. 
     Automated analyzers have been developed, however they are expensive and are best used for the analysis of many similar samples. Such analyzers are not suited for field or educational use, or the analysis of a small number of samples. U.S. Pat. No. 5,817,954 issued to Kahng et. al. on Oct. 6, 1998, shows how the apparatus for automatic titration can be simplified, using some of the same ideas as the present patent. 
     OBJECTS OF THE INVENTION 
     It is the object of this invention to provide an apparatus. Specifically, some advantages are: 
     a) The apparatus is formed as a unitary structure. 
     b) The apparatus is of low cost. The cost of manufacture is substantially less than the existing apparatus. 
     c) The apparatus is compact and easy to store. 
     d) The apparatus is of rugged construction and not easily damaged. 
     e) The apparatus is easily cleaned with a minimal amount of water or other liquid. 
     f) No standard is wasted in rinsing the apparatus before use. The apparatus may be made of such a size that lesser amounts of standard and unknown are required, compared to the standard apparatus. 
     g) No time for the drainage of surfaces is required. 
     h) The amounts of unknown and standard are read from a ruled scale, with or without vernier, or from a digital display. Reading of the position of a meniscus is eliminated. 
     I) There is no concern about any amount of unknown or standard remaining unmixed. 
     j) The correction for standard measured but not mixed is small and of constant value. 
     k) Mixing is accomplished by a stirrer, and no manual mixing is needed. 
     l) There is very good indication of the nearness to the endpoint, and the adjustment to the endpoint is rapid and easily done. Therefore, the time required to do titration is much reduced. 
     m) The apparatus is closed to the atmosphere and is suitable for the titration of moisture or air sensitive substances. 
     n) A large range of concentrations of unknown can be analyzed using a given strength of standard without undue loss of accuracy. 
     o) Reverse titrations are easily done. 
     p) The apparatus is suitable for field use. A minimal work surface is required. 
     q) The apparatus is easily customized or programmed so that the results of titrations of a given sort can be directly read form the scale or digital display, with no computation required. Other objects and advantages will become apparent from the specifications, drawings, and description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section view of one embodiment of the apparatus of the invention; and 
     FIG. 2 shows a perspective view of another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiments of the invention will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements. 
     A syringe barrel  22  is constructed of material resistant to the chemicals used during the titration. Glass and various plastics are suitable. A needle  30  is attached to the syringe barrel  22  by a fitting, or it may be cemented in place. The needle is generally of stainless steel, but small bore plastic tubing may also be used. The inside bore of the needle should be as small in bore as possible without unduly restricting the uptake and discharge of the standard and unknown. The syringe barrel  22  may also have an opening for attachment of a sensor  44  located at the base of the syringe barrel  22  near the syringe inlet. Placement of the sensor near the inlet is important because when placed there, it can give information about the approach of the endpoint. The sensor is most commonly a pH electrode. Sensor  42  is connected to a meter for sensor  46  which may be an integral part of the titration apparatus. A vernier scale  24  imprinted upon barrel  22  is used in conjunction with a scale  28  imprinted on the plunger  26  to read the volumes used. For less accurate work a vernier is not necessary, and a single mark on the barrel will suffice. Because it is the proportion of unknown to standard that is of interest, the divisions of the scale need not correspond to any standard unit of volume. Rather, they are chosen for maximum readability. Division into centimeters and subdivision into millimeters is a good choice. A displacement sensor  50  connected to volume display  52  may also be used to measure the volumes of unknown and standard. These sensors are in common use, the most basic application being a calipers with digital readout. 
     The syringe plunger  26  is constructed of materials suitable mechanical and chemical resistant properties. The portion of the plunger that will be in contact with the liquids must be resistant to the chemicals used. Teflon, polyethylene, and polypropylene are suitable materials. The plunger is machined to provide a leak-proof seal to syringe barrel  22 , or may have a groove fitted for an o-ring or rings which provide the seal. The tightness of fit of plunger  26  in barrel  22  is sufficient as to prevent inadvertent movement of plunger  26 . The plunger may also have a rough surface or rack to be used with a thumb wheel  48  to provide a means of fine movement of the plunger. Small movements of plunger  26  are necessary to get exactly to the endpoint. A magnetic stir bar  32  is located within the syringe. 
     The magnetic stir bar  32  is spun by drive magnets  34  which are spun by a electric motor  36 . Electric motor  36  is powered by a battery  38 , and controlled by a switch with speed control  40  or off/on switch  42 . Alternately, as known in the art the stir bar is controlled by a plurality of NS switched electromagnets. The stir bar must spin at a controlled rate, suitable as to allow easy addition of standard to the endpoint, as explained below. 
     A microprocessor with controls and display  58  may be electrically connected to displacement sensor  50 . The microprocessor may be used to record the volume information, the strength of the standard, and to calculate the strength of the unknown. A holder  54  may be used to store the apparatus between uses, to charge the battery between uses, and to hold the apparatus in a fixed relationship to the unknown or standard in a beaker  56  during the titration. 
     The apparatus is first rinsed with water or other suitable liquid and the syringe plunger is positioned at or near the bottom. The small amount of liquid remaining in the syringe will not interfere with the titration. The beginning position is read. The needle tip is then wiped free of any adhering liquid. The syringe is held in a generally horizontal position, and the tip of the needle placed in a sample of the unknown. If the endpoint is to be detected by means of a color change, the addition of a small amount of an indicator to either the standard or unknown is generally necessary. A volume of the unknown is drawn into the syringe. The needle is withdrawn, wiped clean of unknown, and the volume read from the scale and vernier. The stirrer is then turned on. The needle is then placed in a sample of the standard and the standard is drawn up until the endpoint is reached. The rate of stirring is such that mixing is sufficiently slow so the nearness of the endpoint can be easily ascertained, either by a change in color in the region near the inlet or by a change in the sensor readout, the sensor being placed near the inlet. The importance of a proper rate of mixing and how this makes it easy to rapidly adjust to the endpoint cannot be overemphasized. It the mixing rate is too rapid, there will be little notice of the approach of the endpoint. If the mixing rate is too slow, excessive time is spent waiting for mixing to become complete. The small movements necessary to get exactly to the endpoint are more easily made if a thumb wheel or other means is used. At the end of the titration, the amount of standard is read. A calculation using the amount of unknown, the amount of standard, and the strength of the standard is done to give the strength of the unknown. For the most accurate work, a correction for the amount of standard left in the syringe is made. All liquid is expelled from the syringe and the apparatus is ready for the next titration. 
     If a series of titrations of a given type is planned and a standard of consistent strength is available, a scale may be selected that has a mark showing the amount of unknown to be drawn up, and that will directly read the concentration of the unknown at the end of the titration, making a calculation unnecessary. For example the apparatus can be used to determine the titratable acidity of a wine, or the grape juice or other juice from which a wine is to be made. The syringe is equipped with a mark indicating the amount of unknown to be drawn up. The titration is done with a standard base solution until the endpoint is reached. Marks of the syringe plunger show directly the titratable acidity in any desired units. Thus a series of removable scales can be used with the same plunger to perform different standardized titration. 
     Thus the reader will see that the titration apparatus of the invention provides a highly compact and easy to use device with many advantages over existing apparatus. 
     The apparatus is suitable for almost any type of volumetric titration, with the exception of those which evolve a gas, or form a precipitate which would clog the needle. The apparatus as described titrates a liquid with a liquid. A solid can be titrated if it is dissolved and drawn up in its entirety. The apparatus could also find use in the compounding of solutions, especially those that require a titration. 
     The accuracy attainable is limited principally by the quality of construction and the readability of the volumes. The apparatus will be most accurate when the full volume of the syringe is used, and the amounts of unknown and standard are equal. For example, with a vernier scale and a syringe travel of 70 millimeters, the unknown and the standard could each be read to 0.1 of 35 millimeters, leading to a potential accuracy of about 0.5% for the titration. If the amount of unknown is 10% of the amount of standard, or the reverse, the liquid drawn up in lesser amount could be read to 0.1 of 7 millimeters, leading to a potential accuracy of about 1.4%. Thus, a large range of unknowns can be analyzed with a given standard without great loss of accuracy. If a displacement sensor with digital readout is fitted, the accuracy of reading is increased, as these sensors can detect a change in position of as little as 0.01 millimeter. 
     While the above description contains many specific features, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of one preferred embodiment thereof. Many other variations are possible. For example, the movement of the plunger could be controlled by means of a motor. The stir bar could be driven by NS switched electromagnets located outside the syringe, either fixed at the bottom of the plunger, or following the movement of the plunger. The needle could be straight, and the syringe operated in a vertical position. The scale could be on the barrel and the vernier on the plunger. The stirrer could be powered by AC current instead of a battery. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.