Titration vessel

The characteristics of the vessel where the sample and titrant meet depend to a great extent on those of monitoring system used. Thus, while electroana-lytical or photometric probes do not require special vessels (a simple beaker, thermostated if the chemical system Involves demands It, where the probe can be conveniently submerged), conventional optical (photometric or fluorlmetric) detectors require titration units matched to the design of the measuring compartment of the Instrument, and thermometric detectors require not only the titration cell but also Its environment to be thermally Isolated. 

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Schwartz has published some hypothetical data for the titration of a 1.02 X ICr" M solution of a monoprotic weak acid (pXa = 8.16) with 1.004 X ICr M NaOH. " A 50-mL pipet is used to transfer a portion of the weak acid solution to the titration vessel. Calibration of the pipet, however, shows that it delivers a volume of only 49.94 ml. Prepare normal, first-derivative, second-derivative, and Gran plot titration curves for these data, and determine the equivalence point for each. How do these equivalence points compare with the expected equivalence point Comment on the utility of each titration curve for the analysis of very dilute solutions of very weak acids. 

The platinum wire P dipping into the mercury may be welded to a copper wire, but it is preferable to use a platinum wire sufficiently long to protrude at the top of the electrode tube. The mercury must be pure and clean in case of doubt, the mercury should be washed with dilute nitric acid and then thoroughly rinsed with distilled water. The electrode is filled with mercury so that the wide portion is half-full it is most important that no mercury is spilled into the titration vessel during the titration. After each titration the electrode is repeatedly washed with distilled water. 

With this simplified procedure it is much easier to protect the system from atmospheric moisture, which must obviously be excluded. Modem K-F Titrators are equipped with special titration vessels which are designed to prevent the ingress of atmospheric moisture. Many also have microprocessors attached which will carry out the requisite operations automatically and will often provide a print-out of the results including the percentage moisture content. 

Other detection methods are based on optical transmittance [228-231], Alcohol sulfates have been determined by spectrophotometric titration with barium chloride in aqueous acetone at pH 3 and an indicator [232] or by titration with Septonex (carbethoxypentadecyltrimethylammonium bromide) and neutral red as indicator at pH 8.2-8.4 and 540 nm [233]. In a modified two-phase back-titration method, the anionic surfactant solution is treated with hyamine solution, methylene blue, and chloroform and then titrated with standard sodium dodecyl sulfate. The chloroform passing through a porous PTFE membrane is circulated through a spectrometer and the surfactant is analyzed by determining the absorbance at 655 nm [234]. The use of a stirred titration vessel combined with spectrophotometric measurement has also been suggested [235]. Alternative endpoint detections are based on physical methods, such as stalag-mometry [236] and nonfaradaic potentiometry [237]. 

Standardization was defined in Section 4.2 as a titration experiment in which the concentration of a solution becomes known to a high degree of precision and accuracy. In a standardization experiment, the solution being standardized is compared to a known standard. This known standard can be either a solution that is already a standard solution or an accurately weighed solid material. In either case, the solute of the solution to be standardized reacts with the known standard in the titration vessel. If the solution to be standardized is the titrant, then the known standard is the substance titrated, and vice versa. We will now describe these two methods and the calculations involved. 

It is possible to monitor the course of a titration using potentiometric measurements. The pH electrode, for example, is appropriate for monitoring an acid-base titration and determining an end point in lieu of an indicator, as in Experiment 10 in Chapter 5. The procedure has been called a potentiometric titration and the experimental setup is shown in Figure 14.11. The end point occurs when the measured pH undergoes a sharp change—when all the acid or base in the titration vessel is reacted. The same... 

In addition, potentiometric titration methods exist in which an electrode other than an ion-selective electrode is used. A simple platinum wire surface can be used as the indicator electrode when an oxidation-reduction reaction occurs in the titration vessel. An example is the reaction of Ce(IV) with Fe(II) ... 

If this reaction were to set be up as a titration, with Ce4+ as the titrant and the Fe2+ in the titration vessel and the potential of a platinum electrode dipped into the solution monitored (vs. a reference electrode) as the titrant is added, the potential would change with the volume of titrant added. This is because as the titrant is added, the measured E would change as the [Fe2+] is decreased, the [Fe3+] is increased, and the [Ce3+] is increased. At the end point and beyond, all the Fe2+ is consumed and [Fe3+] and [Ce3+] change only by dilution thus E is dependent mostly on the change in [Ce4+]. At the end point, there would be a sharp change in the measured E. 

In the volumetric method, the titrant can be a solution of iodine, methanol, sulfur dioxide, and an organic base, as described previously. Such a mixture is commonly known as the Karl Fischer reagent and can be purchased from any chemical vendor. It can also be a solution of iodine in methanol solvent. In that case, a Karl Fischer solvent containing the other required components is needed for the titration vessel. 

FIGURE 14.14 Photographs of a typical volumetric unit. The complete unit is shown on the left and a close-up of the titration vessel on the right. The dual platinum wire probe is visible in the right photograph as is the tube (dark in color) that introduces the titrant to the solution. 

Press and hold the IN button on the top rear of the 703 Ti Stand in order to pump solvent into the titration vessel. Continue to add solvent until the electrode pins are completely immersed. 

Press Start. Add the measured water to the titration vessel by piercing the septum in the sample inlet port with the needle of the syringe and pushing the plunger all the way in. Remove the syringe from the sample port. Type in the sample weight on the keypad and press enter. The water will now be titrated by the automatic addition of titrant. At the completion of the titration, read and record the titer on the display. 

Press start. Add the sample to the titration vessel as you did the water in step 6, typing in the sample weight and pressing enter as before. When finished, the percent water in the sample should be displayed. Repeat with a second sample if desired. 

For more determinations, proceed with step 8. When the titration vessel fills (after several runs), eliminate the solution in the titration vessel by pressing the out button on the 703 Ti Stand and holding it in. To perform more determinations after that, fresh methanol must be introduced and conditioned (steps 3 and 4). The titer of the titrant should not change over a short period of time. 

In a Karl Fischer experiment, why is the titration vessel sealed off from the laboratory air ... 

Figure 14.1 illustrates a simple dead-stop end-point assembly or a Karl Fischer titration apparatus. The titration vessel is fitted with a pair of identical platinum electrodes, a mechanical stirrer with adjustable speed, and a burette. It will be observed that absolutely little or no current may flow unless and until the solution is totally free from any polarizing substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the anode and cathode respectively. However, the current shall flow only when the two electrodes... 

Commercially available Modem KF-Titrators are usually equipped with specifically designed titration vessels that are exclusively meant to check and prevent the contact with atmospheric moisture. Quite a few such devices are armed with microprocessors that will perform the requisite operations sequentially in a programmed manner automatically and may also dish out a print-out of the desired results including the percentage moisture content. In fact, these Modem KF-Titrators not only afford greater accuracy and precision in results but also offer much ease and convenience in routine analysis as compared to the classical techniques based on either caulometry or controlled current potentiometiy using two indicator electrodes. 

In this procedure the iodide needed for the reaction with water is normally generated within the titration vessel caulometrically as shown below ... 

Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the amperometric end-point with the Karl Fischer reagent. Quickly add 0.2 g of prednisolone sodium phosphate sample, stir for 1 minute and again titrate to the amperometric end-point with the Karl Fischer reagent. The difference between the two titrations gives the volume (v) of Karl Fischer reagent consumed by the sample. 

It is necessary to limit all possible contact of the sample with air in transferring the sample into the titration vessel Procedure ... 

Introduce 10 to 25ml of the anhydrous "sample solvent (pyridine-glycol, 1 4) into the titration vessel (Fig Et 1), making sure, if an instrument end point apparatus is used, that the electrodes are covered by the amt of "sample solvent introduced. If the color end point is also to be used, introduce 10 to 25ml sample into the 2nd titration flask... 

Finally tip the contents into a titration vessel, and add water rinsings (2-4 ml). Titrate with the standard acid back to the initial color. Use standard alkali to correct any inadvertent overshoot as indicated by a red color. 

In some cases (see below) a KF drying oven is required to get the water from a sample into the titration vessel. For these special cases, a solid sample (usually) is placed into a specially designed KF oven where the sample is heated, and the water goes into the vapor phase. A stream of dry carrier gas (usually, N2 or air) sweeps the liberated moisture into the reaction vessel, where it is titrated by either the coulometric or the volumetric method. It is critical that the carrier gas is dry and that there are no leaks along the pathway to the reaction vessel. Passing the carrier gas over activated molecular sieve prior to the sample will ensure that the gas is dry. 

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