End-point detection

Anhydrous ammonia is normally analy2ed for moisture, oil, and residue. The ammonia is first evaporated from the sample and the residue tested (86). In most instances, the amount of oil and sediment ia the samples are insignificant and the entire residue may be assumed to be water. For more accurate moisture determinations, the ammonia can be dissociated into nitrogen and hydrogen and the dewpoint of the dissociated gas obtained. This procedure works well where the concentration of water is in the ppm range. Where the amount of water is in the range of a few hundredths of a percent, acetic acid and methanol can be added to the residue and a Karl Fischer titration performed to an electrometricaHy detected end point (89—92). 

Conductimetry is used to detect and determine ionic species in solution. It plays important roles in environmental analyses and in the detection of ion chromatography. It is also used in acid-base, precipitation and chelometric titrations to detect end-points. However, the biggest advantage of conductimetry is displayed in the fundamental studies of solution chemistry. Its applications to chemical studies in non-aqueous solutions will he discussed in Chapter 7. 

We often use instruments to detect end points. These instruments respond to properties of the solution that change in a characteristic way during the titration. Among such instruments are colorimeters, turbidimeters, temperature monitors, refractometers, voltmeters, current meters, and conductivity meters. 

Detecting End Points in lodine/Thiosulfate Titrations A solution that is about 5 X 10 M in h has a discernible color, which corresponds to less than one drop of a 0.05 M iodine solution in 100 mL. Thus, provided that the analyte solution is colorless, the disappearance of the iodine color can serve as the indicator in titrations with sodium thiosulfate. 

Both metallic and membrane electrodes have been used to detect end points in potentiometric titrations involving complex formation. Mercury electrodes are useful for EDTA titrations of cations that form complexes that are less stable than HgY- . See Section 21D-1 for the half-reactions involved and Equation 21-5 for the Nernst expression describing the behavior of the electrode. Hanging mercury drop and thin mercury film electrodes appropriate for EDTA titrations are available from a number of manufacturers. As always, whenever mercury is used in experiments like these, we must take every precaution to avoid spilling it, and it must be stored in a well-ventilated hood or a special cabinet to remove the toxic vapors of the liquid metal. Before working with mercury, be sure to read its Materials Safety Data Sheet (MSDS), and follow all appropriate safety procedures. 

An inert indicator electrode constructed of platinum is ordinarily used to detect end points in oxidation/reduction titrations. Occasionally, other inert metals such as silver, palladium, gold, and mercury are used instead. Titration curves similar to those constructed in Section 19D are usually obtained, although they may be displaced along the potential (vertical) axis as a consequence of the high ionic strengths. End points are determined by the methods described earlier in this chapter. 

That is, the strongest acid in water is neutralised with the strongest base that can exist in water. In any other titration case, the end point is not easily detected. In the case of titration of a weak base with a strong acid we have to find a solvent other than water in which a weak base is going to behave as strong one and give us a sharp and easy to detect end point. To achieve this, we have to find solvent more acidic than water, which can be acetic acid. 

The photometric end point has been applied lo many types of reactions. For example, most standard oxidizing agents have characteristic absorption spectra and thus produce photometrically detectable end points. Although standard acids or bases not absorb, the introduction of acid-base indicators permits photoineiric neutralization titrations. The photometric end point ha.s also been used to gicat advantage in titrations with F.DTA (cthylenediaiiiinetciraacetic... 

There are several interesting analogies between coulometric and volumetric titration methods. Both require a detectable end point and are subject to a titration... 

The desired objective is to use sensitive methods to detect end-points at low levels of oxidation relevant to flavor deterioration. To evaluate the oxidative or flavor stability of food lipids, it is critical to establish how the experimental data are influenced by the methodology used and the nature of the sample materials. No short cut approach can be used to food stability tests because it is influenced by complex phenomena. The stability of each system has to be researched under various test conditions. The following factors must be considered ... 

Near-infrared (NIR) is routinely used to qualify monomers prior to polymerization reactions. It is used to measure the kinetics of polymer onset and can be used to detect end-point completion and initiator compound levels in polymerization reactions. NIR spectro- scopy can also be used to sort polymers and to control the quality of incoming raw monomers and finished polymeric materials. Molecular spectroscopy using NIR and IR measurement techniques is often used for competitive analysis and to determine thermal or photo-induced oxidation or degradation reactions in polymers. In general, NIR spectroscopy is valuable for polymer identification, characterization, and quantitation. NIR spectroscopy can be completed for in situ process applications where no sample preparation is a requirement, and where rugged optical systems are a necessity. Some of the earliest work in applying IR and NIR spectroscopy to polymer characterization is found in References 1 to 11. 

In another version of this procedure the nonionic surfactant was first extracted batch-wise with sodium tetraphenylborate into 1,2-dichloroethane. The tetraphenylborate in the isolated organic phase was then titrated with a cationic surfactant, using Victoria Blue B as indicator (70). This titration can also be performed to an electrochemically detected end point. In this version, an excess of anionic surfactant is added to the cationic complex formed by the ethoxylated nonionic surfactant and potassium ion. The ion pair is extracted into dichloroethane, separated from the initial aqueous phase, then titrated with cationic surfactant in the presence of additional water. The ion pair of the anionic surfactant and Fe(II)(l,10-phenanthroline)3 is added as indicator. The end point of the titration is indicated when the last of the anionic surfactant is complexed by the cationic titrant, causing the iron-phenanthroline cation to migrate to the aqueous phase, where it is detected as a change in potential at a platinum electrode (71). 

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