Detection point

Caro s acid, H2SO5 is used as a titrant for determining Fe +. Directions are given for exploring the method of end point detection, the titrant s shelf-life, the method s accuracy and precision, and the susceptibility of the method to interference from other species. 

These titrations can be done manually using a starch indicator for end point detection or more accurately by amperometric methods. 

Ethylenediaminetetra-acetic acid, largely as the disodium salt of EDTA, is a very important reagent for complex formation titrations and has become one of the most important reagents used in titrimetric analysis. Equivalence point detection by the use of metal-ion indicators has greatly enhanced its value in titrimetry. 

F. Alternative methods of detecting the end point. In addition to the visual and spectrophotometric detection of end points in EDTA titrations with the aid of metal ion indicators, the following methods are also available for end point detection. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

Apparatus. Use the apparatus shown in Figs. 14.2(a) and (b). The generator cathode (isolated auxiliary electrode) consists of platinum foil (4 cm x 2.5 cm, bent into a half cylinder) and the generator anode (working electrode) is a rectangular platinum foil (4 cm x 2.5 cm). For potentiometric end point detection, use a platinum-foil electrode 1.25 cm x 1.25 cm (or a silver-rod electrode) in combination with an S.C.E. connected to the cell by a potassium chloride- or potassium nitrate-agar bridge. 

Apparatus. Use the apparatus of Section 14.7. The generator anode is of pure silver foil (3 cm x 3 cm) the cathode in the isolated compartment is a platinum foil (3 cm x 3 cm) bent into a half-cylinder. For the potentiometric end point detection, use a short length of silver wire as the indicator electrode the electrical connection to the saturated calomel reference electrode is made by means of an agar-potassium nitrate bridge. 

Reagent generated Electrolyte composition Notes Substances titrated End-point detection f... 

With today s titrimeters the titration can be programmed so that not only the curve is directly registered but also its first derivative and often even its second derivative. Once the empirical curve has been obtained, a method of end-point detection must be applied, and this should be such that the end-point detected agrees with the true equivalence point. 

Final remarks on end-point detection. In addition to our remarks above on the types of titration curves and the Henderson equation or more extended relationships, we can state that in Gran s method activity coefficients are taken into account however, these were assumed to be constant, which is incorrect, and therefore the addition of an ISA (ion strength adjuster) must be recommended (for errors of the Gran method see ref.66). 

It remains possible to check the correctness of the end-point detection by calibration on samples of known composition under the same measurement conditions a similar procedure consists in the differential titrations introduced by Pinkhof and Treadwell, who used a reference electrode, identical with the indicator electrode, but dipped it into a solution buffered to the end-point potential value67. 

It is certainly clear that a coulometric titration, like any other type of titration, needs an end-point detection system in principle any detection method that chemically fits in can be used, be it electrometric, colorimetric, photoabsorptionmetric, etc. for instance, in a few cases the colour change of the reagent generated (e.g., I2) may be observed visually, or after the addition of a redox, metal or pH indicator the titration end-point can be detected photoabsorptiometrically by means of a light source and photocell combination. Concerning the aforementioned coulometric titration of Fe(II), it is... 

For end-point detection, any method usual in acid-base titration can be used with electrometric indication the precautions for protection against the... 

In the first group the titrant is generated either directly from a participating or active electrode, or indirectly from an inert or passive electrode, in which case it is necessary to add previously an auxiliary substance that generates the titrant by either cathodic reduction or anodic oxidation the end-point detection is usually potentiometric or amperometric. The following selected examples are illustrative of the first group in non-aqueous media ... 

The basic requirements for the CBMS II are to reliably detect and identify with sufficient sensitivity and selectivity both CWA and BWA in point detection and reconnaissance missions, in order to be deployable in wheeled reconnaissance vehicles and be operable by nontechnical personnel wearing, at the extreme, MOPP IV protective gear. Contrary to the usual practice for a military detector system, the CBMS II does not have its own requirements document. Instead, the requirements and specifications for the CBMS II are based on the detector requirements of the host platforms in which it will be deployed. These requirements are described in terms of performance, as opposed to the usual practice of being enumerated in volumes of detailed specifications. As is usual for a complex multiyear program, the requirements changed over the course of the CBMS II program as the requirements for the host platforms evolved. 

Fast solution reactions between analyte and a reagent titration to stoichiometric point by volumetric or coulometric methods end-point detection by visual indicators, precipitation indicators or electrochemical means. 

A prerequisite for a precise and accurate titration is the reproducible identification of an end point which either coincides with the stoichiometric point of the reaction or bears a fixed and measurable relation to it. An end point may be located either by monitoring a property of the titrand which is removed when the stoichiometric point is passed, or a property which can be readily observed when a small excess of the titrant has been added. The most common processes observed in end-point detection are change of colour change of electrical cell potential change of electrical conductivity precipitation or flocculation. (Electrochemical methods are discussed in Chapter 6 precipitation indicators find only limited use.)... 

Table 5.8 gives an indication of the range of elements that may be determined. Most procedures will require an analyte concentration of 10-3 mol dm 3 or more, although with special conditions, notably potentiometric end-point detection, the sensitivity may be extended to 1(H mol dm 3. The analysis of mixtures of metal ions necessitates masking and demasking, pH adjustments and selective separation procedures. Areas of application are spread throughout the chemical field from water treatment and the analysis of refined food and petroleum products to the assay of minerals and alloys. Table 5.10 gives some selected examples. 

Amperometric titrations have an even wider range of application than polarography. Although the titrant may be added from a burette, in many applications it is electrically generated in a coulometric cell (p. 261). Such an arrangement lends itself to complete automation and is particularly valuable for the titration of very small quantities. For examples of coulometric titrations with amperometric equivalence point detection see Table 6.5. 

Working and counter-electrodes source of constant current and timer, or a potentiostat and integrator equivalence point detection system. 

Element or compound determined Titrant generated Means of equivalence point detection Example of application... 

Catalytic Point Detection Moderate Moderate ISA CSA (API) Easily Positioned Industry Standard Detection Device Poisoning Limited to Spot Detection Costly Maintenance Expert Judgment Required for Placement Point Sources (Pumps, Compressors, Seals, etc.)... 

Improved (Chemical Agent) Point Detection System (IPDS) This detection system is a new shipboard point detector and alarm that replaces the chemical Agent Point Detection System. This system can detect nerve and blister agent vapors at low levels and automatically provides an alarm to the ship. 

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