Visual endpoint, titrations

In the light of these observations Olson and Chen [164] decided to use a correction factor for use in their visual endpoint calcium titration method involving titration with EGTA. They found that interferences by magnesium... 

From these equations it can be seen that each mole of water requires one mole of I2. In a visual endpoint Karl Fischer titration, a sample is titrated with the Karl Fischer reagent until a permanent iodine color (indicating that all water has been reacted) is observed. Because of other reaction products, the color change is usually from a yellow to a brownish color, which may be difficult to detect visually. Highly colored samples may affect the visual end point as well. A much sharper end point, known as the dead stop end point, can be obtained if the titration is done electrometrically. Here, two small platinum electrodes dip into the titration cell, a small constant voltage is impressed across these electrodes, and any current that flows is measured with a galvanometer. At the end point of the titration the current either goes to a minimum or else increases suddenly from nearly zero. Commercially available Karl Fischer instruments incorporate semiautomatic microprocessors based on this principle. 

Procedure Transfer about 300 mg of the test article (accurately weighed) to a 150-mL beaker, dissolve in 1.5 mL of anhydrous formic acid, and add 60 mL of glacial acetic acid. Add crystal violet Test Solution, and immediately titrate with 0.1 N HCIO4 to a green endpoint. Perform a blank determination, and make any necessary correction. Each milliliter of 0.1 N HCIO4 is equivalent to 29.43 mg of aspartame. The method description cautions that a blank titration exceeding 0.1 mL may be due to excessive water content, and may cause loss of visual endpoint sensitivity. 

Milk fat Milk fat Blood serum, heparin, CaCL Extraction with isopropanol-heptane-fLSO., titration to visual endpoint Extraction with isopropanol-heptane-fLSO., titration to visual endpoint Milk lipase Bacterial lipase Deeth and Fitz-Gerald (1977) Fitz-Gerald and Deeth (1983)... 

Note Use 0.1 A perchloric acid previously standardized to a green endpoint. A blank titration exceeding 0.1 mL may be due to excessive water content and may cause loss of visual endpoint sensitivity. 

Procedure Where the individual monograph specifies the water content is to be determined by Method lb, transfer 35 to 40 mL of methanol or other suitable solvent into the titration vessel, and titrate with the Karl Fischer Reagent to the electrometric or visual endpoint. Quickly add the Test Preparation, mix, and add an accurately measured excess of the Karl Fischer Reagent. Allow sufficient time for the reaction to reach completion, and titrate the unconsumed Karl Fischer Reagent with standardized Water Solution to the electrometric or visual endpoint. Calculate the water content of the specimen, in milligrams, with the formula... 

A titrimetric method has been reported in which triflupromazine hydrochloride was titrated visually to a colorless endpoint with ceric sulfate. The reaction proceeds through forming a red-colored semiquinone free radical (420 nro ) followed by formation of the colorless sulfoxide derivative. The method can be applied to pharmaceutical dosage forms. 

Halide impurities are probably the most studied of the four general categories of impurities common to ionic liquids and, besides electrochemical analysis, two methods are currently being used to determine the level of residual halide impurities in ionic liquids [12]. The titration of the ionic hquid vnth AgN O3 is still widely used but suffers from a certain solubility of AgQ in the ionic hquid under investigation. This method can be enhanced by the Vollhard method for chlorine determination where the chloride is first precipitated with excess AgNO3 followed by back-titration of the mother liquor with aqueous potassium thiocyanate [13]. This method uses a visual endpoint through the formation of a complex between thiocyanate and an iron (III) nitrate indicator. 

When the aim of the titration is simply to perform an analytical determination, either visual or instrumental indicators will serve the purpose. Titration errors, which are the differences between experimentally determined endpoints and ideal equivalence points, are generally larger and less reproducible for visual endpoints than for instrumental endpoints. Further, only with an instrumental indicator can a titration curve be described. Therefore, instrumental indicators are more suited to the study of the fundamentals of titration processes. 

An acid-base titration is a quick and convenient method for the quantitative analysis of substances with acidic or basic properties. Many inorganic and organic acids and bases can be titrated in aqueous media, but others, mainly organic, are insoluble in water. Fortunately, most of them are soluble in organic solvents hence they are conveniently determined by nonaqueous acid-base titrimetry. Although acid-base titrations can usually be followed potentio-metrically, visual endpoint detection is quicker and can be very precise and accurate if the appropriate indicator is chosen. 

On the other hand, despite interesting studies of visual titrations in acetic acid medium, in actual practice, crystal violet and methyl violet, which exhibit identical color changes, are by far the most widely used indicators. This is probably because their two chromatic transitions in acetic acid cover an unusually large potential range of 200 or 300 mV. However, in titrations of bases with pXb>6.8, the endpoint of crystal violet is assigned to a color that is not in agreement with the complete chromatic transition. Consequently, the visual endpoint location is difficult and depends on the base to be titrated and also on its concentration. 

The Grafting Degree (G) of the samples was determined by the method of neutralization titration. When the grafted LDPE were fully soluble in boiling xylene, the excessive KOH-ethanol was added just after the indicator. Then HCl-ethanol was added in, and the titration was stopped immediately at the visual endpoint when the color changed. G was calculated by the following equation ... 

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

An acid-base titration is a laboratory procedure that we use to determine the concentration of an unknown solution. We add a base solution of known concentration to an acid solution of unknown concentration (or vice versa) until an acid-base indicator visually signals that the endpoint of the titration has been reached. The equivalence point is the point at which we have added a stoichiometric amount of the base to the acid. 

Results of three independent titration studies of silica-alumina cracking catalyst are compared in Fig. 2. Benesi (26) and Hirschler (24) used n-butylamine as the basic reagent and detected endpoints visually by means of Hammett and arylcarbinol indicators, respectively. Drushel and Sommers (21) used diethylamine as the basic reagent and measured end-... 

Optical sensors for ions use indicators, which exist in two different colors, depending on whether the analyte is bound to them. The use of colored indicators is one of the oldest principles of analytical chemistry, used extensively both in direct analytical spectroscopy and in so-called visual titrations. In their sensing application, the indicator is confined to the surface of the optical sensor or immobilized in the selective layer. In that sense, the oldest and most widespread optical sensor is a pH indicator paper, the litmus paper, which is commonly used for the rapid and convenient semiquantitative estimate of pH of solutions or for endpoint detection in acidobasic titrations. Its hi-tech counterpart is a pH optrode (the name of which is intentionally reminiscent of the pH electrode), which essentially does the same thing (Wolfbeis, 2004). The operation principles and limitations of ion optical sensors are common for all ions. 

Electrochemical endpoint detection methods provide a number of advantages over classical visual indicators. These methods can be used when visual methods of endpoint detection cannot be employed because of the presence of colored or clouded solutions and in the case of detection of several components in the same solution. They are more precise and accurate. In particular, such methods provide increased sensitivity and are often amenable to automation. Electrochemical methods of endpoint detection are applicable to most oxidation-reduction, acid-base, and precipitation titrations, and to many complex-ation titrations. The only necessary condition is that either the titrant or the species being titrated must give some type of electrochemical response that is indicative of the concentration of the species. 

Because the generator electrodes must have a significant voltage applied across them to produce a constant current, the placement of the indicator electrodes (especially if a potentiometric detection system is to be used) is critical to avoid induced responses from the generator electrodes. Their placement should be adjusted such that both the indicator electrode and the reference electrode occupy positions on an equal potential contour. When dual-polarized amperometric electrodes are used, similar care is desirable in their placement to avoid interference from the electrolysis electrodes. These two considerations have prompted the use of visual or spectrophotometric endpoint detection in some applications of coulometric titrations. 

The approved variations [14] in the Karl Fischer method include volumetric titration methods to either a visual (excess iodine or addition of an indicator) or volta-metric endpoint detection method. The visual or voltametric endpoint methods usually require 30-40 mg of sample for analysis for freeze-dried biological products containing from 1.0% to 3.0% residual moisture. Coulometric Karl Fischer instruments generate the iodine from potassium iodide for water titration at the electrodes. Only 10-20 mg of freeze-dried sample is required for accurate analysis. 

The determination of ascorbic acid in foods is based, in part, on its ability to be oxidized or to act as a reducing agent. The most common method for determination of vitamin C in foods is the visual titration of the reduced form with 2,6-dichloroindophenol (DCIP) (4-7). Variations in this procedure include the use of a potentiometric titration (6), or a photometric adaptation (S) to reduce the diflSculty of visually determining the endpoint in a colored extract. The major criticisms of this technique are that only the reduced vitamin, and not the total vitamin C content of the food, is measured, and that there can be interference from other reducing agents, such as sulfhydryl compounds, reductones, and reduced metals (Fe, Sn, Cu), often present in foods. The DCIP assay can be modified to minimize the effects of the interfering basic substances, but the measurement is still only of the reduced form. Egberg et al. (9) adapted the photometric DCIP assay to an automated procedure for continuous analysis of vitamin C in food extracts. 

Various methods of extraction of secobarbital prior to titration with nonaqueous titrants have been reported43>44 45>46. Endpoints were determined visually or potentiomet-... 

EIA based on visual titration require clear-cut endpoints. Both horseradish POase and urease produce easily detectable products, whereas the products of APase or BGase are less strongly colored. 

Spectrophotometric titrations are used in cases where it is difficult to determine the endpoint visually as, for example, when there is a permanent change in the colour of the system. Good results are obtained in titrations of rather dilute solutions, of the order of lO " M. Spectrophotometric titrations are often performed in automatic systems. 

Naturally, some means of detecting the endpoint of the titration must be available. Indicators can be used (although their sensitivity is not good at the low levels usually investigated), as well as essentially any other method available for regular titrations. Potentiometry (Chap. 2) or amperometry with two similar electrodes is often used because of increased sensitivity over visual indicators. 

In order for a useful, reliable titration to be achieved, the titration reaction must go to completion in a relatively rapid fashion and be free of any interfering side reactions. The substance used as titrant must be readily available in pure, stable form and be reasonable in cost. Further, it must be possible to readily detect the endpoint, the experimental approximation of the equivalence point, by suitable visual or instrumental indicators. 

Examination of the titration curve visually can locate the endpoint as the midpoint of the nearly vertical portion in the vicinity of the equivalence point. Additionally, if the difference curve in which ApM/AVl is plotted against Vl, is constructed, the maximum is located at the endpoint. 

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