Acidity titration endpoint

Fig. 4-20. The pH values for alkalinity and acidity titration endpoints in a closed system as a function of the initial total carbonate concentration.

Conductometric titration rests on the marked changes that occur near the titration endpoint in the relation between conductivity and the amount of titrant added (an extreme or inflection point). It is used in particular for the titration of acids with base (and vice versa) in colored and turbid solutions or solutions containing reducing and oxidizing agents (i.e., in those cases where the usual color change of acid-base indicators cannot be seen). 

For analysis in solutions, the most frequently used CL reaction is alkaline oxidation of luminol and lucigenin in the presence of hydrogen peroxide as oxidant, although sodium hypochlorite, sodium perborate, or potassium ferricyanide may also be used. CL reactions involving alkaline oxidation have been used to indicate acid-base, precipitation, redox, or complexometric titration endpoints either by the appearance or the quenching of CL when an excess of titrant is present [114, 134], An example of these mechanisms is shown in Figure 14. 

At the titration endpoint, most carbonate in solution is present as C02(aq). We can expect that each mmol of HCO3 originally present in solution will neutralize one meq of acid, according to the reaction... 

Bromocriptine mesilate may be assayed in glacial acetic acid/acetic anhydride 1 7 by titration with 0.1 N perchloric acid. The endpoint may be determined potentiometrically using a glass/calomel electrode system. 

Total Acid. Simple titration procedures are used to determine total acidity. Problems arise because of the widely varying amounts of different acids in wines tartaric, malic, citric, lactic, succinic, acetic, etc. Different pKtt values for these acids make it impossible to predetermine easily the correct pH of the endpoint. Since a strong base is being used to titrate relatively weak acids, the endpoint will be greater than pH 7. In this country phenolphthalein (8.3) or cresol red (7.7) endpoints or a pH meter to 9.0 have been used (3, 6, 12, 76, 77) and the results are expressed as tartaric acid. The result at pH 7.7 X 1.05 approximately equals the result of titrating to pH 8.4. In Europe pH 7 is usually the endpoint, in France the results are expressed as sulfuric acid, and in Germany as tartaric or in milliequivalents (78). 

Two methods are commonly used to determine the endpoint of an acidity titration. The potentiometric method titrates to a predetermined pH and the colorimetric method uses an indicator that changes color at a particular pH to determine the endpoint. Other methods define the endpoint as the inflection of a titration curve, i.e., plots of pH value versus milliliter of NaOH used (Sadler and Murphy, 1998 Hand et al., 1993). However, the increased precision... 

Fluoride addition promotes the cracking (108-110) and isomerization (108, 111) activity of alumina, presumably, because of the formation of Brdnsted acid sites. In a comprehensive study of fluorided aluminas, Antipina et al. (110) demonstrated that there is a close parallelism between generation of cumene cracking activity at 430°C and surface acidity when fluoride content is increased from 0 to 7% wt (see Fig. 15). Acidity was measured by n-butylamine titration endpoints were determined by means of an arylcarbinol indicator that detected Br0nsted acids stronger than those corresponding to a pKa of — 13.3. In a separate article (112), Anti-... 

Dissolve about 240 mg of sample, accurately weighed, in 75 ml of glacial acetic acid. Titrate with standardized 0.1 N perchloric acid in acetic acid to a potenticmetric endpoint using glass-calcmel electrodes. Each ml of 0.1N perchloric acid is equivalent to 25.234 mg of cimetidine. 

Assay Dissolve about 400 mg of sample, accurately weighed, in a mixture of 100 mL of water, recently boiled and cooled, and 25 mL of 2 N sulfuric acid. Titrate the solution immediately with 0.1 N iodine, adding starch TS near the endpoint. Each milliliter of 0.1 N iodine is equivalent to 8.806 mg of C6H806. 

Sodium Chloride Content Accurately weigh about 20 mg of sample, and dissolve it in 50 mL of water contained in a glass-stoppered flask. Add, while agitating, 3 mL of nitric acid, 5 mL of nitrobenzene, 50.0 mL of 0.1 N silver nitrate, and 2 mL of ferric ammonium sulfate TS. Shake well, and titrate the excess silver nitrate with 0.2 N ammonium thiocyanate. The titration endpoint is indicated by the appearance of a red color. Perform a blank determination (see General Provisions). Calculate the content of sodium chloride in the sample by the formula... 

Assay Dissolve about 200 mg, accurately weighed, in 3 mL of formic acid and 50 mL of glacial acetic acid. Titrate with 0.1 A perchloric acid in glacial acetic acid, determining the endpoint potentiometrically. Perform a blank determination (see General Provisions), and make any necessary correction. Each mL of 0.1 Aperchloric acid is equivalent to 10.51 mg of C3H7NO3. 

In calculating the amount of caustic required for caustic neutralization, the oil is titrated to a phenolphthalein end point. This titration endpoint includes not only the FFA, but also the oryzanol compounds. With the higher caustic addition, the ory-zanol is transferred to the soapstock away from the oil. The nutritional benefit of these compounds is lost. An alternative indicator for titration uses alkali blue (8). This indicator reflects the acidity contributed only by the free fatty acids. 

As the titration curve shows (see Fig. 15.13), the pH increases dramatically in the immediate vicinity of the equivalence point [HjO ] changes by four orders of magnitude between 99.98 mL and 100.02 mL NaOH Any indicator whose color changes between pH = 5.0 and pH = 9.0 therefore signals the endpoint of the titration to an accuracy of 0.02 mL in 100.0 mL, or 0.02%. The titration endpoint, the experimentally measured volume at which the indicator changes color, is then almost identical to the equivalence point, the theoretical volume at which the chemical amount of added base equals that of acid present originally. 

Titrating the water with a strong base such as NaOH, the first inflection point around pH 4.5 is taken as the endpoint of the mineral acidity or strong-acid titration. It corresponds to completion of the reaction... 

The titration endpoint for the C02-acidity (i.e., H2CO° content) of natural waters with a strong base such as NaOH is usually taken as pH = 8.3 (pOH = 5.7). This is the pH at which more than 99% of the H2CO3 has been converted to HCO3 based on the titration reaction... 

The exact endpoint pH of the C02-acidity titration may, in fact, differ from 8.3 by several tenths of a pH unit. Its value will depend on the total carbonate (Cj) concentration of the solution (Eq. (5.21 ]), The endpoint is defined by the equality... 

As evident from the definition of total acidity in Eq. (5.41), HCOj is also an acid. At the endpoint of the total acidity titration near pH 11 (the third inflection point in Fig. 5.6) the reaction... 

The alkalinity is measured by titration with a standardized solution of a strong acid such as H2SO4, HNO3, or HCl. The titration curve is identical, but inverse to the acidity titration curve in Fig. 5.6. The caustic alkalinity titration endpoint near pH 11 measures free OH from strong bases. At this endpoint HCO3 = OH . At the carbonate alkalinity endpoint (pH = 8.3) conditions are identical to those defined above for C02-acidity. 

The buret stopcock is opened and titrant is slowly added until the solution permanently changes color or the pH rapidly changes. This is the titration endpoint, and a final buret reading is made. The final state of a titration experiment is shown to the right below. The endpoint occurs when the number of acid and base equivalents in the flask are identical ... 

How does the endpoint pH of a strong base-strong acid titration compare with that of a weak base-strong acid titration ... 

We can make similar definitions and calculations concerning acidity measurements. For a water in which acidity is contributed solely by and carbonate species, the endpoint of the total acidity titration is at a pH where a stoichiometric amount of OH has been added to complete the following three reactions ... 

We have been careful in the preceding discussion of alkalinity and acidity titrations and calculations to refer to the pH of the various endpoints as "approximate values." The actual values that correspond to pHcoj, pHhcos-. and pHcOj - are not truly fixed values, rather they vary with theCj.coj in solution. If we treat the titrations as closed systems the Ct.coj at the endpoint will be the same as theCr.coj in the initial solution. This appears to be a reasonable approach if the solution is not shaken vigorously and if the titration is conducted rapidly. The variation in equivalence point pH values is shown in Fig. 4-20. 

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