Neutralization endpoint

In essence, aqueous titration of surface acidity is an ion-exchange process in which hydrated surface protons are replaced by other hydrated cations (e.g., Na+, NIV") during the course of the titration. The procedure is straightforward. It usually consists of the direct titration of an aqueous suspension of the sample of powdered solid with a dilute base (e.g., sodium hydroxide) to a neutral endpoint. Another commonly used procedure consists of noting the pH of an appropriate salt solution (e.g., ammonium acetate), adding the sample, and measuring the amount of di-... 

Even if you cannot influence the sequence of data collection, you can control the size of the interval between points. The general rule is Take points more closely spaced when the observed quantity is varying more rapidly. Thus data points should be taken more frequently at early times for chemical kinetics and near the neutralization endpoint for an acid-base titration, as illustrated in Fig. 1. 

Suppose we need to find the molarity of a solution of HCl, which has an unknown concentration. We can do this by a laboratory procedure called titration in which we neutralize an acid sample with a known amount of base. In a titration, we place a measured volume of the acid in a flask and add a few drops of an indicator, such as phenolphthalein. An indicator is a compound that dramatically changes color when pH of the solution changes. In an acidic solution, phenolphthalein is colorless. Then we fill a buret with a NaOH solution of known molarity and carefully add NaOH solution to neutralize the acid in the flask (see Figure 14.7). We know that neutralization has taken place when the phenolphthalein in the solution changes from colorless to pink. This is called the neutralization endpoint. From the measured volume of the NaOH solution and its molarity, we calculate the number of moles of NaOH, the moles of acid, and the concentration of the acid. 

FIGURE 14.7 The titration of an acid. A known volume of an acid is placed in a flask with an indicator and titrated with a measured volume of a base solution, such as NaOH, to the neutralization endpoint. 

In the titration, we neutralize the acid by adding a volume of base that contains a matching number of moles of OH. We know that neutralization has taken place when the phenolphthalein in the solution changes from colorless to pink. This is called the neutralization endpoint. From the volume added and molarity of the NaOH solution, we can calculate the number of moles of NaOH, the moles of acid, and then the concentration of the acid. 

Bromopyrogallol red. This metal ion indicator is dibromopyrogallol sulphon-phthalein and is resistant to oxidation it also possesses acid-base indicator properties. The indicator is coloured orange-yellow in strongly acidic solution, claret red in nearly neutral solution, and violet to blue in basic solution. The dyestuff forms coloured complexes with many cations. It is valuable for the determination, for example, of bismuth (pH = 2-3. nitric acid solution endpoint blue to claret red). 

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]. 

The analyst reports the amount of acid required to reach the endpoint, generally expressed in terms of the number of mg of CaC03 that could be dissolved by the acid, per kg solution. Since the mole weight of CaC03 is 100.09 g and each mole of carbonate can neutralize two equivalents of acid, the conversion is,... 

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... 

The equivalence point or endpoint of a titration is the point at which an equivalent amount of acid or base has been added to the base or acid being neutralized. 

Calculations The total volume of 1 N sulphuric acid consumed in the titration was required to neutralize NaOH and Na Oj, thereby converting the latter first to NaHC03 at the phenolphthalein endpoint and then to H2C03 at the methyl orange end-point. 

As effect parameters studied in in vitro assays become more refined, the question becomes more important of how to interpret the findings in terms of toxicity. For example in transcriptomic experiments, very low exposures in the EST which do not affect cellular endpoints of proliferation and differentiation can be shown to affect gene expression (46). The question arises at what level of response the observed effect reaches the level of adversity. Evidently, many physiologic responses are beneficial, neutralizing the hazard by homeostatic control and therefore not all detectable responses should be characterized as toxic or adverse. It may not even be possible to answer this question on the level of an individual test, but this may require a more integrated approach using weight of evidence over a combination of results from different assays. 

Benzoic acid is assayed by titration with 0.1 N sodium hydroxide VS to a pink phenolphthalein endpoint. The procedure calls for the dissolution of about 500 mg of accurately weighed sample in 25 mL of diluted alcohol (previously neutralized with 0.1 N sodium hydroxide). Each milliliter of 0.1 N sodium hydroxide is equivalent to 12.21 mg of benzoic acid. 

Both the Indonesian Pharmacopoeia 1995 [9] and the United States Pharmacopoeia 23 [11 j follow the same procedure. About 500 mg of benzoic acid (accurately weight) is dissolved in 25 mL of dilute alcohol that previously has been neutralized with 0.1 N NaOH, phenolphthalein TS is added, and that solution titrated with 0.1 N NaOH to a pink endpoint. 

For the titration of a strong base with a weak acid, the equivalence point is reached when the pH is greater than 7. The half equivalence point is when half of the total amount of base needed to neutralize the acid has been added. It is at this point that the pH = pK of the weak acid. In acid-base titrations, a suitable acid-base indicator is used to detect the endpoint from the change of colour of the indicator used. An acid-base indicator is a weak acid or a weak base. The following table contains the names and the pH range of some commonly used acid-base indicators. 

At the endpoint, this base will neutralize 0.00546 mol HCI. Therefore, this amount of HCI must have been present in the sample before the titration. 

According to the British Pharmacopoeia 2002, the European and Indian Pharmacopoeias, and the Pharmacopoeia of the People s Republic of China, the assay for mefenamic acid is performed by a titration method. About 0.200 g of the substance to be assayed is dissolved with the aid of ultrasound in 100 mL of warm ethanol that has been previously neutralized to the phenol red endpoint. About 0.1 mL of phenol red solution is added and titrated with 0.1 M sodium hydroxide. Each milliliter of 0.1 M NaOH is equivalent to 24.13 mg of mefenamic acid [2, 5-7],... 

Analyses of NH2-terminated PAMAM dendrimers by titration and concurrent 13C-NMR monitoring of carbons attached to primary and tertiary amino groups were very interesting. Usually, tertiary amines are assumed to be more basic than primary amines. With the present prototypes, the opposite was true. Titrations with hydrochloric acid showed that neutralizations occurred to give rather sharp breaks at pH = 6.85 and pH = 3.86 (Fig. 43), which were in very close agreement with theoretical values for terminal primary amino groups and interior tertiary amino groups, respectively [2, 79]. The distinctive endpoints exhibited by these... 

Assay Transfer about 2 g of sample, accurately weighed, into a 250-mL flask, and dissolve it in 100 mL of water. Add 40 mL of a mixture of equal volumes of formaldehyde and water, previously neutralized to phenolphthalein TS with 1 N sodium hydroxide. Mix, allow to stand for 30 min, and titrate the mixture with 1 N sodium hydroxide to a pink endpoint that persists for 5 min. Each milliliter of 1 N sodium hydroxide is equivalent to 66.06 mg of (NH4)2S04. 

Assay for Calcium Oxide Using 1 N sodium hydroxide, neutralize to litmus the combined filtrate and washings retained in the Assay for Silicon Dioxide (above), and add, while stirring, about 30 mL of 0.05 M disodium EDTA from a 50-mL buret. Add 15 mL of 1 N sodium hydroxide and 300 mg of hydroxy naphthol blue indicator, and continue the titration to a blue endpoint. Each milliliter of 0.05 M disodium EDTA is equivalent to 2.804 mg of CaO. 

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