Indicators, acid-base determination

The non-linear plot in Fig. 11.2A indicates acid-base catalysis (i.e. when both acid and base are involved in the same catalytic mechanism with rate = 1cha.b[HA] [B] [HS]), and the Icha.b parameter can be determined from the slope of a plot of k0bs/(l - FB)( [acid] + [base])2 versus FB (not shown). 

An acid-base titration is a procedure that is used where a base of known concentration is added to an acid of unknown concentration (or vice-versa) in order to determine the concentration of the unknown. In addition, it is possible to determine the Ka of the acid being titrated (or Kb of the base) as well as an appropriate indicator. Acid-base titrations are often the topic of AP test questions and are frequently used in the laboratory questions. You should know about titrations from a conceptual level, be able to perform calculations for titrations, and know how to properly perform one in the laboratory. We ll begin with the conceptual explanation of titrations. 

See also Indicators Acid-Base Redox Complexo-metric. Adsorption, and Luminescence Indicators. pH. Quality Assurance Internal Standards. Water Determination. 

Acid—Base Indicators— Substances whose colors depend on the pH of a solution are known as acid-base indicators. Acid-base indicators exist in solution as a weak acid (HIn) and its conjugate base (InH- Each form has a different color and the proportions of the two forms determine the color of the solution, which in turn depends on the pH of the solution. The pH range over which an acid-base indicator changes color (Fig. 17-7) is determined by the of the specific indicator. 

It has been shown that for most acid-base titrations the inflection point, which corresponds to the greatest slope in the titration curve, very nearly coincides with the equivalence point. The inflection point actually precedes the equivalence point, with the error approaching 0.1% for weak acids or weak bases with dissociation constants smaller than 10 , or for very dilute solutions. Equivalence points determined in this fashion are indicated on the titration curves in figure 9.8. 

The plT at which an acid-base indicator changes color is determined by its acid dissociation constant. For an indicator that is a monoprotic weak acid, ITIn, the following dissociation reaction occurs... 

The acidity constant for an acid-base indicator was determined by preparing three solutions, each of which has a total indicator concentration of 5.00 X 10- M. The first solution was made strongly acidic with HCl and has an absorbance of 0.250. The second solution was made strongly basic and has an absorbance of 1.40. The pH of the third solution was measured at 2.91, with an absorbance of 0.662. What is the value of K, for the indicator ... 

QuaHty control in the production of organic solvent finish removers may be done by gas—Hquid chromatography, which allows the manufacturer to determine the actual ratio of volatile solvent present in the finished product. If the product does not meet specifications, solvents can be added to bring the product to an acceptable composition. A less expensive approach is to use a hydrometer to determine the specific gravity of the product. The specific gravity indicates if the proper blend has been reached. Nonaqueous acid—base titration may be used to determine the amount of acid or alkaline activator present in a remover. 

A double end point, acid—base titration can be used to determine both sodium hydrosulfide and sodium sulfide content. Standardized hydrochloric acid is the titrant thymolphthalein and bromophenol blue are the indicators. Other bases having ionization constants in the ranges of the indicators used interfere with the analysis. Sodium thiosulfate and sodium thiocarbonate interfere quantitatively with the accuracy of the results. Detailed procedures to analyze sodium sulfide, sodium hydro sulfide, and sodium tetrasulfide are available (1). 

Acid-base indicator Acid (or base). Neutralization is complete as determined by color change of indicator. 

The objective of the titration is to determine the point at which reaction is complete, called the equivalence point. This is reached when the number of moles of OH- added is exactly equal to the number of moles of acetic acid, HC O originally present To determine this point, a single drop of an acid-base indicator such as phenolphthalein is used. It should change color (colorless to pink) at the equivalence point. 

As pointed out in Chapter 4, an acid-base indicator is useful in determining the equivalence point of an acid-base titration. This is the point at which reaction is complete equivalent quantities of acid and base have reacted. If the indicator is chosen properly, the point at which it changes color (its end point) coincides with the equivalence point To understand how and why an indicator changes color, we need to understand the equilibrium principle involved. 

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). 

In Sections 10.11-10.16 it is shown how the change in pH during acid-base titrations may be calculated, and how the titration curves thus obtained can be used (a) to ascertain the most suitable indicator to be used in a given titration, and (b) to determine the titration error. Similar procedures may be carried out for oxidation-reduction titrations. Consider first a simple case which involves only change in ionic charge, and is theoretically independent of the hydrogen-ion concentration. A suitable example, for purposes of illustration, is the titration of 100 mL of 0.1M iron(II) with 0.1M cerium(IV) in the presence of dilute sulphuric acid ... 

As indicated in Section 2.4 the strength of an acid (and of a base) is dependent upon the solvent in which it has been dissolved, and in Sections 10.19-10.21 it has been shown how this modification of strength can be used to carry out titrations in non-aqueous solvents which are impossible to perform in aqueous solution. Potentiometric methods can be used to determine the end point of such non-aqueous titrations, which are mainly of the acid-base type and offer very valuable methods for the determination of many organic compounds. 

Discussion. The dissociation of an acid-base indicator is well suited to spectrophotometric study the procedure involved will be illustrated by the determination of the acid dissociation constant of methyl red (MR). The acidic (HMR) and basic (MR-) forms of methyl red are shown below. 

A simple, reliable, and fast method of determining the pH of a solution and of monitoring a titration is with a pH meter, which uses a special electrode to measure H 0+ concentration. An automatic titrator monitors the pH of the analyte solution continuously. It detects the stoichiometric point by responding to the characteristic rapid change in pH (Fig. 11.9). Another common technique is to use an indicator to detect the stoichiometric point. An acid-base indicator is a water-soluble organic dye with a color that depends on the pH. The sudden change in pH... 

The objective of an acid-base titration is to determine the amount of an acid or base in a solution. Because an indicator is itself a weak acid, it may appear that adding an indicator would alter equilibrium concentrations and influence the titration. However, a useful indicator gives a noticeable color to a solution at a concentration of lO M. This is negligible compared with the concentration of the solution being titrated, which is usually in the... 

Alkalinity is measured by acid-base titration with methylorange or phe-nolphthalein as indicator. Phenolphthalein changes color at pH 8.3, whereas methylorange changes color at pH 4.3. At pH 8 the neutralization of the strong alkali ingredients like NaOH is essentially complete. Further reduction of the pH to 4 will also measure carbonates and bicarbonates. Colorimetric tests and glass electrode systems are used to determine pH. 

The potential x as the difference of electrical potential across the interface between the phase and gas, is not measurable. But its relative changes caused by the change of solution composition can be determined using the proper voltaic cells (see Section IV). The name surface potential is unfortunately also often used for the description the ionic double layer potential (i.e., the ionic part of the Galvani potential) at the interfaces of membranes, microemulsion droplets and micelles, measured usually by the acid-base indicator technique (Section V). 

By plotting i versus the ratio R = (CHX)t/(CB)t during the titration, they determined simultaneously the extent of acid-base interaction, the stoichiometry of that interaction and the degree of association of the acid-base adduct. Fig. 4.13 shows hypothetical titration curves line ABC corresponds to the interaction between B and HX as monomers without further reaction between BHX and HX, and the subsequent occurrence of the latter reaction to a small extent is indicated by the line ABC and to the full extent by line ABDE, when no more HX can react with BHX HX line AFDE arises when formation of BHX HX starts right away in the case of previous partial dimerization of B, the various lines will begin at A instead of A. 

Although litmus paper, cabbage juice, and phenolphthalein can indicate whether a substance is acidic or basic, they have limitations in that they cannot determine an exact pH. To do this, an acid-base indicator called universal indicator can be used. Universal indicator is actually a mixture of several different acid-base indicators (usually phenolphthalein, methyl red, bromthymol blue, and thymol blue). This mixture produces a wide range of colors to indicate different pHs. Under very acidic conditions, universal indicator is red. It turns orange and then yellow between the pHs of 3 to 6. It is green at neutral pH and turns greenish-blue as a solution becomes more alkaline. In very basic conditions, universal indicator turns a dark purple color. 

The interest in colour indicators has recently increased as they are used for the direct determination of pH (acid-base indicators) and free calcium ions (fluorescent derivatives based on the calcium chelator EGTA as metallochromic indicators) in biological systems at cellular level. 

In coals alkylated in this manner, the number of acidic sites is substantially reduced, and acid-base associations are virtually precluded. Extracts from alkylated coals should, therefore, be amenable to GPC fractionation. Such fractionation, conducted on Bio Beads S-Xl and S-X2, results in separation by molecular weight and indicates that both benzene and chloroform extracts contain substantial amounts of high ( 6000) and fairly low (560-640) molecular weight fractions (Figure 2). While the extract yields from non-reductively ethylated vitrinite increase in the order benzene extr. chloroform extr. - pyrid. extr > the molecular weights determined by VPO in pyridine, decrease in this order. 

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