〈541〉 Titrimetry

USP 36 Chemical Tests / 〈541〉 Titrimetry 213 is used to correct the fluorometer at frequent intervals for variation in sensitivity from reading to reading within an as- 〈541〉 TITRIMETRY say. Prepare this solution fresh on the day of use. Standard Thiamine Hydrochloride Stock SolutionÐ Transfer about 25 mg of USP Thiamine Hydrochloride RS, Direct TitrationsÐDirect titration is the treatment of a accurately weighed, to a 1000-mL volumetric flask. Dissolve soluble substance, contained in solution in a suitable vessel the weighed Standard in about 300 mL of dilute alcohol so- (the titrate), with an appropriate standardized solution (the lution (1 in 5) adjusted with 3 N hydrochloric acid to a pH titrant), the endpoint being determined instrumentally or of 4.0, and add the acidified, dilute alcohol to volume. Store visually with the aid of a suitable indicator. in a light-resistant container, in a refrigerator. Prepare this The titrant is added from a suitable buret and is so cho- stock solution fresh each month. sen, with respect to its strength (normality), that the volume Standard PreparationÐDilute a portion of Standard Thi- added is between 30% and 100% of the rated capacity of amine Hydrochloride Stock Solution quantitatively and step- the buret. [NOTEÐWhere less than 10 mL of titrant is re- wise with 0.2 N hydrochloric acid to obtain the Standard quired, a suitable microburet is to be used.] The endpoint is Preparation, each mL of which represents 0.2 µg of USP Thi- approached directly but cautiously, and finally the titrant is amine Hydrochloride RS. added dropwise from the buret in order that the final drop Assay PreparationÐPlace in a suitable volumetric flask added will not overrun the endpoint. The quantity of the sufficient amount of the material to be assayed, accurately substance being titrated may be calculated from the volume weighed or measured by volume as directed, such that and the normality or molarity factor of the titrant and the when diluted to volume with 0.2 N hydrochloric acid, the equivalence factor for the substance given in the individual resulting solution will contain about 100 µg of thiamine hy- monograph. drochloride (or mononitrate) per mL. If the sample is diffi- Residual TitrationsÐSome Pharmacopeial assays require cultly soluble, the solution may be heated on a steam bath, the addition of a measured volume of a volumetric solution, and then cooled and diluted with the acid to volume. Dilute in excess of the amount actually needed to react with the 5 mL of this solution, quantitatively and stepwise, using substance being assayed, the excess of this solution then 0.2 N hydrochloric acid, to an estimated concentration of being titrated with a second volumetric solution. This consti- 0.2 µg of thiamine hydrochloride (or mononitrate) per mL. tutes a residual titration and is known also as a ªback titra- ProcedureÐInto each of three or more tubes (or other tion.º The quantity of the substance being titrated may be suitable vessels), of about 40-mL capacity, pipet 5 mL of calculated from the difference between the volume of the Standard Preparation. To each of two of these tubes add volumetric solution originally added, corrected by means of rapidly (within 1 to 2 seconds), with mixing, 3.0 mL of Oxi- a blank titration, and that consumed by the titrant in the dizing Reagent, and within 30 seconds add 20.0 mL of back titration, due allowance being made for the respective isobutyl alcohol, then mix vigorously for 90 seconds by normality or molarity factors of the two solutions, and the shaking the capped tubes manually, or by bubbling a equivalence factor for the substance given in the individual stream of air through the mixture. Prepare a blank in the monograph. remaining tube of the standard by substituting for the Oxi- Complexometric TitrationsÐSuccessful complexometric dizing Reagent an equal volume of 3.5 N sodium hydroxide titrations depend on several factors. The equilibrium con- and proceeding in the same manner. stant for formation of the titrant-analyte complex must be Into each of three or more similar tubes pipet 5 mL of the sufficiently large that, at the endpoint, very close to 100% Assay Preparation. Treat these tubes in the same manner as of the analyte has been complexed. The final complex must directed for the tubes containing the Standard Preparation. be formed rapidly enough that the analysis time is practical. Into each of the six tubes pipet 2 mL of dehydrated alco- When the analytical reaction is not rapid, a residual titration hol, swirl for a few seconds, allow the phases to separate, may sometimes be successful. and decant or draw off about 10 mL of the clear, superna- In general, complexometric indicators are themselves tant isobutyl alcohol solution into standardized cells, then complexing agents. The reaction between metal ion and in- measure the fluorescence in a suitable fluorometer, having dicator must be rapid and reversible. The equilibrium con- an input filter of narrow transmittance range with a maxi- stant for formation of the metal-indicator complex should mum at about 365 nm and an output filter of narrow trans- be large enough to produce a sharp color change but must mittance range with a maximum at about 435 nm. be less than that for the metal-titrant complex. Indicator CalculationÐThe number of µg of C12H17ClN4OS ´ HCl in choice is also restricted by the pH range within which the each 5 mL of the Assay Preparation is given by the formula: complexation reaction must be carried out and by interference of other ions arising from the sample or the buffer. (A − b)/(S − d) Interfering ions may often be masked or ªscreenedº via addition of another complexing agent. (The masking tech- in which A and S are the average fluorometer readings of nique is also applicable to redox titrations.) the portions of the Assay Preparation and the Standard Prep- Oxidation-Reduction (Redox) TitrationsÐDetermina- aration treated with Oxidizing Reagent, respectively, and b tions may often be carried out conveniently by the use of a and d are the readings for the blanks of the Assay Prepara- reagent that brings about oxidation or reduction of the tion and the Standard Preparation, respectively. analyte. Many redox titration curves are not symmetric Calculate the quantity, in mg, of thiamine hydrochloride about the equivalence point, and thus graphical determina- (C12H17ClN4OS ´ HCl) in the assay material on the basis of tion of the endpoint is not possible; but indicators are avail- the aliquots taken. Where indicated, the quantity, in mg, of able for many determinations, and a redox reagent can of- thiamine mononitrate (C12H17N5O4S) may be calculated by ten serve as its own indicator. As in any type of titration, the multiplying the quantity of C12H17ClN4OS ´ HCl found by ideal indicator changes color at an endpoint that is as close 0.9706. as possible to the equivalence point. Accordingly, when the titrant serves as its own indicator, the difference between the endpoint and the equivalence point is determined only by the analyst's ability to detect the color change. A com- mon example is the use of permanganate ion as an oxidizing titrant since a slight excess can easily be detected by its pink color. Other titrants that may serve as their own indicators are iodine, cerium (IV) salts, and potassium dichromate. 214 〈541〉 Titrimetry / Chemical Tests USP 36 In most cases, however, the use of an appropriate redox For the titration of an acidic compound, two classes of indicator will yield a much sharper endpoint. titrant are available: the alkali metal alkoxides and the tetra- It may be necessary to adjust the oxidation state of the alkylammonium hydroxides. A volumetric solution of sodium analyte prior to titration through use of an appropriate oxi- methoxide in a mixture of methanol and toluene is used dizing or reducing agent; the excess reagent must then be frequently, although lithium methoxide in methanol-ben- removed, e.g., through precipitation. This is nearly always zene solvent is used for those compounds yielding a gelati- the practice in the determination of oxidizing agents since nous precipitate on titration with sodium methoxide. most volumetric solutions of reducing agents are slowly oxi- The alkali error limits the use of the glass electrode as an dized by atmospheric oxygen. indicating electrode in conjunction with alkali metal alkoxide Titrations in Nonaqueous SolventsÐAcids and bases titrants, particularly in basic solvents. Thus, the antimony- have long been defined as substances that furnish, when indicating electrode, though somewhat erratic, is used in dissolved in water, hydrogen and hydroxyl ions, respec- such titrations. The use of quaternary ammonium hydroxide tively. This definition, introduced by Arrhenius, fails to rec- compounds, e.g., tetra-n-butylammonium hydroxide and ognize the fact that properties characteristic of acids or trimethylhexadecylammonium hydroxide (in benzene-meth- bases may be developed also in other solvents. A more gen- anol or isopropyl alcohol), has two advantages over the eralized definition is that of BrÈonsted, who defined an acid other titrants in that (a) the tetraalkylammonium salt of the as a substance that furnishes protons, and a base as a sub- titrated acid is soluble in the titration medium, and (b) the stance that combines with protons. Even broader is the defi- convenient and well-behaved calomel-glass electrode pair nition of Lewis, who defined an acid as any material that may be used to conduct potentiometric titrations. will accept an electron pair, a base as any material that will Because of interference by carbon dioxide, solvents for donate an electron pair, and neutralization as the formation acidic compounds need to be protected from excessive ex- of a coordination bond between an acid and a base.

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