Titration requirements

In practice, however, any improvement in the sensitivity of an acid-base titration due to an increase in k is offset by a decrease in the precision of the equivalence point volume when the buret needs to be refilled. Consequently, standard analytical procedures for acid-base titrimetry are usually written to ensure that titrations require 60-100% of the buret s volume. 

Time, Cost, and Equipment Acid-base titrations require less time than most gravimetric procedures, but more time than many instrumental methods of analysis, particularly when analyzing many samples. With the availability of instruments for... 

Conservation of electron pairs for the three titrations requires that for Titration 1 moles Ni = moles EDTAl (Fe, Cr masked)... 

A mixture containing only KCl and NaBr is analyzed by the Mohr method. A 0.3172-g sample is dissolved in 50 mL of water and titrated to the Ag2Cr04 end point, requiring 36.85 mL of 0.1120 M AgNOa. A blank titration requires 0.71 mL of titrant to reach the same end point. Report the %w/w KCl and NaBr in the sample. 

Conservation of charge for this back titration requires that Moles Ag+ = moles K + moles SCN ... 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

Consider an unknown base, RNH. One experiment titrates a 50.0-mL aqueous solution containing 2.500 g of the base. This titration requires 59.90 mL of 0.925 M HC1 to reach the equivalence point. A second experiment uses an identical 50.0-mL sohidon of the unknown base that was used in the first experiment. To this solution is added 29.95 mL of0.925 M HCL The pH after the HC1 addition is 10.77. 

A titration requires a solution whose concentration is known. In Example the NaOH solution used as the titrant was known to be 0.1250 M. A titrant of known concentration is known as a standard solution, and the concentration of such a solution is determined by a standardization titration. In a standardization titration, the solution being titrated contains a known amount of acid or base. An excellent acid for standardization is potassium hydrogen phthalate, KHCg H4 O4. This substance, a carboxylic acid that contains one weakly acidic hydrogen atom per molecule, is easily obtained as a highly pure solid. A known number of moles can be weighed on an... 

A biochemist needed to standardize a solution of KOH. A sample of potassium hydrogen phthalate weighing 0.6745 g was dissolved in 100.0 mL of water and a drop of indicator was added. The solution was then titrated with the KOH solution. The titration required 41.75 mL of base to reach the stoichiometric point. Find the molarity of the KOH solution. 

C04-0097. A student wishes to determine the concentration of a solution of HCl. The student adds 10.00 mL of the HCl solution to 150 mL of water, adds an indicator, and titrates with 0.0965 M NaOH. If the titration requires 32.45 mL of base, what is the concentration of the HCl solution ... 

C04-0157. As a final examination in the general chemistry laboratory, a student was asked to determine the mass of Ca (0H)2 that dissolves in 1.000 L water. Using a published procedure, the student did the following (1) About 1.5 mL of concentrated HCl (12 M) was added to 750 mL of distilled water. (2) A solution of KOH was prepared by adding approximately 1.37 g KOH to 1.0 L distilled water. (3) A sample of potassium hydrogen phthalate (185.9 mg) was dissolved in 100 mL of distilled water. Titration with the KOH solution required 25.67 mL to reach the stoichiometric point. (4) A 50.00-mL sample of the HCl solution prepared in step 1 was titrated with the KOH solution. The titration required 34.02 mL of titrant to reach the stoichiometric point. (5) The student was given a 25.00-mL sample of a saturated solution of Ca (0H)2 for analysis. Titration with the HCl solution required 29.28 mL to reach the stoichiometric point. How many grams of calcium hydroxide dissolve in 1.00 L of water ... 

The pH of a solution changes during titration. As the pH passes the value of p K, the indicator changes color. A good indicator changes color veiy near the pH of the stoichiometric point. Because this pH depends on the p of the substance being titrated, different titrations require different indicators. The best indicator for a titration has a p. in that is as close as possible to the pH of the solution at the stoichiometric point ... 

As titrations require rapid and complete chemical reactions, it is mandatory to work under hydrodynamic conditions hence, with stationary electrodes, which are often used in voltammetric titrations, the solution is stirred by a mechanical or magnetic stirrer with rotating electrodes an additional stirring may sometimes be superfluous. 

Another approach described above, which was employed by Meyerhoff s group [41], requires a set of identical electrodes, in which every electrode is disposed after a single measurement. A typical heparin-protamine titration requires on the order of 10-12 disposable electrodes. 

A dead-stop titration curve is produced if Ag+ is titrated with a halide using a pair of identical silver electrodes. Only whilst both Ag+ and Ag are present will a current flow in the cell, and this is linearly related to the Ag+ concentration. Bi-amperometric titrations require only simple equipment but generally give poorer precision because the currents measured are not necessarily on the limiting current plateau. 

Why does the titration curve of sodium carbonate have two inflection points Why does this titration require that the solution be boiled as you approach the second equivalence point Why can bromcresol green be used as the indicator and not phenolphthalein ... 

Another important point when working at low pH values is the effective concentration of the acid used for titration. Use of 0.0050 mole/liter HCl at pH = 5.0 presents no problems, but at pH = 4.0, the stoichiometric amount of acid is not available for titration. After allowance for the titration required, there must still be enough acid remaining to give the titrant a pH value equal to that of the titrated solution. Taking into account the activity coefficients in a solution of 0.0050 mole/liter HCl and... 

The back-titration requires 9 36 ml of 0 1079 M NaOH Calculate the percentage of Na2C03 in the original sample... 

The evaluation of this equilibrium using pH titration requires the use of certain approximations. Most of the experiments in the literature have dealt with systems which consist of pH measurements of solutions consisting of dissociable monoboric acid, a diol, and sometimes an additional cation such as sodium. Which approximation is made depends upon the examination of these three conservation equations for the titration in question. 

The Karl Fischer titration requires 20 to 45 min (depending on sample being tested) to complete calibration, blank determination, sample equilibration (extraction in Karl Fischer solvent), and final testing. 

Voltammetry (Kauffman, 1998). TBN, TAN. Solvent acetone, ethanol. A three-electrode system, requires lithium perchlorate Uses small samples (5 ml). No titration required. Less time consuming. Eliminates contamination of electrodes. 

Twenty-two mg. of a white solid, compound A, is dissolved in 25 ml of 0.1 N HC1. An odorless but flammable gas is released whose volume, corrected to standard conditions, is 89.6 cc. Methyl orange indicator is added to the solution, and a back titration, using 0,1 N NaOH is performed 15 ml of NaOH are necessary. At this end point, an excess of glycerol and a few drops of phe-nolphthalein are added and the solution again titrated with 0.1 N NaOH, the second titration requiring 10.0 mL What is compound A ... 

Carminic acid, a naturally occurring red pigment extracted from the cochineal insect, contains only carbon, hydrogen, and oxygen. It was commonly used as a dye in the first half of the nineteenth century. It is 53.66% C and 4.09% H by mass. A titration required 18.02 mL of 0.0406 M NaOH to neutralize 0.3602 g of carminic acid. Assuming that there is only one acidic hydrogen per molecule, what is the molecular formula of carminic acid ... 

A 2.20-g sample of an unknown acid (empirical formula = C3H4O3) is dissolved in 1.0 L of water. A titration required 25.0 mL of 0.500 M NaOH to react completely with all the acid present. Assuming that the unknown acid has one acidic proton per molecule, what is the molecular formula of the unknown acid ... 

A potassium permanganate solution is standardized by dissolving 0.9234 g sodium oxalate in dilute sulfuric acid and then titrating with the potassium permanganate solution. The principal products of the reaction are man-ganese(II) ion and carbon dioxide gas. The titration requires 18.55 mL of the potassium permanganate solution to reach the endpoint, which is indicated by the first permanent, but barely perceptible, pink color of the permanganate ion. 

Even with the "well-behaved" silver halides and oxides, indifferent electrolyte. For Insoluble oxides In pure water, containing only H and OH Ions, these Ions constitute not only the surface charge but also the counter charge. (The situation Is a bit academic because carrying out a titration requires the Introduction of other ions anjnvay. Moreover, few oxides are completely insoluble and some Ions may be Introduced by the wall of the vessel (silicates from the glass).) We shall therefore only consider the realistic cases that c ., etc. An additional... 

Selective serotonin reuptake inhibitors (SSRIs) are the most popular treatment option due to safety in overdose situations, low side effect burden, and ease of administration (i.e., once-daily dosing with minimal titration required). SSRIs are also effective treatment for the management of anxiety disorders, a common psychiatric comorbidity among the depressed. 

To obtain accurate values for the parameters of complexation it is necessary to follow the titration curve with sufficient precision and to reach the linear part of the titration curve with certainty (as required by the theory outlined in Section 4.1). To satisfy these requirements numerous metal standard additions (at least 10, but preferably between 15 and 20) are performed throughout the titration experiment. After each standard addition and before the voltammetric measurement a period of 15 min for Cd and Cu, and 25-30 min for Pb are allowed to pass to reach the chemical equilibrium (typically, one titration requires about 20 h). 

Electrolytically generated I2 was used to determine the amount of H2S in 100.0 mL of brackish water. Following addition of excess KI, titration required a constant current of 36.32 mA for 10.12 min. The reaction was... 

An impure sample of (C00H)2 2H20 that had a mass of 2.00 g was dissolved in water and titrated with standard NaOH solution. The titration required 38.32 mL of 0.198 MNaOH solution. Calculate the percent (C00FI)2-2H20 in the sample. Assume that the sample contains no acidic impurities. 

Depending on the instrument and the reagents, volumetric Karl Fischer titrations require samples that contain at least 1 mg of water and more typically 5-10 mg. Coulometric titrators require samples with at least 30-50 JLg of water. Considerations of sample availability, solubility, and water amounts will dictate which technique is best for a particular drug substance. A chart to help one decide where to start is shown in Figure 5. 

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