Stoichiometric equivalence point, titration

When the titration curve is symmetrical about the equivalence point the end point, defined by the maximum value of AE/AV, is identical with the true stoichiometrical equivalence point. A symmetrical titration curve is obtained when the indicator electrode is reversible and when in the titration reaction one mole or ion of the titrant reagent reacts with one mole or ion of the substance titrated. Asymmetrical titration curves result when the number of molecules or ions of the reagent and the substance titrated are unequal in the titration reaction, e.g. in the reaction... 

In such reactions, even though the indicator electrode functions reversibly, the maximum value of AE/AV will not occur exactly at the stoichiometric equivalence point. The resulting titration error (difference between end point and equivalence point) can be calculated or can be determined by experiment and a correction applied. The titration error is small when the potential change at the equivalence point is large. With most of the reactions used in potentiometric analysis, the titration error is usually small enough to be neglected. It is assumed that sufficient time is allowed for the electrodes to reach equilibrium before a reading is recorded. 

The titration of an acid with a base, (a) The titrant (the base) is in the buret, and the beaker contains the acid solution along with a small amount of indicator, (b) As base is added drop by drop to the acidic solution in the beaker during the titration, the indicator changes color, but the color disappears on mixing, (c) The stoichiometric (equivalence) point is marked by a permanent indicator color change. The volume of the base added is the difference between the final and initial buret readings. 

As we saw in Chapter 4, a titration is commonly used to determine the amount of acid or base in a solution. This process involves a solution of known concentration (the titrant) delivered from a buret into the unknown solution until the substance being analyzed is just consumed. The stoichiometric (equivalence) point is often signaled by the color change of an indicator. In this section we will discuss the pH changes that occur during an... 

The acidity of a sample may be determined by titrating it against a standard alkali solution uplo the stoichiometric equivalence point. The accurate identification of the stoichiometric equivalence point being very difficult, the titration is carried to an arbitrary end-point pH. The acidity, therefore, becomes a measure of the amount of a base required to neutralise a given sample to a specific pH ... 

The point in a titration where stoichiometrically equivalent amounts of analyte and titrant react. 

For a titration to be accurate we must add a stoichiometrically equivalent amount of titrant to a solution containing the analyte. We call this stoichiometric mixture the equivalence point. Unlike precipitation gravimetry, where the precipitant is added in excess, determining the exact volume of titrant needed to reach the equivalence point is essential. The product of the equivalence point volume, Veq> and the titrant s concentration, Cq, gives the moles of titrant reacting with the analyte. 

Earlier we made an important distinction between an end point and an equivalence point. The difference between these two terms is important and deserves repeating. The equivalence point occurs when stoichiometrically equal amounts of analyte and titrant react. For example, if the analyte is a triprotic weak acid, a titration with NaOH will have three equivalence points corresponding to the addition of one, two, and three moles of OH for each mole of the weak acid. An equivalence point, therefore, is a theoretical not an experimental value. 

The equivalence point of a redox titration occurs when stoichiometrically equivalent amounts of analyte and titrant react. As with other titrations, any difference between the equivalence point and the end point is a determinate source of error. 

In a titrimetric method of analysis the volume of titrant reacting stoichiometrically with the analyte provides quantitative information about the amount of analyte in a sample. The volume of titrant required to achieve this stoichiometric reaction is called the equivalence point. Experimentally we determine the titration s end point using a visual indicator that changes color near the equivalence point. Alternatively, we can locate the end point by recording a titration curve showing the titration reaction s progress as a function of the titrant s volume. In either case, the end point must closely match the equivalence point if a titration is to be accurate. Knowing the shape of a titration... 

Locate the equivalence point for each of the titration curves in problem 1. What is the stoichiometric relationship between the moles of acid and moles of base at each of these equivalence points ... 

X 10 , 7.8 X 10 , and 6.8 X 10 k The titration curve shown here is for H4Y with NaOH. What is the stoichiometric relationship between H4Y and NaOH at the equivalence point marked with the arrow ... 

Beyond the buffer region, when nearly all of the acetic acid has been consumed, the pH increases sharply with each added drop of hydroxide solution. The titration curve passes through an almost vertical region before leveling off again. Recall from Chapter 4 that the stoichiometric point of an acid titration (also called the equivalence point) is the point at which the number of moles of added base is exactly equal to the number of moles of acid present in the original solution. At the stoichiometric point of a weak acid titration, the conjugate base is a major species in solution, but the weak acid is not. 

An acid-base titration is a laboratory procedure that we use to determine the concentration of an unknown solution. We add a base solution of known concentration to an acid solution of unknown concentration (or vice versa) until an acid-base indicator visually signals that the endpoint of the titration has been reached. The equivalence point is the point at which we have added a stoichiometric amount of the base to the acid. 

At the equivalence point of a titration, exact stoichiometric amounts of the reactants have reacted, i.e., the amount of titrant added is the exact amount required to consume the amount of substance titrated in the reaction flask. If the reaction is one-to-one in terms of moles (moles of titrant equals the moles of substance titrated, as is the case for the reaction represented by Equation (4.4), for example), then the moles of titrant added equals the moles of substance titrated consumed ... 

In an acid-base titration, you carefully measure the volumes of acid and base that react. Then, knowing the concentration of either the acid or the base, and the stoichiometric relationship between them, you calculate the concentration of the other reactant. The equivalence point in the titration occurs when just enough acid and base have been mixed for a complete reaction to occur, with no excess of either reactant. As you learned in Chapter 8, you can find the equivalence point from a graph that shows pH versus volume of one solution added to the other solution. To determine the equivalence point experimentally, you need to measure the pH. Because pH meters are expensive, and the glass electrodes are fragile, titrations are often performed using an acid-base indicator. 

Equivalence point (stoichiometric point) The point in a titration when enough titrant has been added to react exactly with the substance in the solution being titrated. 

In addition to the above sources that should be assessed, for burettes used in titration there is the end point error. There is the repeatability of the end point determination, which is in addition to the repeatability of reading the burette scale, but is part of the repeatability calculated from a number of replicate, independent analyses. There is also uncertainty about the coincidence of the end point, when the color of the solution changes, and the equivalence point of the titration, when a stoichiometric amount of titrant has been added. 

The volume of reagent (titrant) required for stoichiometric reaction of analyte is measured in volumetric analysis. The stoichiometric point of the reaction is called the equivalence point. What we measure by an abrupt change in a physical property (such as the color of an indicator or the potential of an electrode) is the end point. The difference between the end point and the equivalence point is a titration error. This error can be reduced by subtracting results of a blank titration, in which the same procedure is carried out in the absence of analyte, or by standardizing the titrant. using the same reaction and a similar volume as that used for analyte. 

We will do one sample calculation for each region. Complete results are shown in Table 11-1 and Figure 11-1. As a reminder, the equivalence point occurs when the added titrant is exactly enough for stoichiometric reaction with the analyte. The equivalence point is the ideal result we seek in a titration. What we actually measure is the end point, which is marked by a sudden physical change, such as indicator color or an electrode potential. 

There are several minimum requirements for a successful titration (a) the reaction taking place must proceed quantitatively according to a particular stoichiometric equation (b) the reaction must be sufficiently rapid (c) there must be a satisfactory way of locating the equivalence point and (d) it must be possible to prepare (and maintain) a standard solution of the titrant of precisely known concentration, although this restriction is overcome in other ways in the practice of cou-lometric titrations. Clearly condition (a) implies that the titrant must not enter into any side reactions and that no extraneous material is present that could alter the stoichiometry of the desired titration reaction. 

The iodine liberated is stoichiometrically equivalent to the DO in the sample. It is titrated against a standard solution of Na2S203 or phenyl arsine oxide using starch indicator to a colorless end point. 

An acid-base titration is a procedure that is typically used to determine the concentration of an unknown acid or base solution. In a titration, a strong or base or known concentration is added to a base or acid of unknown concentration. An indicator solution is typically utilized to mark the equivalence point, or the point at which the stoichiometric amounts of acid and base are equivalent. 

In the previous Sample Problems and Practice Problems, you were given the concentrations and volumes you needed to solve the problems. What if you did not have some of this information Chemists often need to know the concentration of an acidic or basic solution. To acquire this information, they use an experimental procedure called a titration. In a titration, the concentration of one solution is determined by quantitatively observing its reaction with a solution of known concentration. The solution of known concentration is called a standard solution. The aim of a titration is to find the point at which the number of moles of the standard solution is stoichiometrically equal to the original number of moles of the unknown solution. This point is referred to as the equivalence point. At the equivalence point, all the moles of hydrogen ions that were present in the original volume of one solution have reacted with an equal number of moles of hydroxide ions from the other solution. 

In order to determine the equivalence point (the point at which exactly stoichiometric quantities of sample and titrant have been brought together), it is necessary to find a chemical or physical property that changes very rapidly at this point. Many properties have been used successfully, but the most common method is the visual observation of a color change in a chemical indicator present in very small concentration. This observable change takes place at the end point, which must lie very close to the equivalence point. The technique of titration is concerned principally with approaching the end point with reasonable speed without running over it is best learned by practice, but there are descriptions in the literature that may be helpful. 

A titration involves the measurement of a titrant with a known concentration when reacted with an analyte (the substance being analyzed). The equivalence point or stoichiometric point of the reaction is where enough titrant has been added to exactly react with the analyte. The equivalence point of any base is the amount in grams that can be neutralized by 1 mole of H+ ions. The equivalence point of any acid is the amount in grams that can be neutralized by 1 mole of OH ions. Usually, a substance called an indictor is added at the beginning to show a change by color in the equivalence point. 

Acid-base titrations are an example of volumetric analysis, a technique in which one solution is used to analyze another. The solution used to carry out the analysis is called the titrant and is delivered from a device called a buret, which measures the volume accurately. The point in the titration at which enough titrant has been added to react exactly with the substance being determined is called the equivalence point, or the stoichiometric point. This point is often marked by the change in color of a chemical called an indicator. The titration procedure is illustrated in Fig. 4.18. 

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