The equivalence point of\na chemical reaction is the point at which equal quantities of reactants are\nmixed chemically. <\/p>\n\n\n\n
In other words, while titrating, it is a point where the amount of added titrant is enough to neutralize the analyte solution completely. The number of moles of titrant i.e. standard solution is equal to the moles of a solution having an unknown concentration. It is also known as the stoichiometric point because it is a point where the moles of acid is equal to the moles of the base that are needed to neutralize the solution. Note that acid to base ratio doesn’t need to be 1:1. This acid-base ratio is explained by the balanced acid-base chemical equation. Indicators can be used for this purpose, for example, methyl orange or phenolphthalein. <\/p>\n\n\n\n
<\/p>\n\n\n\n The equivalence point\ncannot be taken the same as the endpoint of a titration. The endpoint is\nreferred to as the point at which the used indicator changes its color. These\ncolor changes appear after reaching the equivalence point. If we use the\nendpoint to determine equivalence, it will induce an error. <\/p>\n\n\n\n There are different\nmethods for the determination of the equivalence point. They are discussed as\nfollows:<\/p>\n\n\n\n pH indicator:<\/strong><\/p>\n\n\n\n a\npH indicator is a chemical substance that changes its color in reaction to any\nchemical change. An acid-base indicator, for example, phenolphthalein changes\nits color which depends on its pH. Redox indicators can also be used for this\npurpose. Initially, just a single drop of indicator solution is added to the\ntitration. The change in color will show that the endpoint has been attained.\nThis will be an estimation of the equivalence point.<\/p>\n\n\n\n Conductance:<\/strong><\/p>\n\n\n\n The\nelectrical conductivity of a solution is affected by the ions. Hence the\nconductivity changes when they react with each other. (for example, while\nperforming an acid-base titration, the H3O+, and OH- ions reacts and form\nneutral water, H20 solution). Conductance is relatively a difficult method to\noperate, especially when the ions that are present in the solution can\nparticipate in conductivity. Conductance can be used for a few acid-base\nreactions.<\/p>\n\n\n\n Color change:<\/strong><\/p>\n\n\n\n At\nthe equivalence point, the solution will change its color naturally without any\naddition of an indicator in some reactions. This may be observed in transition\nmetals where the oxidation state consists of different colors.<\/p>\n\n\n\n Precipitation:<\/strong><\/p>\n\n\n\n During titration, the precipitate will form if\nthe reaction forms a solid. A good example of precipitation is the reaction\nbetween silver, Ag+, and Chlorine, Cl- that results in the formation of an\ninsoluble salt, Silver Chloride, AgCl. Unexpectedly, this makes it very hard to\ndetermine the endpoints accurately because of particle size, the rate of\nsedimentation and color makes it very difficult to see. This is the reason why\nprecipitation titration is done as back titrations.<\/p>\n\n\n\n Isothermal Calorimetry:<\/strong><\/p>\n\n\n\n The\ndetermination of the equivalence point is done by calculating the amount of heat\nthat is produced or absorbed by using a device known as an isothermal titration\ncalorimeter. This type is usually used in titrations that involve biochemical\nreactions i.e., as enzyme binding. <\/p>\n\n\n\n Thermometric Titrimetry:<\/strong><\/p>\n\n\n\n Thermometric\ntitrimetry is an exceptionally multifaceted technique. Here the equivalence\npoint is determined by measuring the rate of temperature change produced by a\nchemical reaction. This property differentiates it from calorimetric\ntitrimetry. Because thermometric titrimetry is a relative technique, it is not\nnecessary to perform the titration under isothermal conditions. This type of\ntitration can be conducted in plastics or even in glass vessels. To prevent\nstray draughts, these vessels are usually enclosed by causing any noise that\ndisturbs the endpoint. Because of the ability of this type of titration to be\nconducted under ambient conditions, they are appropriate for routine process\nand quality control in the industry. The temperature will either increase or\ndecrease during the titration process, depending on whether the reaction taking\nplace between the titrant and analyte is exothermic or endothermic. It\ntitration when all analyte has been consumed by the reaction, rate of\ntemperature changes i.e. an increase or decrease determines the equivalence\npoint and inflection in the temperature curve can be observed. The equivalence\npoint can be observed precisely by employing the second derivative of the\ntemperature curve. The software which is used in a modern automated\nthermometric titration system consists of regular advanced digital algorithms\nso that the noise generating from highly sensitive temperature probes does not\ncause any interference with the appearance of a regular, uniform second\nderivative peak that describes the endpoint. This technique has the ability of\nvery high precision and the coefficient of variance (CVs) of less than 0.1 are\nusual. The latest thermometric titration temperature probes have a thermistor\nthat forms one arm of a Wheatstone bridge. the best thermometric titration\nsystem can resolve temperature to 10-5 K if coupled to high-resolution\nelectronics. If the temperature changes while performing titration become as\nlittle as 0.001K a sharp equivalence point will be obtained. Where there is an\nenthalpy change, this technique can be applied necessarily to any chemical reaction\nin a fluid, though the reaction kinetics plays an important role in calculating\nthe sharpness of the endpoint. This point of titrimetry has been substantially\napplied to acid-base, EDTA, REDOX and precipitation titration. Important\nexamples of precipitation titration include:<\/p>\n\n\n\n As\nnon-aqueous titrations can easily be carried out as aqueous titration because\nthe temperature probe does not require to be electrically connected to the\nsolution as it is required in potentiometric titrations. Solutions that are\nhighly turbid or colored can be easily analyzed by thermometric without any\nfurther treatment of a sample. The probe is maintenance-free. Now a day by\nusing the latest and highly pr\u00e9cised stepper motor driven burettes,\nthermometric titrations are completed within a few minutes, making this\ntechnique a perfect choice where high production in a laboratory is needed.<\/p>\n\n\n\n Spectroscopy:<\/strong><\/p>\n\n\n\n in\nthis type, spectroscopy is used to determine the equivalence point if the spectrum\nof the reactant, product or titrant is known. A specific amount of the product\nand reactant is used to find the equivalence point. A very low level of the\nfree titrant’s presence can also be determined. In short, this method is used\nto determine the existence of semiconductors. <\/p>\n\n\n\n Amperometry:<\/strong><\/p>\n\n\n\n Amperometry\nis a detection technique that is used to measure the change in the current.\nAmperometry is mostly used in those titrations where the excess titrant can be\nreduced. This method is helpful while titrating a halide with Ag+ because the\nformation of precipitates will not be affected.<\/p>\n\n\n\n Examples of equivalence point:<\/strong><\/p>\n\n\n\n Let suppose hydrochloric acid HCl (a strong acid) is taken as an analyte and sodium hydroxide NaOH (strong base) is taken as a titrant. If we plot a graph between analyte pH and a titrant NaOH which can be added from the burette, a titration graph will be formed like is given below:<\/p>\n\n\n\n Figure 1.1<\/p>\n\n\n\n The above figure 1.1 shows at point 1 that when no base like NaOH is added. The pH of the analyte is low as it mostly contains H3<\/sub>O+<\/sup> from the separation of HCl. <\/p>\n\n\n\n As sodium hydroxide NaOH is added drop by\ndrop, it starts getting absorbed by OH–<\/sup> slowly that is produced by\ndissociation of NaOH. The produced analyte will be acidic due to the dominant\npresence of hydronium ions, H3<\/sub>O+<\/sup>. <\/p>\n\n\n\n In figure 1.1 point 2 indicates the time point at which the pH is recorded just before the neutralization takes place completely. While point 3 shows us the equivalence point.<\/em><\/strong> Here the moles of sodium hydroxide added is equal to the moles of the hydroxyl chloride in the analyte. This is the point where hydronium ions, H3<\/sub>O+<\/sup> are completely neutralized by hydroxyl ion, OH–<\/sup>. The pH of the solution is neutral i.e. pH=7 because it has salt, NaCl and water H2<\/sub>O.<\/p>\n\n\n\n Point 4 of figure 1.1 shows that as we keep on adding NaOH, the pH of the solution starts becoming basic because of the complete neutralization of the HCl. Now there are plenty of OH–<\/sup> ions are present in the solution which dissociates from NaOH.<\/p>\n\n\n\n Let\u2019s consider a weak acid, an acetic acid CH3<\/sub>COOH and a strong base sodium Hydroxide NaOH as a titrant. If we plot a graph between the pH of the analyte and the volume of NaOH, we will get a titration curve as shown below.<\/p>\n\n\n\n Figure 1.2<\/p>\n\n\n\n Figure 1.2 shows at point 1 that NaOH is not added yet, so right now the pH of the analyte is low as it mostly contains H3<\/sub>O+<\/sup> when CH3<\/sub>COOH dissociates. But as acetic acid is a weak acid, hence the initially its pH will be higher.<\/p>\n\n\n\n As\nwe keep on adding NaOH dropwise, H3<\/sub>O+<\/sup> will start getting\nconsumed by OH– <\/sup>slowly. This will be produced by the dissociation of\nNaOH. But here the analyte is still acidic due to the presence of H3<\/sub>O+<\/sup>\nions mostly.<\/p>\n\n\n\n In\nfigure 1.2 point 2 indicates the pH recorded at a time just before a\nneutralization takes place completely.<\/p>\n\n\n\n Point 3 represents the equivalence point. This is the point where moles of NaOH is equal to the moles of CH3<\/sub>COOH in an analyte. Hydronium ions are completely neutralized by hydroxyl ions. The solution will then contain CH3<\/sub>COONa salt and H2<\/sub>O.<\/p>\n\n\n\n Note:<\/p>\n\n\n\n You\ncan notice a difference here as compared to a case 1 with a having a strong\nacid titrated against a strong base. In this case (weak acid and a strong\nbase), the pH is not neutral at a point of equivalence. The solution is having\na pH~9 at the equivalence point. Let’s figure out the reason below. <\/p>\n\n\n\n From the above equation, it is shown that the solution contains CH3<\/sub>COONa at the equivalence point. This dissociates into a sodium ion and acetate ions. Acetate ion is the conjugate base of the weak acid CH3COOH. Hence, CH3<\/sub>COO–<\/sup> is relatively a strong base (i.e. weak acid has a strong conjugate base) therefore they react with water to produce hydroxide ions that increase the pH to near to 9 at the point of equivalence. <\/p>\n\n\n\n Point 4 in figure 1.2 shows that when sodium hydroxide is in greater amount, the gained titration curve will be identical to HCl-NaOH.<\/p>\n\n\n\n Here, let\u2019s consider hydrochloric acid as a strong acid as an analyte and ammonia as a weak base as a titrant. If we plot a graph between the pH of the analyte solution and volume of the titrant NH3<\/sub>, we will get a titration curve as given below:<\/p>\n\n\n\n Figure 1.3<\/strong><\/p>\n\n\n\n In figure 1.3 point 1 shows that when no ammonia is added, the pH of the analyte will be low as it mostly contains H3O+ from the dissociation of Hydrochloric acid. <\/p>\n\n\n\n As ammonia is added drop by drop, H3<\/sub>O+<\/sup> starts to get consumed by ammonia slowly. The analyte will be still acidic due to the majority of H3<\/sub>O+<\/sup> ions.<\/p>\n\n\n\n Point\n2 in figure 1.3 indicates that the pH is recorded at a point just before a\nneutralization takes place completely.<\/p>\n\n\n\n Point\n3 shows the equivalence point. Here the number of moles of added NH3<\/sub>\nis equal to the moles of HCl in the analyte. The hydronium ions are fully\nneutralized by NH3<\/sub>.<\/p>\n\n\n\n Note:<\/p>\n\n\n\n It\nis noticed that in the case of the weak base against a strong acid, the pH is\nnot neutral at the point of equivalence. Hence the solution that is achieved\nwill be acidic having a pH around 5.5 at the point of equivalence.<\/p>\n\n\n\n The reason for this is that at a point of equivalence the solution has only ammonium ions NH4<\/sub>+<\/sup> and Chloride ions, CL-.<\/sup> As the ammonium ions are the conjugate acid of a weak base i.e. NH3<\/sub>. Therefore, NH4<\/sub>+<\/sup> is a strong acid and thus NH4<\/sub>+<\/sup> will react with water to produce hydronium ions which makes the solution acidic.<\/p>\n\n\n\n Point 4 shows that after achieving an equivalence point, we will keep on adding ammonia and when in excess, the pH will start increasing. Ammonia is a weak base so its pH is above 7 but it as lower as compared to a strong base NaOH shown in case 1. <\/p>\n\n\n\n Let\u2019s consider ammonia, a weak base as an analyte and an acetic acid which is a weak acid as a titrant. If we plot a graph between the pH of the analyte solution versus the volume of the acetic acid as a titrant, we will get a titration curve as below: <\/p>\n\n\n\n Figure 1.4<\/p>\n\n\n\n We\nhave noticed that there is no steep in this graph plot. This is what we call a\n“point of inflection” at the point of equivalence. The absence of the\nrequired steep change in pH does not give us much information through such a\ncurve.<\/p>\n\n\n\n References:<\/strong><\/p>\n\n\n\n https:\/\/www.khanacademy.org\/test-prep\/mcat\/chemical-processes\/titrations-and-solubility-equilibria\/a\/acid-base-titration-curves<\/a><\/p>\n\n\n\n https:\/\/www.thoughtco.com\/definition-of-equivalence-point-605101<\/a><\/p>\n\n\n\n https:\/\/en.wikipedia.org\/wiki\/Equivalence_point<\/a><\/p>\n","protected":false},"excerpt":{"rendered":" In chemistry, an equivalence point is a term that is used while performing titration. It applies to any acid-base or neutralization reaction technically. Definition: The equivalence point of a chemical reaction is the point at which equal quantities of reactants are mixed chemically. In other words, while titrating, it is a point where the amount … Read more<\/a><\/p>\n","protected":false},"author":6,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"rating_form_position":"","rating_results_position":"","mr_structured_data_type":""},"categories":[2],"tags":[],"multi-rating":{"mr_rating_results":[{"adjusted_star_result":5,"star_result":5,"total_max_option_value":5,"adjusted_score_result":5,"score_result":5,"percentage_result":100,"adjusted_percentage_result":100,"count":3,"post_id":904}]},"_links":{"self":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/904"}],"collection":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/comments?post=904"}],"version-history":[{"count":2,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/904\/revisions"}],"predecessor-version":[{"id":939,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/904\/revisions\/939"}],"wp:attachment":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/media?parent=904"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/categories?post=904"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/tags?post=904"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-10.png\" "\\"\\"")
<\/figure>\n\n\n\nMethods\nof Determining the Equivalence Point:<\/strong><\/h3>\n\n\n\n
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