Learning Objectives Understand the basics of acid-base titrations Understand the use of indicators Perform a titration calculation correctly. The reaction of an acid with a base to make a salt and water is a common reaction in the laboratory, partly because so many compounds can act as acids or bases. Another reason that acid-base reactions are so prevalent is because they are often used to determine quantitative amounts of one or the other. Performing chemical reactions quantitatively to determine the exact amount of a reagent is called a titration . A titration can be performed with almost any chemical reaction for which the balanced chemical equation is known. Here, we will consider titrations that involve acid-base reactions. During an acid-base titration, an acid with a known concentration (a standard solution ) is slowly added to a base with an unknown concentration (or vice versa). A few drops of indicator solution are added to the base. The indicator will signal, by color change, when the base has been neutralized (when [H + ] = [OH - ]). At that point - called the equivalence point or end point - the titration is stopped. By knowing the volumes of acid and base used, and the concentration of the standard solution, calculations allow us to determine the concentration of the other solution. It is important to accurately measure volumes when doing titrations. The instrument you would use is called a burette (or buret). For example, suppose 25.66 mL (or 0.02566 L) of 0.1078 M HCl was used to titrate an unknown sample of NaOH. What mass of NaOH was in the sample? We can calculate the number of moles of HCl reacted: # mol HCl = (0.02566 L)(0.1078 M) = 0.002766 mol HCl We also have the balanced chemical reaction between HCl and NaOH: HCl + NaOH → NaCl + H 2 O So we can construct a conversion factor to convert to number of moles of NaOH reacted: \[0.002766\cancel{mol\, HCl}\times \frac{1\, mol\, NaOH}{1\cancel{mol\, HCl}}=0.002766\, mol\, NaOH\] Then we convert this amount to mass, using the molar mass of NaOH (40.00 g/mol): \[0.002766\cancel{mol\, HCl}\times \frac{40.00\, g\, NaOH}{1\cancel{mol\, HCl}}=0.1106\, g\, NaOH\] This is type of calculation is performed as part of a titration. Example \(\PageIndex{1}\): Equivalence Point What mass of Ca(OH) 2 is present in a sample if it is titrated to its equivalence point with 44.02 mL of 0.0885 M HNO 3 ? The balanced chemical equation is as follows: \[\ce{2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O}\] Solution In liters, the volume is 0.04402 L. We calculate the number of moles of titrant: # moles HNO 3 = (0.04402 L)(0.0885 M) = 0.00390 mol HNO 3 Using the balanced chemical equation, we can determine the number of moles of Ca(OH) 2 present in the analyte: \[0.00390\cancel{mol\, HNO_{3}}\times \frac{1\, mol\, Ca(OH)_{2}}{2\cancel{mol\, HNO_{3}}}=0.00195\, mol\, Ca(OH)_{2}\] Then we convert this to a mass using the molar mass of Ca(OH) 2 : \[0.00195\cancel{mol\, Ca(OH)_{2}}\times \frac{74.1\, g\, Ca(OH)_{2}}{\cancel{mol\, Ca(OH)_{2}}}=0.144\, g\, Ca(OH)_{2}\] Exercise \(\PageIndex{1}\) What mass of H 2 C 2 O 4 is present in a sample if it is titrated to its equivalence point with 18.09 mL of 0.2235 M NaOH? The balanced chemical reaction is as follows: \[\ce{H2C2O4 + 2NaOH → Na2C2O4 + 2H2O}\] Answer: 0.182 g Exercise \(\PageIndex{2}\) If 25.00 mL of HCl solution with a concentration of 0.1234 M is neutralized by 23.45 mL of NaOH, what is the concentration of the base? Answer: 0.1316 M NaOH Exercise \(\PageIndex{3}\) A 20.0 mL solution of strontium hydroxide, Sr(OH) 2 , is placed in a flask and a drop of indicator is added. The solution turns color after 25.0 mL of a standard 0.0500 M HCl solution is added. What was the original concentration of the Sr(OH) 2 solution? Answer: \(3.12 \times 10^{-2}\;M\) Sr(OH) 2 Video 14.6.1: https://www.youtube.com/watch?v=IKP9UayAmH0 Indicator Selection for Titrations What indicator is used depends on what type of titration you are performing. An indicator should be chosen that will change color when enough of one substance (acid or base) has been added to exactly use up the other substance. Only when a strong acid and a strong base are produced will the resulting solution be neutral. The three main types of acid-base titrations, and suggested indicators, are: Titration between . . . Indicator Explanation strong acid and strong base any NaN strong acid and weak base methyl orange changes color in the acidic range (3.2 - 4.4) weak acid and strong base phenolphthalein changes color in the basic range (8.2 - 10.6) Summary A titration is the quantitative reaction of an acid and a base. Indicators are used to show that all the analyte has reacted with the titrant.