Using english to report

Experiment 9: Acid / base titration

Introduction

            This laboratory exercise relies on a titrationtechnique to determine an unknown  concentration of monoprotic acid in solution. Inthe process oftitration, a basic solution is gradually added to the acidic solutionuntil complete neutralization is obtained. The ‘end point’ ofthe titration isdetected with thehelp ofan indicatoras colorofthe solution changes upon neutralization.

            By measuring the volume of the titrant required to reach the ‘end point’, it is possibleto relate the concentrationofthe acidto theconcentration ofthe base. In this manner, the unknownconcentration can be expressed through the knownconcentration. The concentration determination isrepeated several times in order to improve the precision of the measurements and to estimate the experimental error.

Results and discussion

            The experiment involves two steps: (i) Standardization of sodiumhydroxide (NaOH) solution using potassiumhydrogen phtalate (KHP) solution, and (ii)titrationofan unknown monoprotic acid solution using the standardized NaOH solution. The two steps, (i) and (ii),are essentially similar.Therefore, only the first step isbriefly described below. The neutralization reaction proceeds as follows:

NaOH + KHP →Na+ + K+ + P2-+ H2O

Once this reaction is complete, anexcess of NaOH starts building up, triggeringthe response from the indicator:

NaOH + HIn(colorless) →Na+ + In-(pink) + H2O.

            An interesting question is why NaOH first reacts with KHP and only then, after KHP is consumed, it starts reacting with HIn. Atthis point I do not know the answer, but hope that this question will beaddressed later in class.

            Another question arisesas to why step (i) isneeded at all. Indeed, one could envisage a simpler measurement schemewhere the solutionof NaOH isprepared with known concentration and used to titrate anunknown acid. Bear in mind, however, that NaOH is a poor primary standard: it is highlyhygroscopic, chemically unstable (reacts with CO2of air), typically low-purity (if purchased cheap), and has low molecular weight (which leads to higher relative error when the compound isweighed out). Conversely, KHP has many desirable characteristics which make it a good primary standard. This dictatesa choice of the two-step scheme, withKHP as a primary standard and NaOH as a secondary standard.

Standardization

            For the standardization step, the KHP solution has been prepared by weighing out 4.8149 g of (dried) KHP and dissolving it in distilled water to a volumeof 250 mL. Consideringthat the molecularweight ofKHP is MWKHP  = 204.23 g/mol, the concentration ofthe KHP solution is:

 C KHP = (4.8149[g]/204.23 [g/mol)/0.250[L] = 0.0943035[M] = 94.3035[mM]

            The titration (standardization)resultsusing 25.00 mLaliquots of the KHP solution are summarized in Table 1 below.

            Table 1. Volumedata fromNaOH standardization measurements using KHP solution. The data are obtained from the buret readings.

* Trial 1 was preceded withthe scout titration (trial 0). The results fromthe scout titration are not included in this table since theyare not quantitatively accurate.

            Already a cursory inspection of Table 1 shows that the results are highly reproducible – the uncertainty in the volume of the titrant is on the order of 0.1 mL (0.4%).

            The data fromTable 1 can be used to determine the concentration of sodium hydroxide solution,  . The molar balance conditions corresponding to the complete neutralization (end pointof the titration) can be written as:

The mean concentration, c NaOH can be calculated on the basis of this formula, using c KHP = 94.3035 Mm, V KHP = 25Ml and V NaOH as listed in Table 1. The values of C NaOH calculated in this manner are 98.2328, 98.0286, and 98.5614 mM for trials i= 1, 2, and 3, respectively. The mean concentration, C NaOH, is 98.2742 mM.

The uncertainty in c NaOH  is reported according to the standard recipe:

Here  is the number of measurements,  N 3 N= , and  tis the so-called ‘Student’s coefficient’,  assuming that the confidence level is 95% and that the number of degrees of freedomis  . The sample standard deviation,  , was calculated according tothe above formula using the data fromthe individual trials,  , and their mean, t = 4.30 assuming that the confidence level is 95% and that the number of degrees of freedom is N -1 = 2. The sample standard deviation, s , was calculatedaccording tothe above formula using the data fromthe individual trials, cⁱNaOH, and their mean, c _NaOH. This calculation produced  s= 0.2688mM.

            After rounding off the result and retaining the significant digits I obtained:

The concentration of sodiumhydroxide obtained in this fashion is further used to determine the concentration of the unknown.

Titration of the unknown

The titration results using standardized NaOH solution are listed in Table 2.

Table 2. Volume data fromthe titration ofunknown monoprotic acid using standardized

NaOH solution. The data are obtained fromthe buret readings.

* Trial 1 was preceded withthe scout titration (trial 0). The results fromthe scout titration are not included in this table since theyare not quantitatively accurate.

The subsequent calculations followexactly the samesteps as described above in the ‘Standardization’ section. The balance condition at the end point of the titration is now used to determine the concentration of the unknown, c unknown = c NaOH (V NaOH/v unknown). The volume  used in this titration is 25 mL.

The concentration c unknown determined in this fashion is the main result ofthis study:

C unknown = 6.02±0.5 Mm @95% confidence level

Control questions:

(a) Why it is necessary to measure volumes very carefully when preparing the KHP solution(solution5), but NOT necessary  to measure volumes too carefully when preparing the dilute NaOH solution (solution4)?

The molarity of the dilute NaOH solution,C NaOH, is not determined fromthe volumes of the concentrated NaOH solutionandwater added together. Instead, it is determined from the volumes of the concentrated NaOH solutionandwater added together. Instead, itisdetermined via the KHP titration. On the other hand, the molarity of KHP solution, C KHPis determined based on the mass of the KHP and the volume of water it is dissolved in.

(b) Why in step 2 should the pipet and beaker berinsed with the KHP solution (not with distilled water), whereas the Erlenmeyer flasks should be rinsed with distilled water (not with the KHP solution)?

Using the distilled water for rinsing may cause (unwanted)dilution of the KHP solution, and hence it is better to washwith the sameKHP solution.

            The situation with the Erlenmeyer flask isdifferent. It is only important that 25 mL of KHP solution with concentrati of C KHP are transferred to the Erlenmeyer flask. At this point, extra water lefton the walls of the Erlenmeyer flask does not matter. Indeed, addition of water does not change the number of KHP molecules in the flask and, therefore, has no effecton the calculation of C NaOH.

            On the contrary, rinsing the flask withKHP solution would increases the number of KHP molecules in the flask and thus compromise the method.

(c) Give at least two reasons why it is bettertoread theinitial volume of a buret than to adjust the volume to some round value such as zero.

First, it is generally impossible toachieve a good accuracy by trying to fill a buret to someround value (human hands are not steady enough forthis purpose). Reading the volume of a buret is more accurate (eyes are a more reliable instrument). Second, trying to fill a buret to a round value is time-consuming. Indeed, one has to add solution to a buret in small increments, constantly checking the resulting fluid level.

Conclusion

This study demonstrates that the titration method relying on visual detection of the end point allows for fairly precise determination ofthe unknown concentration (relative error less than 1%). Bothbases andacids canbe analyzed in thismanner, as illustrated in this report:  and  c NaOH = 98.3± 0.7mM =± 62.0 0.5mM (confidence level 95%). The precision and accuracy of the method can be improved by (i) fitting the buret with an electronic device to control the flow of the fluid, (ii) using a spectrophotometer to detect the color change at the end point ofthe titration,and (iii) using a special small-volume chamber where two solutions – basic and acidic– canberapidly mixed inordertoprevent a timelag from slow diffusion.