Gravimetric titration: simple, buret-free, and high precision

Introduction

In this article two example applications of gravimetric titration — acid content of fruit juices/soft drinks and pH titration curves — are presented. Both are suitable for laboratory experiments in senior secondary school chemistry courses or in college general chemistry courses. This material is excerpted or adapted from two articles^1,2 that may be downloaded from my web pages. On this site, student-ready versions of these labs with detailed procedures can be found.

At Mohawk College, it is considered a program objective to train graduates to use a buret because it is felt that employers demand this skill. So gravimetric titration is used only for special purposes, such as a potentiometric or pH titration. Here, it certainly is less frustrating than using a buret. It is my opinion that a secondary school or university teacher should try using the gravimetric method for all their titrations. It is so much easier and faster (and cheaper), so students can do more experiments, or perhaps attempt some labs never tried due to lack of time and money.

Gravimetric titration

Gravimetric titration with a 60-mL polymer controlled drop-dispensing squeeze-bottle and a 2-place digital balance is simpler, faster, less costly, and more precise than titration with a 50-mL buret (Graphic 1). The amount of titrant used is determined by measuring the mass of the reagent dropper bottle before and after each titration, instead of measuring the volume used in a buret. Burets are expensive, breakable and can be awkward for students. A complete description of the method, its advantages, and the equipment used are found in reference 1.

Acid content of fruit juices and soft drinks

The total acid content of a fruit juice or a soft drink is easily determined by gravimetric titration with 0.1 M NaOH solution, using either phenolphthalein as indicator or a pH meter to pH>8 as the end-point. Some typical titration results for titration of acids in fruit juices and soft drinks are found in Table 1. The titration mass values stated are approximate because the 0.1 M NaOH solutions used were not standardized. All of the results are the means of at least two titrations. The major acid present is noted in each case. Full details of this experiment and student instructions are found in reference 1.

Table 1. Titrations of 10-mL samples with 0.1 M NaOH solution

* pH meter titrations to > pH 8     ** carbonated drinks degassed

What precision can students achieve on a gravimetric titration experiment?

Michael Jansen of Crescent School in Toronto kindly invited me to his classroom in February 2012 to try the experiment with a group of eight grade 11 chemistry students who participated in a weekly extracurricular chemistry session. The group (Owen, Carter, Ryan, Jonathan, Nick, Kevin, Scott, and Jorgen) worked as pairs to analyse Tim Hortons® and SunRype® apple juices, which proved to be very similar in acid content.

In an 80-minute period, without any advance preparation, the students performed some rough trials and then five or six analyses per group. The analysis of 5-mL samples of juice, measured by graduated cylinder, was performed by both drop count and gravimetrically, using a 2-place digital balance. Drop count is used as an aid when doing replicate titrations of the same sample solution. The best set of results, from one pair, is listed in Table 2. The abbreviation “Con In” stands for confidence interval. The value of the 95% confidence interval of the data set was obtained using the statistics engine at the website WolframAlpha.³

The students obtained very good precision on a ≈ 2.7-g titration, as indicated in the table. The range of the 95% confidence interval was ≈ 3.4% of the mean value of the titration.

Table 2. Gravimetric titrations of 5-ml samples of Tim Hortons apple juice with 0.1 M NaOH

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Comparing the precision of the gravimetric and the volumetric methods

In order to compare the relative precision of the gravimetric method and the volumetric method of titration, both methods were used to standardize a 0.1 M NaOH solution. Samples of the primary standard substance, potassium hydrogen phthalate (KHP), were weighed by 4-place analytical balance, and titrated with the 0.1 M NaOH solution using phenolphthalein indicator.

The results of this experiment indicated that the gravimetric method is more precise than the volumetric method. The gravimetric method needs just less than half the sample amount to give the same precision as the volumetric method. The full results are described in reference 2.

Gravimetric pH titration curves

The gravimetric titration method is well suited to the manual development of a pH or other potentiometric titration curve. Two important factors here are the physical simplicity of the method, and the ease with which the titrant solution may be metered into the titration vessel. A concrete description of a before and after case from the second term analytical chemistry laboratory for the two-year technician program at Mohawk College will be used to illustrate these points. This is described fully in reference 2.

Graphic 3. Gravimetric

pH titration

pH titration
Before: Previously, the students were asked to use the volumetric method to record the titration curve of a solution containing an unknown concentration of carbonate and bicarbonate ion. Although they were familiar with the use of a stirring motor and PTFE-coated stirring-bar, a combination electrode, and a pH meter, they had difficulty putting the 50 mL volumetric buret in place over the titration beaker
(Graphic 2).

They then had great difficulty getting up high enough over the bench to read the level in the buret, and found it nearly impossible to control the small additions of titrant desired for the titration curve measurements. The exercise was overlong for a two-hour period; it was hazardous because the students were climbing up on the bench to read the buret; and the titration curves produced were of poor quality.

After: Switching to the gravimetric titration method solved these problems. Using a polymer drop-dispensing squeeze-bottle instead of a buret allowed the titrant solution to be easily metered directly into the beaker by counting drops (Graphic 3). The mass of the added reagent was determined by measuring the mass of the squeeze-bottle after each addition on a 2-place top-loading balance. When the pH was changing slowly, more drops (e.g., 30 drops), and when the pH was changing rapidly, fewer drops (e.g., 5 drops) were added between successive pairs of pH - mass measurements.

It was found that in the same 2 hours of laboratory time, two different pH titration curves of good quality could be determined, as illustrated by the plots shown. The full experiment script can be downloaded from my web pages.⁴

Gravimetric titration of carbonate ion solution with 0.1 M HCl solution

Mass of 0.1 M HCl solution (g)

Acknowledgements

The author thanks Bill Rolfe, Fiona Anthony, and Randy Travis, the technologists of the Department of Chemical, Environmental, and Biotechnology of Mohawk College, for their invaluable work in the development of the experiments described in this article.

References (website retrieved November 2012)

1. Article by David Cash
   http://www.uclmail.net/users/dn.cash/articles.html
   http://www.uclmail.net/users/dn.cash/GravTitr3.pdf.
2. Article by David Cash
   http://www.uclmail.net/users/dn.cash/articles.html
   http://www.uclmail.net/users/dn.cash/GravTitr2.pdf.
3. Statistical values obtained by the use of the site WolframAlpha
   http://www.wolframalpha.com/examples/Statistics.html.
4. Mohawk College experiment script
   http://www.uclmail.net/users/dn.cash/experiments.html
   http://www.uclmail.net/users/dn.cash/pHMeter.pdf.