KoolAid and similar drinks as convenient laboratory reagents: The weak acid-strong base titration of citric acid or malic acid

Replacing a strong acid in an existing titration experiment. If you are already using a strong acid for a titration experiment, substitute the drink solution for your previous analyte solution, and use/switch to phenolphthalein as the indicator. As shown by the Trial Titrations section above, a 10 mL sample of drink solution will likely require about 2 to 4 mL of 0.1 M NaOH solution for neutralization, varying with brand and flavour. If this titration volume is smaller than you would wish, you have two options:

• use a larger volume of the regular drink solution
• or perhaps more conveniently, use a multiple-strength drink solution as the sample. For example, a triple-strength (3×) KoolAid Orange or Strawberry-Kiwi flavour drink solution will have a titratable concentration of H⁺ very near 0.1 M, and could replace 0.1 M HCl without any change in your experiment instructions

Using a polymer squeeze-bottle titration method. For a new experiment, consider using the drop-dispensing polymer squeeze-bottle method of titration described below. In my own experiments a good quality capillary-tip polymer drop-dispensing squeeze-bottle is used to dispense 0.1000 M NaOH¹⁰ solution to titrate the sample of drink solution. A bottle such as the 6 fl. oz. (60 mL) size Nalgene bottle shown in the photograph is ideal for this purpose¹¹. My titrations employ a gravimetric determination of the amount of 0.1000 M NaOH used. However, if you do not wish to employ a balance or balances for student use, the method for drop-count titrations is identical.

A dropper bottle titration method offers convenience, speed, and good precision. Student instructions for the experiment follow. The titration instructions are identical for either a dropcount or a gravimetric titration. The difference is the analytical measurement of the amount of 0.1 M NaOH solution used: the number of drops of titrant or the mass in grams of titrant respectively.

A study¹³ showed that for a bottle held vertically, the drop count for a 100 drop sample was 23 drops per mL. The mean drop size was 0.044 g with a standard deviation of 0.001 g and a range of 0.005 g. Measurements were made in g of distilled water, but may be taken as either g or mL values (See Notes: Mass (g) and Volume (mL) Equivalence of Solutions below).

A dropper-bottle method has been used successfully for gravimetric pH titrations in an analytical chemistry course¹⁴. I have also used the method with 12 and 13 year old students on open house days at Mohawk College, and on a visit to a school in Toronto¹³. Almost all of these students were able to use a dropper-bottle proficiently for titration within a few minutes.

You can try out a dropper bottle titration without having to buy Nalgene bottles. Over-the-counter eye-drop squeeze bottle empties can be obtained from users or from a local optometrist or ophthalmologist. These bottles are not large enough and are difficult to fill/refill so they are not convenient for student use in this experiment. But they are actually more precise than the Nalgene bottles. A 15 mL artificial tear solution eye-drop bottle can hold enough 0.1 M NaOH solution to perform at least three titrations.

Experiment Instructions

Note: If you foresee that the Standard Labware Method of dispensing the drink solution and preparing the flasks for titration will be too time-demanding, consider using the Two Dropper-Bottle Method of preparation. It differs only in that the analyte sample is dispensed into the Erlenmeyer flask using a dropper-bottle. The titration method is exactly the same for both.

 

EITHER

Standard Labware Method – Preparing to Titrate

This is the method used for my own titrations. This method allows for more than one different drink sample to be analyzed during a session. In the instructions given here, there is one procedure omitted from my own method – pre-rinsing of the beaker, dropper pipet, and graduated cylinder with the analyte solution (see Notes: Pre-Rinsing of Wet Labware below). If the labware used is wet with distilled water when the analyte solution is first placed into the beaker and other labware, this will result in an inaccuracy of unknown magnitude in the result (see Question No. 1). This step has been deleted in order to simplify the experiment and shorten the time needed. If you want your students to do this procedure the instructions are given in the Notes. 

Preparing the Titration Flasks

1. Place about 50 to 60 mL of the supplied drink solution into your beaker.
2. Pour about 10 mL of the drink solution into the graduated cylinder. Use the dropper pipet to adjust the level in the cylinder to the 10.0 mL mark. Hold the graduated cylinder at eye level to eliminate parallax error.
3. Pour the drink portion into one of the Erlenmeyer flasks. Do not rinse out the cylinder.
4. Repeat, adding a second portion into the second Erlenmeyer flask.
5. To each flask, add approximately 15 mL of distilled water from the wash bottle, and 4 or 5 drops of phenolphthalein acid-base indicator solution. Swirl gently to mix the contents of each flask. The titration station should be well lit. Observe the solution colour in the flask in front of or over a white background.

OR

Two Dropper-Bottle Method (Speed Titration) – Preparing to Titrate

In this method, the analyte solution is added to the titration flask using a dropper bottle. This method is inspired by the traditional low-technology field test kits¹⁵ that pre-date the hand-held electronic instruments now available for many of these tests (see Notes: Drop-Count Field Test Kits below).

There are two major reasons for using a multiple-strength drink solution for this version of the experiment:

• Adding 10 mL of regular-strength drink solution from a dropper bottle requires about 240 drops – this is very tiring and time-consuming;
• The 60 mL bottles would have to be refilled too often; certainly after each class does the experiment – this is not good planning.

This method assumes that the multiple-strength drink has an acid (H⁺) molarity of about 0.1 M, equalling the concentration of the NaOH solution. That way, both the drop counts will be about equal, and equally precise. The target would be for about an 80 drop sample of the analyte solution. This is enough of a drop count to provide reasonable precision, without being overly tiresome. The required multiple will have to be determined by trial titration of the drink sample used, and adjusted by experience. 

Rinse and Waste Disposal. All waste is non-toxic and may be discarded to a municipal wastewater system. Either discard directly to a sink or use collection vessels. Clean up spills as instructed as quickly as possible.

Preparing the Titration Flasks

1. To each titration flask, add about 80 drops of multiple-strength drink solution. Record the number of drops added to each flask. Hold the dropper bottle vertically over the flask mouth. When the bottle becomes hard to squeeze, turn it upright and allow air to enter to equalize the pressure inside to atmospheric.
2. To each flask, add approximately 25 mL of distilled water from the wash bottle, and 4 or 5 drops of phenolphthalein acid-base indicator solution. Swirl gently to mix the contents of each flask. The titration station should be well lit. Observe the solution colour in the flask in front of or over a white background.

THEN

Titrating

If titrating gravimetrically, the NaOH solution bottle must be kept clean and dry. The drop count is determined and recorded for both drop count and gravimetric titrations. In both cases, this will allow the second and subsequent titrations to be slowed as the end-point volume is neared.

1. If titrating gravimetrically, tare the balance to zero, then place the NaOH bottle on the pan. Read and record the mass to two places after the decimal point (e.g. 75.61 g).

2. For both drop count and gravimetric titration, the drop count to the end-point colour change will be determined and recorded.

3. Trial 1. Add 0.1 M NaOH solution titrant to one of your flasks 5 drops at a time. Hold the bottle in a vertical position above the mouth of the flask.

4. Watch carefully to see if a colour change occurs where the drops enter the solution. Swirl carefully without splashing after each addition. Compare the colour in the flask to that in the as yet unreacted flask. When the bottle becomes hard to squeeze, turn it upright and allow air to enter to equalize the pressure inside to atmospheric.

5. Continue until the initial reddishness around the added titrant lasts more than a few seconds. Gradually slow down the rate of titration, observe, and then swirl before another addition. When the colour change observed requires several seconds or longer to fade, slow to adding one drop at a time. Use a stream of distilled water from the wash bottle to rinse all splashed solution down into the solution at the bottom of the flask. The objective is to stop the titration at the point where one more drop will cause a significant colour change to a more reddish hue. This is not easy on the first titration, but will be easier when you know what to look out for. 

6. For a gravimetric titration, repeat the mass determination of Step 1. For both types of titration, record the drop count to the point before one more added drop causes the large colour change. The ideal will be to measure the mass of the bottle or the drop count before adding the drop that causes a major colour change.

7. Trial 2. Repeat steps 3 to 6 for the sample in the second flask. You can safely add the titrant up to about 10 drops ahead of the expected end-point volume all in one go. Then slow down to adding one drop at a time. Try to stop when the next drop will cause the big colour change. Measure and record the end mass of the bottle if required, or the titration drop count if required.

8. Discard the contents of the flasks to waste. Rinse the flasks with two small portions of distilled water.

9. Subsequent Trials. Prepare samples for two more trials.

10. Continue doing trials until you have three satisfactory results in all. For drop count titrations, three satisfactory results will have a range of no more than 5 drops from least to most. For gravimetric titrations, three satisfactory results will have a range of no more than 0.20 g from least to most.

Teaching Note: adjust the target ranges as needed based on experience with your students.

Calculations

Determine the mean value of the three (or more) titrations within the accepted range. Use this value for your calculations.

• If using the gravimetric method, convert the mass of titrant NaOH solution in g units to a volume in mL units (see Note: Mass (g) and Volume (mL) Equivalence of Dilute Aqueous Solutions).
• If using the drop-count method, use the mean drop volume value provided by your teacher to convert the mean number of drops into a volume in mL units (see Note: Calibration of Dropper Bottles for Drop-Count Titration Calculations).

Use the calculation method suggested by your teacher. You should be able to use the balanced chemical equation of the analytical reaction to determine: the molarity of H⁺ ion in the drink solution; the molarity of the fruit acid; and the g/L concentration of the fruit acid.

Notes

Preparation of a Standard Drink Solution. A re-purposed 1 gallon (4 L) distilled water jug is a suitable vessel for the preparation and storage of a drink solution. Clean and then rinse the jug and cap with small portions of tap water then distilled water. Add the required amount of powder or liquid concentrate to make 2 quarts (2 L) of the drink. Add the required amount of distilled water. The amount of water added need not be precisely metered. Even if you could add the water with high precision, replicate batches of solid powder or additions of liquid concentrate are unlikely to be precisely similar. Mark the fill line on the outside of the container for future batch preparation. Cap securely. Mix until all of the solid has dissolved. Ensure uniformity by making 10 slow inversions. Label the jug.

Preparation of a Multiple-Strength Drink Solution. A re-purposed 1 or 1.5 quart (1 or 1.5 L) wide-mouth juice bottle with a screw cap is a suitable vessel for the preparation and storage of a drink solution. Clean and then rinse the bottle and cap with small portions of tap water then distilled water. Add the required amount of powder or liquid concentrate to make 2 quarts (2 L) of the drink. But in this case, add only the amount of distilled water required to make to make a your required multiple-strength drink solution. For example, about 500 mL to make a quadruple-strength solution. The amount of water added need not be precisely metered. Even if you could add the water with high precision, replicate batches of solid powder or additions of liquid concentrate are unlikely to be precisely similar. Mark the fill line on the outside of the container for future batch preparation. Cap securely. Mix until all of the solid has dissolved. Ensure uniformity by making 10 slow inversions. Label the bottle.

Preparation of 0.1 M NaOH solution. Preparation of a standardized solution of 0.1 M sodium hydroxide from solid pellets of NaOH that is uncontaminated by sodium carbonate is both a hazardous and a complex procedure. Solid sodium hydroxide is the exemplar of a hazardous solid¹⁶. The solid adsorbs both water and carbon dioxide from the air. It is safer to purchase and dilute a concentrated NaOH solution of known molarity, for example 3.0 M¹⁷. A 1 L bottle of a 3 M solution would provide up to 30 L of 0.1 M NaOH solution for this exercise.

Standardization of 0.1 M NaOH Solution. There is no compelling reason to standardize the sodium hydroxide solution used by the students. You can tell the students it is 0.1 M. However, it would be wise to prepare a solution that is close enough to 0.1 M that the students’ results are credible. The amount of solution prepared should be sufficient for the experimental work for at least one class so that all of the results are comparable. Once prepared, the drink solution to be used should be trial titrated beforehand using the sodium hydroxide solution that has been prepared for use, to ensure that the resulting values are acceptable.

If you are a perfectionist, purchase a bottle of certified 0.1000 M NaOH for your own use¹⁰. Use this standardized 0.1000 M solution to titrate and standardize your prepared drink solution. Very little volume will be required. Then titrate the now standardized drink solution against the sodium hydroxide solution prepared for student use in the experiment, thus determining its NaOH concentration.

Polymer Capillary-Tip Drop-Dispensing Squeeze-Bottles. Capillary-tip controlled drop-dispensing polymer squeeze-bottles can be used for either gravimetric titration⁹ or drop-count titration¹³. The titration method is identical. Unless you have sufficient two-place balances available, drop-count titration is recommended. My preferred bottle type is the Nalgene¹¹. The polymer wall of this bottle is thick and stiff, so the bottles are very durable and ideal for titrations. The bottles are hard to squeeze, but nothing comes out even when the bottle is fully inverted. We have used these bottles at Mohawk College for many years for all manner of purposes and solutions. They are indestructible. Examples of use are: indicator dispensing; microscale experiments with indicators, buffers, redox reagents, precipitations, complexation, corrosion, etc.; dispensing small volumes of acids and bases in numerous experiments.

For drop-count titrations, compared to using eye-droppers or capillary tip pipets, using squeeze-bottles reduces both the likelihood of reagent spills and the chance of dripping reagent, since the bottles are capped and no reagent can be lost when a bottle is upright.

Calibration of Dropper Bottles for Drop-Count Titration Calculations. There are two required properties of the bottles to be used. Firstly, the dispensing of drops must be easily controlled, even when the bottle is inverted. Secondly, the drop size must be consistent, not variable. Using the drop count of base required to neutralize samples of acid for calculation can only make sense if the drop size from the dropper bottles is consistent and reproducible.

The Table 2 gives the result of weighing successive drops of distilled water from each of three types of sample bottle on a 4-place analytical balance. The balance door was open, and the last place (0.1 mg) was unsteady, so each reading was rounded to the third place (to the nearest 1 mg).

The percent relative standard deviation (% RSD) of both types of 60 mL dropper bottles was about ± 2.5 %. Notice that the ‘medical grade’ Tears II bottle, an overthe-counter synthetic-tear eye-drop, had a much more consistent drop volume. This type of delivery system for medication is subject to FDA/USP regulation 905^18,19, which mandates that the amount of medication delivered in successive doses be within an RSD of ± 2.5 %, and that includes variability of the solution in different bottles as well as both inter-bottle and intra bottle variability.

You can have your students use my values from the table for mean drop volume for a bottle. But if time permits the students can easily calibrate the bottle(s) they used for the experiment. A calibration method is given in Question 5: measuring the volume of a 100 drop sample. An alternative is to count drops to a precise volume, e.g. 5.0 mL.

Pre-Rinsing of Wet Labware.

1. The beaker, the graduated cylinder, the dropper pipet, and the Erlenmeyer flasks should be cleaned, then rinsed with two small (10 mL) portions of tap water, then rinsed with two small (10 mL) portions of distilled water. Leave wet. Do not lay the dropper pipet on an unclean surface.
2. Rinse 1. Place about 10 mL of regular-strength analyte drink solution in the beaker. Swirl, tip and rotate the beaker to wet the walls with this solution. Use the rinse portion to fully wet the inside of the dropper pipet. Return the rinse to the beaker. Pour the rinse portion into the graduated cylinder to fully wet the inside walls. Discard the rinse portion as instructed.
3. Rinse 2. Repeat the rinse and discard as before.

Drop-Count Field Test Kits. Field test kits are made by a number of laboratory supply companies¹⁵. These test are portable, and are based on simple wet chemistry. They are either based on a drop-count titration using an eye-dropper, or are based on a colour development reaction with a colour comparison chart. The drop-count titrations are usually in the range of 15 – 25 drops, so while the chemistry of a test may be highly accurate, the precision is low, ± 5 % or even ± 10 % being typical. Field tests are designed to be low precision, but easy to perform rapidly, so that many tests may be performed in a short time. They are mainly used to survey or detect problems, in which case documented samples are gathered and returned to a testing laboratory for further analysis.

Mass (g) and Volume (mL) Equivalence of Dilute Aqueous Solutions. The densities of most dilute aqueous solutions are very near 1 g/mL. For gravimetric titrations with dilute aqueous solutions, it causes only a very small error to take the volume of solution used as an amount in mL units equal to the mass in g units of the solution used¹².

Example Data: density of 0.125 M NaOH at 20 ºC = 1.0039 g/mL density of 0.052 M CA at 20 ºC = 1.0022 g/mL

The error introduced by equating mass (g) to volume (mL) for 0.1 M NaOH and 0.03 M citric acid is therefore below 1% and is not significant for this experiment.