Manufacture and Use of an Effervescent Antacid

David Cash | Sun, 06/30/2024 - 10:13

Two Reactions of Sodium Bicarbonate

Effervescent antacid formulations nominally contain sodium bicarbonate and citric acid. In the first step of manufacture of the antacid, heat-drying of sodium bicarbonate above 100 °C unavoidably causes a partial decomposition into sodium carbonate. On using the antacid, neutralization of the sodium bicarbonate and sodium carbonate mixture by citric acid in the formulation creates a carbon dioxide effervescence. A question set about a fictitious effervescent antacid formulation unpacks what is occurring both qualitatively and quantitatively. The student will review and apply formulas, balanced equations, percentage elemental compositions, mass-mass calculations, mole-mole calculations, theoretical yield, and limiting reagent.

Overview

A plausible fictitious formulation for a sodium bicarbonate-citric acid effervescent antacid pharmaceutical is the basis for a set of questions. The two important chemical reactions of sodium bicarbonate are features of this question set. The first reaction is the low-temperature decomposition of sodium bicarbonate into sodium carbonate, beginning at 80 °C. The second reaction is the aqueous neutralization of the sodium bicarbonate - sodium carbonate mixture by the acids in the formulation. These latter reactions create a carbon dioxide effervescence.

The novel aspect of this article is the recognition that solid sodium carbonate makes up a minor part of the heat-dried sodium bicarbonate content. Given the necessary information about the sample it is possible to determine the mass of sodium carbonate present using the chemical relationships. Two examples of doing this are described in detail in this article. The student answering the question set is guided step-by-step through the required calculations.

Manufacture

In the manufacturing stage of a solid pharmaceutical product the ingredients are powdered and dried using various methods. A dried powder flows smoothly. This allows the later operations of formulating, blending, transporting, and dispensing into unit doses to be carried out efficiently. Adsorbed moisture may also alter physical properties, interfere with chemical analysis, or cause undesired chemical reactions in storage. Some solids cannot be dried using elevated temperature (heat-drying) for a variety of possible reasons. More complex and costly methods may be used if necessary.

The bicarbonate (hydrogen carbonate) ion is thermally unstable. In aqueous solution, it exhibits observable decomposition at 50 °C and is fully decomposed at 100 °C. The products are carbonate ion, water, and carbon dioxide. This reaction is used to remove the temporary hardness of water caused by the presence of bicarbonate salts. Removal of the insoluble carbonates leaves only permanent hardness.

Ca²⁺ (aq) + 2 HCO₃^1- (aq) → CaCO₃ (s) + CO₂ (g) + H₂O (aq)

Bicarbonate ion in the solid state also undergoes this decomposition reaction. For solid sodium bicarbonate, the decomposition occurs at a higher temperature than for the aqueous ion. Decomposition is under way at 80 °C and is measurably occurring at 100 °C. This reaction makes sodium bicarbonate useful as a dry powder fire extinguishing chemical. The carbon dioxide produced excludes air and smothers the fire.

2 NaHCO₃ (s) → Na₂CO₃ (s) + CO₂ (g) + H₂O (g)

The production of carbon dioxide from sodium bicarbonate (baking soda) can be used in baking. Baking soda can be purchased in the baking aisle of a grocery store. Using sodium bicarbonate alone to produce carbon dioxide for baking is not a good choice. The residue of sodium carbonate produces an unpleasant taste. Mixing the sodium bicarbonate with an acid in what is called baking powder is more efficient, producing twice as much carbon dioxide and no carbonate residue.

Sodium bicarbonate cannot be dried as a pure substance using heat-drying by heating to above 100 °C due to its decomposition reaction. Sodium bicarbonate begins to decompose at 80 °C. The consequence of heat-drying sodium bicarbonate at a temperature above 100 °C is at least a partial decomposition into sodium carbonate.

This does not always prevent using the method, as the presence of some sodium carbonate does not necessarily cause a problem. In particular, the presence of sodium carbonate as a minor component of an effervescent antacid formula is allowed. The amount of sodium carbonate in a sample of heat-dried sodium bicarbonate can be calculated if sufficient information about the sample is known. See the calculation section below for examples.

Use

A pharmaceutical effervescent antacid as a consumer product is sold either as a powder mixture or a tablet. It is used by adding the solid to water and dissolving completely before ingesting. The ingredients dissolve and react readily in aqueous solution. These reactions are standard and will not be detailed in this article. In the resource question set the student is guided step-by-step to answer questions about the four neutralization reactions occurring.

Formulation:

The formulation of the fictitious pharmaceutical for this article:

ASA 0.325 g

Anhydrous citric acid 1.000 g

Heat-dried sodium bicarbonate 1.950 g

Sodium content 0.580 g

(There is no other ingredient containing sodium.)

The formulation given in this article has slightly different quantities to those given in the question set formulation. Both formulations contain two acids, ASA (acetylsalicylic acid) and citric acid. This is plausible. There has for many years been a consumer effervescent antacid sold containing ASA.

Calculations

The student resource question set (see the Supporting Information) has similar questions to those here. The students are given a more comprehensive set of guidelines accompanying their questions. The acid-base neutralization equations and calculations are standard and need not be detailed here. Adding ASA to the fictitious antacid formulation requires the student to write four neutralization equations each with a different stoichiometry in the use section. There is also an additional gram-to-mole calculation to be performed for the ASA.

The methods used to calculate the amount of sodium carbonate in a heat-dried sample of sodium bicarbonate are not standard applications in introductory chemistry. In answering the question set, the student is given a step-by-step guide.

Preliminary Calculations

The student first assembles a chemical tool kit by filling in a blank version of the table shown:

Name	Sodium Bicarbonate	Sodium Carbonate	Water	Carbon Dioxide
Formula	NaHCO₃	Na₂CO₃	H₂O	CO₂
Molar Mass	84.00 g	105.99 g	18.02 g	44.01 g
Sodium Content (percent)	27.366 %	43.381 %	X	X
Sodium Content (decimal)	0.27366	0.43381	X	X

The student then is asked to write and balance the decomposition equation, and add the relevant mass values* as shown:

2 NaHCO₃ (s)	→	Na₂CO₃ (s)	+	CO₂ (g)	+	H₂O (g)
2 x 84.00 g = 168.00 g	→	105.99 g	+	44.01 g	+	18.02 g

* All formula masses and percentage element compositions used in the examples are quoted from the 1973-74 edition of the CRC Handbook of Physics and Chemistry.

Wherever possible, the student is asked to double-check calculated values -e.g. ‘check that the left-side and right-side masses are equal’.

Example 1

A 152.35 g mass of solid NaHCO₃ was briefly heated to above 100 °C in a stream of warmed air. After cooling to room temperature, the product solid remaining had a mass of 145.83 g. The solid sodium bicarbonate has been partially decomposed by heating.

Calculate: the mass of NaHCO₃ that has decomposed; the mass of sodium carbonate formed by decomposition; the mass of sodium bicarbonate remaining unreacted; the percentage by mass of sodium bicarbonate decomposed; the percentage by mass of sodium carbonate in the final mixture.

At this point the student is instructed to rewrite the equation as shown here. They are also asked to calculate the theoretical yield of sodium carbonate to show that thus is a partial decomposition. They are asked to recognize that the loss of the volatile products leads to a mass decrease of the solid.

2 NaHCO₃ (s)	→	Na₂CO₃ (s)	+	CO₂ (g) + H₂O (g)
2 x 84.00 g = 168.00 g	→	105.99 g	+	44.01 g + 18.02 g = 62.03 g
				(volatile products)

Theoretical yield of Na₂CO₃ (using the equation data) = (105.99 g) x (152.35 g/168.00 g) = 96.117 g of Na₂CO₃

This mass is less than the mass remaining (145.83 g) after heating.

The decomposition is incomplete.

Answers to Example 1

Step 1 Determine the mass decrease on heat-drying the 152.35 g initial precursor mass of sodium bicarbonate. This is the total mass of the volatile products.

Mass decrease = 152.35 g – 145.83 g = 6.52 g

Step 2 Use the mass-mass relationships in the balanced chemical equation to calculate the mass of solid sodium carbonate produced by the partial decomposition and the mass of solid sodium bicarbonate reacted when the mass of volatiles (from Step 1) was formed.

Na₂CO₃ produced = (6.52 g) x (105.99 g/62.03 g) = 11.14 g of Na₂CO₃ produced

NaHCO₃ reacted = (6.52 g) x (168.00 g/62.03 g) = 17.66 g of NaHCO₃ reacted

Step 3 Calculate the mass of NaHCO₃ remaining from the initial mass.

NaHCO₃ remaining = 152.35 g – 17.66 g = 134.69 g of NaHCO₃ remaining

Step 4 Check that the total mass value is correct.

Total mass = 134.69 g of NaHCO₃ + 11.14 g of Na₂CO₃ = 145.83 g (correct)

Step 5 Calculate the percentage of sodium bicarbonate decomposition.

Percent Na₂CO₃decomposed = (17.66 g/152.35 g) x 100 = 11.59 %

Step 6 Calculate the percentage of sodium carbonate in the final mixture.

Percent Na₂CO₃in the final mixture = (11.14 g/145.83 g) x 100 = 7.64 %

Notice that these two percentage values are not identical.

Sodium Content

The amount of sodium present in a formulation is a very important and useful piece of information, as shall be seen below. Since this is a pharmaceutical that will be ingested, the manufacturer is required by regulation to provide the sodium content of the dosage to the consumer. This information is needed when the initial mass of the sample is not known.

1. Calculate the sodium content of the initial mass of sodium bicarbonate.

Na content of the initial mass of NaHCO₃ = (152.35 g) x (0.27366) = 41.69 g of Na

2. Calculate the sodium content of the theoretical yield amount of sodium carbonate.

Na content of the theoretical yield amount of Na₂CO₃ = (96.117 g) x (0.43381) = 41.70 g of Na

3. Calculate the mass of sodium in each of the two components of the final mixture and the total mass of sodium in the final mixture.

Na content = (134.69 g) x (0.27366) + (11.14) x (0.43381) = 36.86 g + 4.83 g = 41.69 g of Na

These calculations are intended to be a further check that the student has not made any errors in calculation to this point, and to make sure that the student realizes that the sodium content does not vary as the decomposition is occurring. The answers to questions 1, 2 and 3 must agree except for rounding differences in the last digit.

The student is now ready to deal with the information presented in the fictitious antacid formulation. The method shown in Example 2 which follows below may also be applied to information from a consumer product (see Question 35 in the Question set that you will find in the Supporting Information).

Example 2

The fictitious antacid formulation contains 1.950 g of heat-dried sodium bicarbonate. The sodium content is 0.580 g. The sodium bicarbonate is the only ingredient containing sodium. Determine the amount of sodium bicarbonate and the amount of sodium carbonate present. Calculate the percent by mass of sodium carbonate in the two-component mixture.

The method used here is based entirely on chemistry. There are alternative methods that use percentage compositions followed by simple arithmetic. Some of these methods are described in the second resource file that is found in the Supporting Information, Alternate Methods of Calculation. This file is also intended for students as a follow-up to Part 1 of the Question set.

Preliminary Calculations

The sodium content does not change during the heat-drying of the sodium bicarbonate.

Calculation of the percentage by mass of sodium in the heat-dried sodium bicarbonate will establish that the latter is a mixture.

Percentage by mass of sodium = (0.580 g / 1.950 g) x 100 = 29.74 %

This is between 27.366 % (0.0 % Na₂CO₃) and 43.381 % (100.0 % Na₂CO₃) - This is a mixture.

Using the mass of sodium and the percent sodium content of pure sodium bicarbonate the initial mass of sodium bicarbonate can be calculated.

Initial mass of sodium bicarbonate = 0.580 g sodium x (100 g sodium bicarbonate / 27.366 g sodium) = 2.1194 g sodium bicarbonate

Since the initial mass of sodium bicarbonate is now known (2.1194 g) and the final mass is known (1.950 g), the method of Example 1 may be used.

Step 1 Determine the mass decrease on heat-drying the 2.1194 g initial mass of sodium bicarbonate. This is the total mass of the volatile products.

Mass decrease = 2.1194 g – 1.950 g = 0.1694 g

Na₂CO₃ produced = (0.1694 g) x (105.99 g/62.03 g) = 0.2895 g of Na₂CO₃ produced

NaHCO₃ reacted = (0.1694 g) x (168.00 g/62.03 g) = 0.4588 g of NaHCO₃ reacted

Step 3 Calculate the mass of NaHCO₃ remaining from the initial mass.

NaHCO₃ remaining = 2.1194 g – 0.4588 g = 1.661 g of NaHCO₃ remaining

Step 4 Check that the total mass value is correct.

Total mass = 1.661 g of NaHCO₃ + 0.2895 g of Na₂CO₃ = 1.9501 g (correct)

A second check can be performed by calculating the sodium content of the mixture using the calculated mass values:

Sodium content of the mixture = sodium content of NaHCO₃ + sodium content of Na₂CO₃

= (1.661 g) x (0.27366) + (0.2895 g) x (0.43381) = 0.4545 g + 0.1256 = 0.5801 g Na (correct)

Calculate the percentage of sodium bicarbonate decomposition.

Percent NaHCO₃ decomposed = (0.4588 g/2.1194 g) x 100 = 21.6 %

Calculate the percentage of sodium carbonate in the final mixture.

Percent Na₂CO₃ in the final mixture = (0.2895 g/1.950 g) x 100 = 14.8 %

Using Consumer Product Effervescent Antacid Tablets or Powders in General Chemistry

Knowing that there is some sodium carbonate in each tablet or powder dosage and being able to calculate the amounts of both the sodium bicarbonate and the sodium carbonate present makes these products more complex, interesting, and useful case studies for the teaching of general chemistry. In addition to the theory topics, there are opportunities for demonstrations, experiments, and projects.

1. Sodium Carbonate Content
Theory Topics (as suggested above)	Demonstrations, Experiments, and Projects
Chemical names and formulas (NaHCO₃ and Na₂CO₃). Elemental percentage compositions. Balanced equation of the decomposition reaction. mass - mass relationships. theoretical yield. calculation of the amount of Na₂CO₃present.	Solid NaHCO₃ added to warm / hot water at varying temperatures. CAUTION for very hot water. Solid NaHCO₃ heated to convert entirely to Na₂CO₃; percentage yield calculation. Solid NaHCO₃ heated to measure mass change over time (in a thermostated oven if possible). Project: reverse engineer the conditions for production of a specific heat-dried sodium bicarbonate.
2. Acid – Base Reactions
Theory Topics	Demonstrations, Experiments, and Projects
Neutralization Reactions: Chemical names and formulas (NaHCO₃, Na₂CO₃, acetylsalicylic acid, and citric acid). Balanced equations of the four neutralization reactions. Mol – mol relationships. Conversion of mass to mol values. Limiting reagent and excess reagent calculation.	Dissolving of tablet(s) in water; testing pH of resulting solution. Testing the pH of solutions of citric acid, sodium bicarbonate, sodium carbonate, ASA. Boiling a solution of sodium bicarbonate and testing the pH of the resulting solution.
Conjugate Acids and Bases: For example, use balanced equations to explain how a dissolved antacid containing mostly sodium citrate acts to neutralize HCl in the stomach.	pH of a sodium phosphate (TSP) solution. pH of a detersive solution containing sodium citrate.
3. Gas Laws
Theory Topics	Demonstrations, Experiments, and Projects
Calculate the mass and mol of carbon dioxide produced by decomposition of a given mass of sodium bicarbonate. Calculate the mol of carbon dioxide produced by reaction of either sodium bicarbonate or sodium carbonate with given number of mol of acid equivalents. Determine the theoretical volume of carbon dioxide produced in each case.	Design experiments to capture carbon dioxide produced by decomposition and/or neutralization reactions. Design a method of estimating the volume of gas produced in each case. Design experiments to use mass change to measure the amount of carbon dioxide produced.
4. Equilibrium and Rate of Reaction
Theory Topics	Demonstrations, Experiments, and Projects
Is the increase of the rate of decomposition of sodium bicarbonate as the temperature increases an equilibrium point change or an activation energy effect? What is activation energy? Why do reaction rates increase with temperature?	Carry out an experiment to determine the activation energy of the neutralizations occurring when an effervescent tablet dissolves in water. By analogy, design a method to estimate the activation energy of the decomposition of sodium bicarbonate in warm water and hot water (CAUTION for very hot water).

You will find two student resources included in the Supporting Information documents listed. “Manufacture and Use of an Effervescent Antacid - Information and Questions for Students” and a resource titled “Alternate Methods of Calculation”.

Sources

Sachin Kothawade, Vishal Pande, Vaibhav Wagh, Kalyani Autade, Sandesh Bole, Rajashri Sumbe and Shubhangi Albhar, Perspective Chapter: Pharmaceutical Drying, a chapter from Drying Science and Technology, November 2023 https://www.intechopen.com/chapters/88568#.
Temporary hardness - Wikipedia, https://en.wikipedia.org/wiki/Hard_water#Temporary_hardness.
Sodium bicarbonate - Wikipedia.
Science Buddies and Svenja Lohner , Vanishing Baking Soda, Scientific American, August 2017, https://www.scientificamerican.com/article/vanishing-baking-soda/.
Baking Powder, Wikipedia, https://en.wikipedia.org/wiki/Baking_powder.
Sodium Bicarbonate Market, Future Market Insights, https://www.futuremarketinsights.com/reports/sodium-bicarbonate-market.
Chemistry Experiment: Does Alka-Seltzer Tell the Truth?, The Science Classroom YouTube Channel (Josh Kenney), November 2020. - (starts at 4 min 20 sec)
Kevin Bryan & George Lyle, Determination of the mass of carbon dioxide evolved by Alka Seltzer using an electronic analytical balance, https://www.csun.edu/scied/4-discrpeant-event/co2_evo_alka_seltzer/co2_evo_alka_seltzer.htm
Robert Perkins, The Chemistry of Alka-Seltzer, Chem13News, October 2017.

Supporting Information:

Alternate Methods of Calculations

Manufacture and Use of an Effervescent Antacid - Information and Questions for Students.pdf

Concepts:

Collection:

Community:

Safety

General Safety

For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).

For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.

Other Safety resources

RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies