Introduction

There are many brands of antacid indigestion medications on the market and through the following experimental investigation you can check out their value for money. There are various ways you can approach the investigation e.g. you can compare tablet with tablet in terms of the recommended dose as to which neutralises the most acid. You can compare the cost of each tablet with the amount of acid it neutralises.

Experimentally, the investigation involves titrating an indigestion tablet (weighed) with standardised hydrochloric acid (in the burette, to represent stomach acid) using methyl orange indicator to observe when the tablet has been completely neutralised, that point is known as the end-point of the titration.

The apparatus, chemicals and indicator colours are illustrated in the diagram on the left and the procedure described in five stages below.

Preparing the sample and titration procedure

Initially the burette is clamped carefully in a vertical position and filled with standard hydrochloric acid of known concentration. A burette is a long glass tube (open at the top), and accurately calibrated for volume in cm³ an ¹/₁₀th cm³ intervals, with a tap and tip at the lower end.

For safety reasons wear safety glasses, initially work below eye level, using a funnel, the acid is carefully added from the stock bottle down in to the burette, avoid spillage, until the level is above the 0.00 cm³ scale mark.

This burette filling is done below eye level in case of spillage down into your eyes - goggles/safety glasses, which you should be wearing, do not protect from liquid falling onto your forehead! 

The acid is run through to expel any air bubbles in the tip or tap until the reading below the meniscus is 0.00 cm³ (the reading on the above diagram is 7.00 cm³, which could represent a small titration value).

All burette readings should be made at eye level and taking the value exactly below the meniscus on the burette scale.

The burette is usually calibrated to a maximum 50.00 cm³ (only 10.00 cm³ in diagram - I couldn't fit rest of scale on!). Now we are ready to take the antacid indigestion tablet!

The weighed tablet is crushed up and dissolved in e.g. 25 cm³ or 50 cm³ of pure water (for other tablets keep to the same volume of water as part of the fair test).

Add a few drops of methyl orange indicator to the tablet solution and it should turn yellow for an alkali. Carefully place the conical flask under the tip of the burette so drops don't go astray!

The titration: You carefully add small portions of the hydrochloric acid, swirling after each addition and checking the colour of the indicator (although not shown in the diagram, its good to stand the flask on white tile to see the colour changes better).

At the start of the titration the methyl orange indicator is yellow. As you add the acid you get 'splurges' of reddish-orange colour until the mixture is swirled in the conical flask and the yellow is temporarily restored.

The swirling of the flask contents is important, it ensures all the added hydrochloric acid reacts with the antacid indigestion tablet solution i.e. everything gets well mixed up and reacted.

Try to add drop wise (to avoid overshooting) when you seem to be near the orange colour at the end-point as the yellow indicator colour begins to fade.

The indicator colour at the end-point is orange and indicates all the dissolved tablet has been neutralised.

At the end-point you take the titration reading by carefully reading the burette scale under the meniscus (think of the underside of the meniscus as sitting on your reading - see diagram on the right illustrating a reading of 24.5 cm³).

To get the titration value you subtract the1st reading from the 2nd.

The first reading might be zero (0.00 cm³) BUT you can do subsequent titrations without refilling the burette every time, again you just subtract the 1st reading from the 2nd.

You continue to use the burette like this until in needs refilling for further titrations.

If you 'overshoot' the titration with excess acid, the methyl orange indicator turns red and the result is invalid.

The first titration you do is likely to be the most inaccurate until you 'get your eye in'. This is called the rough titration and you can do the next titration quite rapidly until near the end-point and then proceed slowly and accurately to the end-point itself. 

The titration should be repeated several times with the same brand of and the average (mean) titration value calculated to use in any subsequent calculations.

This makes the experimental results more valid and reliable, as will any subsequent calculations and conclusions based on the data recorded.

The procedure should then be repeated with different brands of antacid indigestion tablets and the results compared.

You should keep to the same volumes of water and the same concentration of hydrochloric acid throughout the whole class/individual investigation.

Using a whole class you could amass quite a bit of data by dividing the work up amongst the pupils.

Data and analysis of the results

There are various ways in which you can interpret the results, so here are a few ideas.

(a) Initially you can compare the volume of acid needed to neutralise an individual tablet, which is simply X cm³ of HCl neutralised per tablet. This gives a straightforward comparison, the bigger the titration the more stomach acid would be neutralised.

(b) If you have weighed the tablet, which I recommend you do, you can compare the effectiveness of the antacid tablets in terms of acid per mass of tablet i.e. X cm³ of HCl neutralised per gram tablet (cm³/g). So this measure the effectiveness of the tablet based on mass ('weight').

(c) If you know the cost of the packet of indigestion tablets, you can work out the cost of an individual tablet.

Then you can calculate the 'cost effectiveness' of the medication by dividing the titration value by the cost per tablet e.g. X cm³ of HCl neutralised per cost of tablet (cm³/p)

• Summary of procedure to compare the effectiveness of ant-acid indigestion tablets
  • These tablets are designed to neutralise excess acid in the stomach.
  • A known and equal mass of each brand of indigestion tablet is crushed and mixed with some water e.g. 20 cm³ (fair test points).
  • Make sure the mixture is gently swirled to completely dissolve the crushed tablet powder.
  • The burette is filled with a standard solution of hydrochloric acid and zeroed to the top calibration mark of 0.00 cm³.
  • Universal indicator is added to the flask and the indigestion powder should turn it blue - alkaline.
  • The acid is carefully and slowly added until the indicator turns green - neutral at the end-point of the titration.
  • You then read the volume of acid required to neutralise the ant-acid powder.
  • The bigger the volume of acid required for neutralisation, the more effective the indigestion powder per mass of powder.
  • Repeat the procedure with another brand of indigestion powder using the same standard acid solution (fair test).

Q20 ASPIRIN ASSAY ANALYSIS This question follows on in some respects from Q9b which I'd forgotten I'd already written, apologies for some repetition!

2-ethanoylhydroxybenzoic acid (acetylsalicylic acid), known commercially as aspirin, can be analysed by titration with standard sodium hydroxide solution when a sample of it is dissolved aqueous alcohol (a mixture of ethanol and water) and using phenolphthalein indicator (pKind = 9.3, useful range pH 8.3-10).

In the pharmaceutical industry, aspirin is manufactured by reacting 2-hydroxybenzoic acid (salicylic acid) with ethanoic anhydride.

Prior to this reaction, 2-hydroxybenzoic acid is manufactured by reacting carbon dioxide with phenol, the mixture is heated under pressure sodium hydroxide in the so called Kolbe Reaction.

Aspirin, therefore, always contains a small percentage of 2-hydroxybenzoic acid as an impurity!

(a) Give the equation for the Kolbe synthesis of 2-hydroxybenzoic acid.

(b) Give the equation for the formation of aspirin from 2-hydroxybenzoic acid.

(c) Give the molecular formulae and calculate the molecular masses of 2-hydroxybenzoic acid and aspirin.

Accurate relative atomic masses: A_r(C) = 12.01, A_r(H) = 1.01, A_r(O) = 16.00

(d) Why must ethanol be added to the water prior to doing the titration?

(e) Five samples of aspirin were titrated with commercially purchased precisely 0.1000 mol dm^-3 (0.1000M) sodium hydroxide solution and the results are given below.

The titration values were recorded to the nearest 0.05 cm³, which is reasonable of a burette calibrated in 0.1 cm³ increments.

In each case calculate the titre/mass and work out its average value for the five titrations.

(f) Assuming that only aspirin was titrated (though not true), from the average titre/mass figure calculate the 'theoretical' % purity of the aspirin by the following sequence:

(i) What volume of 0.1000 M NaOH is equivalent to 1.000 g of aspirin?

(ii) Give the reaction equation for the titration.

(iii) How many moles of aspirin can be titrated by your answer to (i)

(iv) from (iii) calculate the theoretical mass of aspirin titrated.

(v) From (iv) calculate the theoretical % purity of the aspirin!

(g) Why is the theoretical % purity based on this titration method always likely to be over 100%?, ignoring any titration errors - which does not necessarily explain why via this method of analysis you will always tend to get >100%, especially if you do the titration very accurately!

(h) Assuming that 2-hydroxybenzoic acid is titrated with NaOH on a 1 : 1 molar basis, calculate the % of this impurity in the aspirin by the follow sequence:

(i) From your answer to (f)(iii) calculate an average molecular mass

(ii) From the average molecular mass, and a little bit of algebra, using x as the % of the 2-hydroxybenzoic acid impurity, calculate the value of x.

(i) Suppose for the sake of argument, there was an error of 0.1 cm³ on the titration value which is likely to be the biggest source of error. Obviously there are errors associated with the NaOH molarity, the weighing, burette reading.

(i) What is the approximate % error on the titration value?

(ii) What error range of values for M_r(av) would this give?

(iii) Using the minimum and maximum values from (ii), recalculate the % of 2-hydroxybenzoic acid in the aspirin using the method indicated in (h) and quote the range of possible values.

(iv) Comment on the results of your calculations, a bit worrying for some coursework projects! yes?

(v) In principle, what must an alternative method be capable of doing? Can you suggest an appropriate method - and forget acid-alkali titrations!

ANSWERS to Q20 SET 1. questions

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Quiz 12 on the basics of acid-alkali titration calculations Good practice questions for GCSE level