Discussion of Back Titration Report

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The acid neutralising capacity (ANC) of 3 brands of calcium carbonate (CaCO3) tablets was determined by reacting the tablets in excess standardized hydrochloric acid (HCl) and then back-titrating with a standardized sodium hydroxide (NaOH) solution. Back titration was required for two reasons. Firstly, CaCO3 tablets are poorly water-soluble but dissolve rapidly in acid. Secondly, CaCO3 is a weak base so it is difficult to determine the end point of the reaction if titrated directly.

Assuming good manufacturing practice, the amount of active ingredient described on the product label should conform to quality control standards and accurately reflect the experimentally determined content of CaCO3 in each tablet. Titralac antacid had a measured mass of 0.4031g and this was determined to be 96% accurate based on the labelled mass. Quick-eze antacid had reasonable quality control with a measured mass of 0. 7491g, this being 93.6% accurate based on the label. Tums displayed lower quality control as the antacid had a measured mass of 0.7491g, which was only 84.1% accurate compared to the labelled mass. Overall, tablet quality control appeared high for Titralac and Quick-eze but was lower for Tums. The poor quality control results for Tums is not consistent with a Canadian study of antacids, which found a similar ANC for Titralac (8.5 vs 8.1 mmol HCl), but a higher ANC for Tums (10.5 vs 8.4 mmol HCl ) (Marcra et al CAN med Assoc March 1985) This may indicate an anomalous result.

Utilising the back titration the percentage of CaCO3, the active ingredient, in the tablets could be determined. Consistent with the hypothesis for the experiment the ANC of the tablet increased with increasing active ingredient. Quick-eze neutralised the most HCl, 15mmol, and contained the most active ingredient, 750mg ± 13mg of CaCO3. Tums was ranked second, neutralising 8.4mmol of HCl and containing 420mg ±18mg of CaCO3. Titralac neutralised the least HCl, 8.1mmol, and contained 400mg ±3mg of CaCO3. It is evident that as Quick-eze had the highest active ingredient content it was able to neutralise the highest quantity of acid. This was expected according to the equation for neutralisation displayed below: CaCO3(s) + 2 HCl(aq) --> CaCl2(aq) + CO2(g) + H2O(l)

It is evident that each mole of CaCO3 reacts with two moles of acid so as the amount of CaCO3 increases so does the tablet’s ANC.

However, the ANC is not the only factor to consider in determining the best antacid tablet. The amount of active ingredient compared to weight of the tablet indicates the value of the tablet. The Titralac antacid tablet was calculated to be 59.3% CaCO3 indicating that the remainder, 40.7% contains filler, as CaCO3 is the active ingredient. The CaCO3 component of the Tums was lower with only 31.9% being the active ingredient and the remainder filler. However, the Quick-eze tablets had the lowest percentage of active ingredient, 14.8% with the majority, and 85.2%, being filler. Although Quick-eze has the largest mass of CaCO3 it is the smallest percentage composition. Contrasting this, Titralac, which is the smallest tablet by mass, had the largest percentage of CaCO3 in the tablet therefore less filler, followed by Tums. It would therefore appear that Titralac would be the best value but cost and dissolution time must also be considered.

Active ingredient content versus cost was assessed using a number of methods. Cost effectiveness is a method for representing how effective the tablet is at neutralizing HCl relative to its cost by using the number of moles of active ingredient. A higher cost effectiveness value is desired. The cost of CaCO3 per g was $0.22 for Titralac antacid tablets and the cost effectiveness was 89.6(mmol/$). Tums was more expensive at $0.38 per g and had a lower cost effectiveness of 52.6(mmol/$). The cost of Quick-eze per g was $0.33 and the cost effectiveness was 60.1(mmol/$). Based on cost effectiveness Titralac would be...
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