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Determination of the content of Mg (OH)2 in an indigestion remedy by back titration

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Introduction

Determination of the content of Mg (OH)2 in an indigestion remedy by back titration Abstract Milk of Magnesia (Magnesium hydroxide mixture) is a mildly basic mixture, which is commonly used in the treatment of constipation and indigestion, because it neutralises excess acid it is called an antacid. Magnesium hydroxide draws water into the colon by osmosis and induces peristalsis and is therefore also known as an osmotic laxative. An accurate analysis of Milk of Magnesia must measure the total Mg (OH)2 both dissolved and suspended in solution. This is difficult because Milk of Magnesia is poorly soluble in water and exists as a white opaque solution so each sample taken for titrimetric analysis may not be representative of the whole bottle. To make the solution as homogeneous as possible, the bottle was shaken thoroughly each time an amount was taken. A direct titration of Milk of Magnesia is difficult due to the cloudy suspension and the fact that some of the solution may stick to the sides of the conical flask, preventing complete titration and altering the measurement of the endpoint. The inherent opaque solution also makes it difficult to detect the color change of the endpoint, because of these reasons a back titration is preferred to measure the %w/w of Mg (OH)2 in Milk of Magnesia. Excess HCl was reacted with Mg (OH)2 to yield a clear solution, an indicator was added and then the excess acid was back titrated with NaOH. The results obtained from the titration and the subsequent calculations determined whether the Milk of Magnesia mixture that was titrated actually did contain the required amount of Mg (OH)2 (%w/w) ...read more.

Middle

Question 2 % w/w content for Mg (OH)2 % w/w = x 100 14. (0.674g/4.118g) x 100 = 16.367% (3.d.p.) 15. (0.717g/4.705g) x 100 = 15.229% (3.d.p.) 16. (0.707g/4.399g) x 100 = 16.072% (3.d.p.) Question 3 Average of %w/w values 17. (16.367 + 15.229 + 16.072)/3 = 15.889% (3.d.p.) The B.P. states that hydrated Magnesium oxide should be between: 7.45 - 8.35% w/w. The average value that was derived (15.899%) from this titration did not fall in the range specified by the B.P. Discussion 1. The volume of NaOH delivered varied considerably, the greatest volume delivered was 3.75 cm3 the least volume delivered was 0.8cm3, this gives a range of 2.95cm3. The B.P states that the titration readings should be within 0.1cm3 of each other, however the mass of magnesium hydroxide mixture also varied considerably between titration, therefore the volume of NaOH delivered would reflect this as smaller quantities of the magnesium hydroxide mixture would have more excess HCl left over and would therefore require more NaOH to turn the solution basic again, this trend is observed in the results table. 2. The three %w/w values which were calculated from the experiment had a range of 1.138% from the highest value (16.367%) to the lowest value (15.229%), the three masses of the magnesium hydroxide mixture had a range of 0.59g, the highest value was 4.708g and the lowest value was 4.118g. As the values were relatively close together there was no reason why an average could not be taken, however if there had been a value that was very far out, then it would have been wise to omit it from the average as it would skew the result. ...read more.

Conclusion

Conclusion The purpose of this experiment was to determine whether the contents of "Phillips Milk of Magnesia" actually did contain the required amount of Mg (OH)2 as stated in the B.P. (7.45 - 8.35% w/w). This could not be proven in this experiment using a back titration, as the Milk of Magnesia mixture was not a pure compound consisting only of Mg (OH)2 . A second base NaHCO3 was also discovered to be present in the mixture, which interfered with the result significantly. The HCl and NaOH were also not titrated to a volumetric standard, prior to conducting the titration. It is also highly likely that the indicator used (Methyl orange) was not suitable to indicate the end point, so the results of the experiment are inconclusive. Therefore it can be concluded that the titration method used was not suitable in producing accurate results to determine the %w/w. In retrospect, a complexometric titration would have been more suitable in determining the %w/w more accurately. 1 British Pharmacopoeia. Volume III, 2008.p.2856. MHRA.London. 2 www.avogadro.co.uk [accessed on 12.02.2010] http://www.avogadro.co.uk/miscellany/titration/apparatus.gif 3 British Pharmacopoeia. Volume III, 2008.p.2856. MHRA.London 4 www.medicines.org.uk [Accessed on 12.02.2010] 5 Ramsden EN. Calculations for A Level Chemistry.4th Ed. Nelson Thornes. Cheltenham. 2001. p.92. 6 Earl B & Widford LDR. Introduction to Advanced Chemistry .John Murray. London. 2001. P.42 7 British Pharmacopoeia. Vol IV.2008. Appendix I B. A134. MHRA.London 8 Harwood P. A2 Chemistry for AQA. Collins. London.2008p.54. 9 British Pharmacopoeia. Vol IV.2008. Appendix I B. A134. MHRA.London ?? ?? ?? ?? ...read more.

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