Experiments in Analytical Chemistry -Mg(OH) 2 determination in milk of magnesia

Definition of Acids and Bases  Arrhenius Acid : H + donor Base : OH - donor  Brønsted and Lowry Acid : H + donor Base : H + acceptor  Lewis Acid : electron pair acceptor Base : electron pair donor  Acid-base reaction Acid1 + Base2 ↔ Base1 + Acid2  Conjugate acid-base pair

Acid-base indicators are also acid or base. Principles of acid-base indicators

Phillips’ milk of magnesia 위염 (gastritis) 이나 위궤양 (stomach ulcer) 등으로 인 한 속쓰림 증상이 있을 때 이를 해소하기 위하여 복 용 주성분은 Mg(OH) 2 산 - 염기 반응으로 정량할 수 있음 반응 : Mg(OH) 2 (s) + HCl(l)→ MgCl 2 (aq) + H 2 O(l)  Mg(OH) 2 제조 마그네슘 염에 OH - 작용시켜 콜로이드 형태로 침전시킨다. Mg 2+ (aq) + 2OH − (aq) → Mg(OH) 2 (s) 의학적 용도 제산제 (antacid), 설사 (diarrhea) 약, 고칼륨증 치료, 겨드랑이 냄세 탈취제 (deodorant), 구내 염, 구강궤양에 사용 부작용 : 저칼륨증, 근육통 산업적 용도 산성폐수 중화 인공산호 제조 발화점 상승, 연기발생 억제 화재방지용 코팅제

Milk of magnesia Milk of magnesia is a viscous, mildly basic mixture that is used to treat upset stomach and occasionally to prevent constipation. According to standards set by the pharmaceutical industry, milk of magnesia should contain at least 7% Mg(OH)2 by weight. We will be doing detective work to see if the pharmaceutical industry is being truthful in their claim by determining the weight percent of magnesium hydroxide in different bottles of milk of magnesia. An accurate analysis of milk of magnesia must measure the total Mg(OH) 2 both dissolved and suspended in solution. This is tricky because milk of magnesia exists as a white opaque solution so each sample may not be representative of the whole bottle. To make the solution as homogeneous as possible, the bottle must be shaken thoroughly.

Standardization of NaOH by potassium hydrogen phthalate (KHP) KHP (weak acid) KHP  K + + HP - HP - + OH -  P 2- + H 2 O  Procedure 1.Weigh ~0.8 g of dried KHP (MW = g/mol) into an Erlenmeyer flask and dissolve in mL of distilled water. 2.Record the amount of KHP and water used. 3.Add 4 drops of indicator into the flask and titrate to the first permanent appearance of pink. 4.Near the endpoint, add the NaOH dropwise to determine the total volume most accurately.

KHP needs to be dried before use as a primary standard  Technique tip for drying and weighing samples 1.Place your sample in a weighing bottle and put the weighing bottle, sample and lid into a beaker. Write your name on a small piece of paper and place it in the beaker with your sample. 2.Place a watch glass over the top of the beaker so dirt will not fall into the sample and put the whole thing in the oven. When you remove the beaker and sample (use gloves!), place the sample with cap into a desiccator with dry desiccant to cool, ~1hr. 3.Be careful not to contaminate your sample. The desiccator should have a light coating of grease or petroleum jelly on contact surface with the lid to keep from having air exchange. 4.Weighing by difference involves weighing the weighing bottle, sample, and cap on the Analytical balance, then dispensing a small amount of sample by carefully pouring some sample from the weighing bottle into a second container. 5.Put the lid back on and reweigh the sample and container. The difference between these two masses is the amount of sample transferred to the flask or beaker. You should handle the sample (weighing) bottle and lid only with kim wipes (or tongs) to avoid fingerprints, which can affect your masses.

Standardization of HCl by Na 2 CO 3  Reaction 2HCl + Na 2 CO 3  NaCl + H 2 O + CO 2 (g)  Procedure 1.Weigh ~0.2 g Na 2 CO 3 into an Erlenmeyer flask and dissolve it in 50 mL of boiled, cooled distilled water. Record the exact amount of Na 2 CO 3 used in your notebook. (The water is boiled to expel CO 2 from the solution.) 2.Add 4 drops of phenolphthalein to the solution and record the color. 3.Titrate with the HCl until just before the endpoint (when the solution is very light pink) and then gently boil the solution to expel the CO 2 from solution that has been produced during the reaction. 4.Cool the solution to room temperature and then wash the sides of the flask with a small amount of H 2 O to get all of the sample back into solution. 5.Finish the titration. (This will take VERY little HCl so go slow!) 6.Record the color of the solution and the volume of HCl used.

Titration of milk of magnesia by HCl Direct titration of milk of magnesia is difficult due to the cloudy suspension and the fact that some of the solution may cling to the sides of the flask, preventing complete titration and altering the measurement of the endpoint. The opaque solution may also make it difficult to detect the color change of the endpoint. For this reason, a back titration will be used to measure the weight percent Mg(OH) 2 in milk of magnesia. Excess HCl will react with all of the Mg(OH) 2 to yield a clear solution and then the excess unreacted acid will be back titrated with standardized NaOH.  Reaction Ma(OH) 2 + 2H +  Mg H 2 O OH - + H + (excess)  H 2 O

 Procedure 1.Obtain a vial filled with between g milk of magnesia and weigh the vial and sample without the lid on. Record the value. 2.Rinse the contents of the vial into a 250 mL Erlenmeyer flask with distilled water and dilute the solution to 50 mL total volume. Extra water can be used if milk of magnesia remnants remain in the vial. 3.Add a measured amount of HCl (from a buret) into the beaker until the solution is clear and then add an additional 1-2 mL HCl to ensure there is an excess of acid. Record the volume of HCl added. (For a 1 g sample of milk of magnesia, this should be at least 30 mL HCl.) Measure the pH of the solution using a pH meter or pH paper and record the value. 4.Add 4 drops of indicator and titrate the solution with standardized NaOH. The titration will go quickly, with less than 5 mL of NaOH required. Record the volume of NaOH used and the color of the solution. Measure the pH of the solution again and record the value. 5.Rinse the vial with distilled water and dry in an oven or in a microwave for 1 minute. When dry and cool, weigh the vial to determine the actual weight of the milk of magnesia sample. Make sure there is no water left in the vial! This will skew your results! Record the weight.

 Questions for study 1.Would the weight percentage of Mg(OH) 2 be higher or lower if some of the milk of magnesia sample got stuck on the sides of the flask and was not titrated? Show the thought process for your answer. 2.Was there a consistent amount of Mg(OH) 2 in each bottle of milk of magnesia, or did the values change? Was the change dramatic or small? 3.How would the weight percentage of Mg(OH) 2 change if the weight of the sample was measured incorrectly (for example, the vial was not washed thoroughly or some water remained, changing the weight)? 4.If you determined a different weight percent of Mg(OH) 2 other than ~ 7%, what do you think is the reason? 5.Is there something in your determination that went wrong or are pharmaceutical companies giving us false information?