Question

Average composition of Milk Average Composition of Ash Weight (%) Weight (%) 20.8 14.3 Water Fat Protein Lactose Ash Other Solids 87.3 3.67 3.42 4.78 0.73 0.09 Ca Na Mg Fe 7.4 1.46 0.09 PO43 32.5 14.3 Cl- SO426.9 2.2 Other The dry non-fat milk sample that you will analyze contains the protein, sugar, ash, and other solids present in the original whole milk, plus traces of fat and water. The water content of the sample is typically about 2 % by weight, an amount which is negligible for our purposes. The last traces of moisture cannot be removed by heating without simultaneous decomposition of other sample components. Since the water and fat have been removed, the ash is actually 8.1% (not 0.7%) of the dry milk sample. In the absence of interferences, and with proper choice of indicator, calcium can be determined quite precisely by titrating with EDTA anywhere in the pH range from 9.5 to 13.0. Of the many different compounds in milk, only three (magnesium, iron, and phosphate) B. Some Data Pertaining to this EXxperimenl The following information may be helpful in performing feasibility calculations: pKa values (ionic strength = 0.1) EDTA Calcon pKi = 2.0, pK2-27, pK3-62, pKa = 10.3 pK2 = 7.4, pKg = 13.5 Ksp values Ca3(PO4)2 Ca(OH)2 Mg(OH)2 1 x 10-26 5.5 x 10- 5.6 × 10-12 Logarithms of known formation constants, log Kr (ionic strength 0.1) CaEDTA MgEDTA 10.7 8.8 CaCalcon 5.6 MgCalcon 7.7 C. Pre-Laboratory Calculations (should be completed prior to arriving in lab) To understand the problems described in the introduction, both theoretically and experimentally, and to devise a suitable method for determining calcium in milk, carry out the following preliminary calculations. Calculate the anticipated percentage of Ca in milk using the tables provided in the introduction to this experiment. Remember, non-fat dried milk does not contain water or ft (ie. 90.67% of the mass of whole milk is not present Your answer should be between 1 and 2 % calcium. 1. 2. After the nonfat milk is diluted with water (oriration,ts concentrationis approximately 0.010 M Ca?+. The solution is buffered to a pH of 12.5 to remove the Mg* by hydroxide precipitation. Using the Ksp for Ca(OH)2, determine what concentration of Ca+ can exist in solution at pH 12.5. Can 0.010 M Ca2 exist at this pH? a. b. Using the Ksp for Mg(OH)2, determine what concentration of Mg?+ can exist at pH 12.5. Has the Mg2+ been effectively removed from solution?

Answer 1

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