The Gravimetric Determination of Calcium Prelab Introduction There are a few main objectives for this lab The first main objective is to understand the theory of gravimetric analysis which will be done experimentally using CaO Another concept in this lab is the use of homogeneous precipitation to crystallize a sample Finally the last main goal of this lab is to calculate the percent CaO in an unknown sample1 In this lab we will be creating a monohydrate salt by converting calcium carbonate to calcium oxalate This will be done and proceeds by the following reaction Ca2 aq C2O42 aq H2O aq CaC2O4 H2O s 1 A hot solution is used to form this monohydrate salt a salt which contains one H2O molecule per mole of the crystal This is done in a hot solution of ammonium oxalate rather than a cold one so that the di or trihydrate does not form Also it is in the presence of a very acidic solution so that the product is not exceeded and precipitation does not occur The main concept used in this lab is gravimetric analysis which uses masses of reactants and products to determine properties of a substance1 There are three main parts to the gravimetric analysis The first is the species must be converted to an insoluble compound The Ksp determines this The higher the Ksp the higher the higher the solubility The second is the compound must be pure and finally the final compound must be able to be measured These three concepts are the main necessities for the process of gravimetric analysis For isolation of crystals a slow filtration technique is necessary Most of the techniques in this lab emphasize slowness The slowness ensures that the regions of supersaturation which is contains more of a dissolved substance than is normally able under normal circumstances do not coprecipitate This is also so the crystals are large enough because the smaller the crystals the more unstable they are in the drying process which causes the loss of some of the crystals or water of hydration which is the hydrated water in the insoluble compound This again shows the necessity of slowness If this is done slow a small number of crystalline nucleation centers which is when droplets of water form a crystal will form causing a better purity of the final product Urea in this case is added for the slowness or another substance can be added to ensure that the pH is slowly raised and so that the crystals are formed slowly A hydrolysis reaction then occurs as shown in the next equation after the solution is heated NH2 2CO s H2O S CO2 g 2NH3 aq 2 2 The above reaction is a hydrolysis reaction which is a reaction that reacts in the presence of water This then reacts with the very acidic 1 pH solution to form ammonium in a neutralization reaction which is a reaction that neutralizes a base or acid and is as follows NH3 aq H aq NH4 aq 3 Then the H ions decrease and the reaction shifts to the right as shown HC2O4 aq C2O42 H aq 4 This is process is called homogeneous precipitation which is the process of generating crystals at a slow and homogenous rate The regions are localized and causes them to not form rapid small crystals In this process once the pH is raised to around 6 there are enough ions to form crystals Even though some solutions may be very hydroscopic the one dealt with in the lab can be heated to lose the water in is because for complete combustion it must be heated until 500 C so a simple heating will not combust it These concepts are very often used in the real world One example of this is at Mawson Institute in Australia a Vacuum filtration system was used with gravimetric analysis to form flexible PEDOT thin films The gravimetric analysis was used to obtain the root source 2 Also in an experiment that the American institute for chemical engineers conducted the gravimetric analysis was used in an experiment used to determine effects of the heating rate in nanoparticles of thermite reactions The gravimetric analysis was used for the comparison of the nanoparticles and crystalline structures3 The use of monohydrate salts can also be seen in many different places in science The department of physics in the national institute of India performed one experiment that exemplifies this concept A monohydrate salt was synthesized by a slow evaporation process similar to the gravimetric process4 3 Table of Reagents Reagent HCl Molar Mass g mol 36 46 Density g mL 1 49 Melting Point C 114 2 Boiling Point C 85 05 Methyl Red 269 3 791 179 182 N A Ammonium Oxolate Urea Calcium Carbonate Acetone 124 1 1 577 N A N A 60 06 100 09 1 32 2 71 132 7 825 N A N A 58 08 791 95 35 56 2 DI H2O 18 02 1 00 0 100 Hazards Corrosive irritant to skin and eyes Hazardous if ingested skin and eye irritant Hazardous if ingested and skin and eye irritant Skin and eye irritant Skin and eye irritant and hazardous if ingested Skin and eye irritant hazardous if ingested Hazardous in copious amounts Procedure 1 Obtain dried impure calcium carbonate and three glass crucibles a Label the crucibles with a pencil b Dry the crucibles by placing them in a 400mL beaker and heat at 110 C i Cover with a watch glass ii Heat for one hour c Cool them in desiccator until they are room temperature d Clean them with a kim wipe 2 Weigh the crucibles on an analytical balance then repeat the heating and cooling process and weigh again 3 Place between 35 38g of the sample in a beaker and store the pre weighed crucible weight in the desiccator 4 Weigh by difference the samples for the three samples To Each of the Three Samples 5 Add 100 mL of DI water 6 Add 6 0 mL of 6M HCl 7 Check the pH using pH paper it should be less than 1 8 Add 4 5 drops of methyl red which should turn the solution red 9 Add 20 mL of ammonium oxalate to a 50mL beaker 10 Add 1 mL of 6M HCl to the ammonium oxalate solution test the pH so that it is below 1 11 Add the solution the the adjusted calcium solution with indicator in it 12 Add 25g of urea and stir until it is dissolved a This is stopping point 1 i When stopping here cover the solution with parafilm 4 13 Cover the beaker with a clean watch glass and heat solution until it is close to boiling but do not let it boil a Heating should take from 25 35 min b After doing this to all solutions this is stopping point 3 14 Set up a vacuum filter apparatus 15 Filter the hot solution through the apparatus a This is stopping point 3 16 Wash the crystals 15mL of ice cold DI water 17 After this rinse with 10mL of acetone 18 …