Question

In one of my lab, I needed to know the amount of calcium in a milk beverage. To do so, I needed first to standardize a solution of EDTA, then do a blank titration, and finally titrate the milk solution with the EDTA. To perform all this, we added a buffer and an Mg indicator (EBT) to the titrand. I understand the necessity of the blank titration, and I know that I need to substract its concentration to the concentration of the milk beverage and that is going to be my final answer. But does the concentration of the blank titration affect the concentration of the standardization of EDTA? Do I need to substract this concentration to both milk titration and EDTA standardization?

Answer 1

For EDTA standardisation you don't have to perform blank titration, it doesn't affect the concentration of EDTA. But for milk you have to perform blank titration. Here is an image of referencereference.

Method Sample Preparation For samples that are already in solution, such as freshwater, seawater and milk, no further preparation Is needed. For solid samples such as eggshells and limestone, the samples must first be dissolved in acid. Accurately welgh about 0.5 g of the solid into a small beaker or conical flask, add about 20 mL dilute hydrochloric acid and allow the solid to completely dissolve (this may take several minutes). Neutralise the unreacted acid with dilute sodium hydroxide solution until the pH of the solution is almost 7 (according to pH Indicator paper). For eggshells, the inner membrane will remain undissolved and may be carefully removed from the solution. Transfer the solution to a 100 mL volumetric flask and make up to the mark with distilled water. Standardisation of the EDTA Solution 1. Plpette a 10 ml sample of the EDTA solution into a conical flask. 2. Add 10 mL of ammonia buffer solution and 1 mL of Erlochrome Black T Indicator solution. 3. Titrate the EDTA with the magnesium chloride solution until the endpoint is reached - a permanent colour change from blue to pink. 4. Having determined the average titre of the magnesium chloride solution, determine the number of moles used. 5. Given the Mg*: EDTA ratio of 1: 1, calculate the concentration of your EDTA solution. Titration Method for Seawater, Milk and Solid Samples 1. Pipette 10 mL of the sample solution into a conical flask. 2. Add 20 mL of 0.05 mol L'EDTA solution. 3. Add 10 mL of ammonia buffer, 50 mL of distilled water and 1 mL of Eriochrome Black T Indicator solution. 4. Titrate the sample with the standard 0.025 moll- magnesium chloride solution until a permanent pink colour appears. Titration Method for Fresh or Tap Water Samples 1. Add a 100 mL of the sample solution into a 250 mL conical flask. 2. Prepare a 0.005 mol L'EDTA solution by diluting the 0.05 mol L'EDTA solution by a factor of 1/10. Add 20mL of this diluted EDTA to the sample solution. 3. Add 10 mL of the ammonla buffer and 1 mL of Erlochrome Black T Indicator solution. 4. Prepare a 0.0025 mol L magnesium chloride solution by diluting the 0.025 mol L 'magnesium chloride solution by a factor of 1/10. 5. Titrate the sample solution with this 0.0025 moll- magnesium chloride solution until a permanent pink colour appears. Repeat the titration with further samples until concordant results (titres agreeing within 0.1 mL) are obtained. Figure 1 Colour changes for magnesium chloride back-titration in clear solution using Eriochrome Black T indicator. Left flask: blue colour well before endpoint (all Cal/Mg'lons complexed by excess EDTA, all indicator molecules uncomplexed). Centre flask: last trace of blue/purple colour just before endpoint (excess EDTA almost totally complexed by added Mg?). Right flask: pink/red colour at endpoint (all EDTA complexed by added Mg, indicator also complexed). Figure 2 Same colour changes for magnesium chloride back- titration as in Figure 1, but for cloudy (opaque) sample solution, eg milk. Left flask: blue colour well before endpoint. Centre flask: last trace of blue/purple. Right flask: pink/red colour at endpoint