Sodium Chloride Content in Ketchup by Precipitation Titration

Transcription

1 Background Sodiu Chloride Content in Ketchup by Precipitation Titration Sodiu chloride is one of the ost coon substances found in nature. Knowing the salt content in food products is iportant not only for taste but also for health and copliance of recognized standards. One of the ethods ost coonly used to deterine the NaCl content is the precipitation reaction with silver nitrate in an acidic ediu. The result is usually calculated as percent of sodiu chloride. Reaction NaCl + AgNO 3 AgCl + NaNO 3 analyte titrant precipitate g/ol g/ol H + Safety Always take the necessary safety precautions when working in the laboratory: work carefully; wear a lab coat, protective gloves and safety goggles. Silver nitrate causes serious skin and eye irritation. It stains skin and other aterials black. Potassiu dichroate is very toxic and corrosive. Please read the aterial safety data sheet (MSDS) carefully before working with it. Potassiu dichroate ust only be used for anual titration. Titration solutions are dangerous to the environent as they contain containants such as heavy etals. Please dispose of solutions according to the instructions below. Tasks 1. Titer deterination: Perfor a titer deterination (standardization) of the titrant AgNO 3 c = 0.1 ol/ by titrating a standard sodiu chloride solution (c = 0.1 ol/). Add exactly 5 of the standard solution into a titration beaker by using a pipette. For a anual titration with visual equivalence point deterination add 40 of deionized water and a few drops of saturated potassiu dichroate indicator solution. Place the beaker under the burette and start the titrating by slowly adding titrant to the standard sodiu chloride titration solution under peranent stirring. Titrate until the equivalence point is reached as is evident by a color change fro yellow to orange-red. Note the volue consued. Calculate the titer by using the correct forula. Workshop 2 1 Powered by METTER TOEDO

2 If you use an autoatic titrator with a potentioetric electrode (silver ring electrode) add 40 of H 2 SO 4 solution, c = 0.02 ol/, to the titration beaker. This dilutes and acidifies the standard solution. Ierse the titration tube and the electrode in the titration solution and start the titration. The titration will stop autoatically when the endpoint is reached or when the axiu volue of titrant is added. The result, consued titrant and titer, will be displayed. Deterine the titer at least three ties. Calculate the ean value, standard deviation (s) and relative standard deviation (srel). 2. Sodiu chloride content deterination: To deterine the sodiu chloride content in ketchup first weight about 1.5 g of ketchup accurately into the titration beaker. Dissolve the saple in deionised water. Then follow the sae saple preparation and titration procedure as for the titer deterination above. Deterine the sodiu chloride content (in %) at least three ties and calculate the ean value, standard deviation (s) and relative standard deviation (srel). Waste disposal Filter the titration solutions and dispose of the precipitate as special waste in the bins labelled heavy etal waste. The liquid phase ust be neutralized (ph 7) before it can be disposed of as waste water. Note Be careful not to add too uch acid to titration solution. The optial value is ph 4-5. If the solution is too acidic it will begin to dissolve the silver in the electrode thus releasing silver ions. These silver ions bind the chloride in solution and give a falsely low chloride concentration easureent. Workshop 2 2 Powered by METTER TOEDO

3 Sodiu Chloride Content in Ketchup by Precipitation Titration Equipent Manual titration: - 1x Analytical balance - 1x Accurate pipette (5 or 1 10 ) - 1x Manual burette (10 ) - 6x Titration beakers (e.g. 100 glass beakers) - 1x Magnetic stirrer - 3x Magnetic stirrer bars Autoated titration: - 1x Analytical balance - 1x Accurate pipette (5 or 1 10 ) - 1x Mettler-Toledo Easy Cl or Easy Pro titrator with 10 burette and tubing - 1x EM45-BNC ph Electrode - 6x Titration beakers - 3x Magnetic stirrer bars Cheicals The quantities below were roughly estiated for 5 titer deterinations and 5 acetic acid deterinations. - 1 deionized water (ainly for anual titration and rinsing) sulphuric acid solution, c = 0.02 ol/ (2.06 g H 2 SO 4 95 % in 1 solution, for autoated titration) silver nitrate solution, c = 0.1 ol/ (16.99 g solid silver nitrate in 1 solution) - 10 of saturated potassiu dichroate solution (about 1.2 g potassiu dichroate in 10 solution, only for anual titration) - 30 sodiu chloride standard solution, c = 0.1 ol/ (titration standard) - 10 g ketchup as saple Preparation - Prepare the titrant and indicator solutions. - Rinse the burette of the autoatic titrator with titrant at least twice to dispel any air bubbles trapped in the burette and tubing. Coents - This ethod ay be slightly adapted depending on the saple used and its chloride content. Teacher Inforation 3 Powered by METTER TOEDO

4 Solution 1. Titer deterination: Calculation: t = VEQ c C 1 cst z t: Titer of the titrant (no unit) : Volue used (in, here: NaCl standard solution) VEQ: Titrant consuption at the equivalence point (in ) c: Noinal concentration of the titrant (in ol/, here: AgNO 3, c = 0.1 ol/) C: Constant (in /ol, see equation above) cst: Concentration of the standard solution (in ol/, here: NaCl, cst = 0.1 ol/) z: equivalence nuber (here: 1) Expected result: The titer should be around 1 for a fresh titrant solution. 2. Sodiu chloride content deterination: Calculation: R = VEQ c t C M 10 z R: Result, content (in %, here: sodiu chloride content) VEQ: Titrant consuption at the equivalence point (in ) c: Noinal concentration of the titrant (in ol/, here: AgNO 3, c = 0.1 ol/) t: Titer of the titrant, as deterined before (no unit) C: Constant (in g %/ol, see equation above) : Saple weight (in g) M: Molar ass of the analyte (in g/ol, here: NaCl, M = g/ol) 10: Factor for g to g and % conversion (in g/(g %)) z: equivalence nuber (no unit, here: 1) Expected result: The sodiu chloride content in ketchup is usually around 3%. The expected concentration is declared on the label of the bottle. Teacher Inforation 4 Powered by METTER TOEDO

5 Exaple 1. Titer deterination: Three titer deterinations were perfored by titrating 5 of sodiu chloride standard solution (c = 0.1 ol/). The saple volue and results are shown in the following table. Nr. VEQ t The titer of the first easureent was calculated as follows: 1 cst z = ol t = VEQ c 1 = 10 ol ol 10 = ol The ean, standard deviation (s) and the relative standard deviation (srel) were calculated for the three easureents: Mean: s: srel: 0.21 % 2. Sodiu chloride content deterination: Three easureents of a ketchup saple were perfored to deterine the sodiu chloride content. The following table shows the saple weight and results. Nr. VEQ R g % g % g % Teacher Inforation 5 Powered by METTER TOEDO

6 The detailed calculation for the first easureent is: M 10 z = g ol g % 10 g = g % 1 ol R = VEQ c t C = ol g % ol = % g For these three easureents the ean, s and srel are as follows: Mean: 3.187% s: % srel: 0.26 % Teacher Inforation 6 Powered by METTER TOEDO