PRACTICUM 05 QUANTITATIVE ANALYSIS: DEPOSITION METHOD (ARGENTOMETRY) Brief Theory Argentometry titration is a titration using silver nitrate solution to determine halogen levels. An example of the reaction is: NaX(aq) + AgNO3(aq) AgX(aq) + NaNO3(aq). Argentometric titration using the Mohr method means that the added Ag+ first reacts to form a white AgCl precipitate. When the Cl- has finished reacting, the excess Ag+ then reacts with CrO42- which comes from the added K2CrO4 indicator and forms a brick red Ag2CrO4 precipitate, meaning the end point of the titration has been reached. As an example of the use of the argentometric method, it can be seen in determining the best surfactant concentration bound to the kaolin compound, determined by the Cl- test using the argentometric titration method. Based on the indicators used, argentometry can be divided into : 1. Mohr's method (formation of colored precipitate) The Mohr method can be used to determine chloride and bromide levels in a neutral atmosphere with a standard AgNO3 solution and the addition of K2CHO4 as an indicator. Titration in this way must be carried out in a neutral or slightly alkaline atmosphere, pH 6.5 – 9.0. In an acidic environment, silver chromate dissolves because it forms dichromate and in an alkaline environment a silver hydroxide precipitate forms. The reaction that occurs is: Acid: 2CrO42- + 2H-↔CrO7 2- + H2O Base : 2 Ag+ + 2 OH-↔2 AgOH 2AgOH↔Ag2O + H2O Both solutions can be measured with sodium bicarbonate or calcium carbonate. The alkaline solution is first acidified with acetic acid or boric acid before being neutralized with calcium carbonate. Although according to the product of the solubility of iodide and thiocyanate it is possible to determine the concentration in this way. However, because silver lodide and thiosanate strongly attack chromate, the results are not satisfactory. Silver also cannot be determined by titration using NaCl as a titrant because the silver chromate precipitate that is initially formed is difficult to react at the end point. The chloride or bromide solution in a neutral or slightly catalyzed atmosphere is titrated with a titered solution of silver nitrate using a chromate indicator. If the chloride or bromide ions have been completely precipitated by the silver ions, the chromate ions will react to form a brown/brick red silver chromate precipitate as the end point of the titration. As an indicator, 0.003M or 0.005M K2CrO4 chromate solution is used which with silver ions will form a red brown precipitate in a neutral or slightly alkaline atmosphere. The advantages of the indicatoryellow color will disturb the color, this can be overcome by dissolving the indicator blank in a titration without the test substance with the addition of calcium carbonate as a substitute for the AgCl precipitate. 2. Valhard model (Determinant of soluble dyes). This method is used to determine Cl+, Br-, and I- ions by adding a standard AgNO3 solution. The indicator used is Fe3+ with NH4CNS titrant, to neutralize the silver salt content by titrating again after adding an excess of standard solution. The excess AgNO3 is titrated with a standard KCNS solution, while the indicator used is Fe3+ ions where the excess KCNS solution will be bound by Fe3+ ions to form the blood red color of FeSCN. 3. Fajans Method (Absorption Indicator) Argenometric titration using the Fajans method is the same as using the Mohr method, there is only a difference in the type of indicator used. The indicators used in this method are absorption indicators such as cosine or fluonescein according to the type of anion precipitated by Ag+. The titrant is AgNO3 until the violet suspension turns red. pH depends on the type of anion and indicator used. Absorption indicators are substances that can be absorbed by the surface of the sediment and cause color to appear. This precipitation can be adjusted to occur at the equivalent point, among other things, by choosing the type of indicator used and the pH. Before the equivalence point is reached, Cl- ions are in the primary layer and after equivalence is reached, a small excess of AgNO3 5 causes the Cl- ions to be replaced by Ag+ so that the Cl- ions will be in the secondary layer. Vitamins are additional food components that play a very important role in human nutrition. Synthetic vitamins are widely used to replace lost vitamins and to control the taste of vitamin content in foods. Thiamin or vitamin B1 is a combination of a compound with a pyrimidine main ring and a compound with a thiazole main ring. Because of its role as a coenzyme in the intermediate metabolism of alpha-keto acids and carbohydrates, thiamine is found in almost all plants and animals. Vegetables and fruit contain small amounts of vitamin B1. Vitamin B1 is found in high amounts in grains, especially in the sprouts and rice bran. Vitamin B1 (thiamyne) is one type of vitamin that has a low level of stability. Various food processing operations can greatly reduce the vitamin B1 content in foodstuffs. Heat, oxygen, sulfur dioxide, and neutral or alkaline pH can cause the destruction of vitamin B1, while light does not reduce this vitamin. Thiamin is a water-soluble vitamin that is stable in acidic conditions and unstable in neutral or basic conditions. The optimum pH is 2-3. Vitamins, in general, can be grouped into two groups, namely fat-soluble vitamins, namely vitamins A, D, E, K; as well as water-soluble vitamins such as vitamin B and vitamin C. Vitamin B1 functions to treat vitamin B1 deficiency in conditions: beriberi, Wernicke's encephalophyta syndrome, peripheral neuritis accompanied by pregnancy in alcoholics with complications in the sensory nerves, sufferers of metabolic disorders. Tools and materials Tool - Test tubes and test tube racks - Stirring rod - Beaker - Measuring cup - Erlenmeyer - Burette - Statives and Claims Material - AgNO30.1N - Sample of Vit.B syrup (thiamine HCl) - Dilute nitric acid - K2CrO4 - NH4CNS Work procedures - Pipette 10 mL of the Vit syrup preparation. B1 is equivalent to 100 mg of thiamine hydrochloride, put it in an Erlenmeyer, add 20 mL of distilled water. - The solution was acidified with dilute nitric acid and then 10 mL of 0.1 N AgNO3 standard solution was added. - The precipitate that occurs is filtered until the solution does not contain chloride. - The filtrate containing excess AgNO3 standard solution was then titrated with 0.1 N ammonium thiocyanate standard solution using the iron (III) ammonium sulfate indicator. - The end point of the titration is marked when the color of the solution changes to red. - Each mL of 0.1 N silver nitrate is equivalent to 16.86 mg of thiamine HCl. - Calculate the level of thiamine HCl in the preparation using the equation.