EXPERIMENT 4SOLUBILITY OF IONIC SALTS IN SEAWATER Objectives 1. To standardise silver nitrate solution using a known chloride solution. 2. To determine the chloride concentration in seawater. Introduction Precipitation Titrations A titration process that is carried out such that the stoichiometric reaction between two substances produces a slightly soluble salt that precipitates out in the solution is called a precipitation titration. A common example of a precipitation titration is when the chloride ion in a sample is determined by titrating it with a standard solution of silver nitrate. The equation to show the precipitation reaction is Ag+(aq) + Cl-(aq) → AgCl(s) The chloride in the sample reacts with the added silver nitrate which will then produce a white precipitate of AgCl that settles to the bottom of the flask. The amount of silver nitrate required to completely react with the chloride can be calculated frim concentration and volume used to reach the endpoint. According to the stoichiometric equation, the reaction between the two is one to one; hence the amount of chloride sample equals that of the silver nitrate added. Endpoints In any titration, the completion of a reaction is often physically detected. This point in the titration is called the endpoint; point which just enough titrant has been added. The endpoint that signals the completed reaction between the silver ion and the chloride can be shown by the presence of indicator. In this experiment, chromate ion, CrO42- is used as the indicator. A solution of chromate ion is yellow while the precipitate Ag2CrO42- is blood-red. Chromate ion is added at the beginning of titration. Presence of CrO 42- initially will turn the solution yellow. Upon addition of standard sliver ion, the silver ion will react with the chloride in the sample solution. White precipitate of silver chloride will form. This will continue until there is no more chloride left in the flask. Silver ion still added after this point will react the chromate ion instead, to produce a precipitate of Ag2CrO4 which is brick red in colour. This indicated the endpoint for the titration. What is the minimum concentration of free silver ion that must be present in the solution before it will react with the chromate ion? Let us assume that the [CrO42-] in the solution is 0.001 M. then the solubility product constant, Ksp for silver chromate is Ksp = 1.12 x 10-12 = [Ag+]2[CrO42-] [Ag+] = [Cl-] and [Ag+]2 = Ksp = 1. will Ag2CrO4 precipitate form.01 M) you wish to standardise. hence [Ag+]2 = 1. The pH were adjusted to between 7 and 10 by adding sulphuric acid or sodium hydroxide if necessary.76 x 10-10 / 1 x 10-2 = 1. signalling the endpoint of the titration is now reached. 2. the solubility product constant of silver chromate will be exceeded and you will see the red precipitate due to Ag2CrO4. A 50 mL burette were cleaned thoroughly with tap water and rinse a few times with distilled water. Thus. Before doing the analysis.Since [CrO42-] = 0. you will carry out the Mohr method to determine the concentration of the chloride ion in the seawater.76 x 10-8. At this point. The solubility product then is Ksp = 1. So at the start of the titration. Thus no precipitate of Ag2CrO4 will form. 3. that is why no precipitate of Ag2CrO4 can form at the beginning of the titration. In this experiment. no silver chromate precipitate will form.01 M and we add silver ion on it. The pH were checked using a pH paper. a method for determining chloride in a sample solution using chromate ion as the indicator. Now let us look at what happens when the concentration of chloride ion in sample is 0. addition of even one drop of silver ion will increase the concentration of the silver greatly. This is of course very much less than the minimum concentration of [Ag+] above. [Ag+]2 = 1. Add approximately 100 mL distilled water to dissolve the salt. Then rinse and fill the burette with the silver nitrate solution (approximately 0.33 x 10-5. [Ag+] = 1. At the equivalence point. you will first standardise the silver nitrate solution which is to be used in the determination of chloride ion in the sample. Standardisation of the silver nitrate solution 1.35 x 10-5. About 0. After this equivalence point when all chloride is used up.76 x 10-1. .020 g of dry sodium chloride were weighted accurately. which is still lower than 3. 4. only when the concentration of silver ion in the solution exceeds 3.35 x 10-5. This method is known as the Mohr method.12 x 10-9. Titrate the salt solution earlier with the titrant until a permanent orange pink colour is obtained.76 x 10-10 = [Ag+][Cl-].001 M. and [Ag+] = 3.35 x 10-5M. 1.0 mL of 5% potassium chromate indicator were added. Below this concentration value. Transfer quantitatively into a 250 mL conical flask. Procedure A. Filter the sample if it is cloudy. The titration were repeated for another three samples. 10.0 mL of 5% potassium chromate were added and titrate against the standardised silver nitrate solution. Analysis of seawater 1. Use a dropper towards the end to avoid exceeding the line. The pH of the solution is between 7 and 10 were ensured using a pH paper as above. 1.5. This standardisation process were repeated until you have three values that agree with each other. 2. B.0 mL portion of seawater were measured accurately. 5. 4. 3. 10. .0 mL aliquot of the diluted seawater sample were pipetted into a 250 mL conical flask. Transfer the sample to 100 mL volumetric flask and dilute to the mark with distilled water.