advertisement

1 Preparation and Standardisation 0,1 N Solution of Hydrochloric Acid Quantitative Determination of Sodium Hydroxide Hydrochloric acid is widely used for titration of bases. Standard acid solutions are ordinary prepared by diluting an approximate volume of the concentrated reagent and subsequently standardising the diluted solution against a primary-standard base. The composition of the concentrated acid is established through careful density measurement; a weighed quantity (measured volume) is than diluted to an exact volume. Diluted solution of the reagent is stable indefinitely. Acids are frequently standardised against accurately weighed quantities of sodium tetraborate decahydrate (borax). Borax is a primary standard. Borax end point occurs at about pH 5,12. The sample is titrated to the appearance colour of the indicator (methyl orange pT 5,5): 2HCl + Na2B4O7 + 5H2O = 2NaCl + 4H3BO3 For primary standard solutions allow the correction factor. Correction factor shows how many times the prepared solution is more or less concentrated than theoretically calculated. I. Preparation the primary standard solution (0,1 N borax) 1. Calculate a primary standard sample (mBT) for preparation 100 ml of solution: N ⋅ V ⋅ meq B mBT = . M.m. Na2B407×10H2O = 381,37 g; mass of borax equivalent meqB = M/2. 1000 2. Weight borax sample on hand balance with 0,1 g accuracy and put powder into weighting bottle. 3. Weight weighting bottle with borax on analytical balance. 4. Transfer the borax into volumetric flask and weight empty weighting bottle on analytical balance. Calculate the borax sample weight mBP. 5. Add 40-50 ml of hot distilled water to volumetric flask, dissolve borax and establish exact volume of solution with water. m ⋅ 1000 6. Calculate precision normal concentration of prepared solution of borax: NB = BP . V ⋅ meq B mBP 7. Calculate the correction factor for prepared 0,1 N borax solution: CFB = . mBT II. Preparation the 0,1 N hydrochloric acid solution 1. Calculate mass of hydrochloric acid sample mA for preparation of 250 ml of solution. M.m. HCl = 36,5 g; meqA = 1. 2. Measure density (d) of commercial hydrochloric acid. Calculate mass of commercial concentrated hydrochloric acid solution (mAconc), using density tables (from chemical textbooks). Calculate volume of commercial acid will be used VAconc = mAconc/d. 3. Measure concentrated hydrochloric acid aliquot, transfer it into volumetric flask and dilute with distilled water. 2 III. Standardisation the solution of hydrochloric acid 1. Load a burette with prepared hydrochloric acid solution. 2. To conical flask pour in 10 ml aliquot of standard borax solution (VB). 3. Introduce 2-3 drops of methyl orange solution, and titrate with prepared hydrochloric acid solution until the solution just begin to change colour from yellow to red. Read the burette mark. 4. Repeat titration also two times. Calculate the approximate value of used hydrochloric acid solution (VA). 5. Calculate the exact concentration of the hydrochloric acid solution accordance to equivalents law (NB⋅VB = NA⋅VA): NA = NB⋅VB/VA. IV. Quantitative determination of sodium hydroxide 1. Add to conic flask 10 ml aliquot of problem solution (VAlk). 2. Introduce 2-3 drops of phenolphthalein solution, and titrate with prepared hydrochloric acid solution until the solution lost the red colour. 3. Repeat titration also two times. Calculate the approximate value of used hydrochloric acid solution (VA). NA ⋅ VA ⋅ meqAlk 4. Calculate the sodium hydroxide amount: mAlk = . VAlk 5. Calculate the sodium hydroxide percentage: x% = M.m. NaOH = 39,9997. meqAlk = 1; pAlk = 5,00; W = 1000. NA ⋅ VA ⋅ meqAlk ⋅ W ⋅100% . VAlk ⋅ pAlk