04 How To Prepare Standard Solutions
04 How To Prepare Standard Solutions
04 How To Prepare Standard Solutions
Table of contents
Page
1 2 3 4 5 6 7 8
Module context Module profile Session plan Overhead/flipchart masters Evaluation sheets Handout Additional handout Main text
2 3 4 6 16 18 22 23
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1 Module context
This module describes procedure and a laboratory exercise for preparation of standard solutions. Modules in which prior training is required to complete this module successfully and other available, related modules in this category are listed in the table below. While designing a training course, the relationship between this module and the others, would be maintained by keeping them close together in the syllabus and place them in a logical sequence. The actual selection of the topics and the depth of training would, of course, depend on the training needs of the participants, i.e. their knowledge level and skills performance upon the start of the course. No. 1 Module title Basic water quality concepts Code WQ -01 2 Basic chemistry concepts WQ -02
WQ -03
Objectives Discuss the common water quality parameters List important water quality issues Convert units from one to another Discuss the basic concepts of quantitative chemistry Report analytical results with the correct number of significant digits. Apply the adopted standard practices in laboratory operations
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2 Module profile
Title Target group Duration Objectives : : : :
Key concepts
: :
Volumetric glassware: graduated flask, pipettes, burette, graduated cylinder. Analytical balance with operation manual. Required chemicals, dried, cooled and stored in desiccator. Support of a basic chemical laboratory.
: :
Analytical Chemistry: An introduction, D.A. Skoog and D. M. West/1986. Saunders College Publishing Chemistry for Environmental Engineering, C.N. Sawyer, P.L. McCarty and C.F. Parkin. McGraw-Hill, 1994
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3 Session plan
No 1 Activities Preparations Use your standard ToT checklist Make sufficient copies of suppliers instruction manual for the available balance/s Dry and cool beforehand sufficient quantity of potassium dichromate and ferrous ammonium sulphate Collect and arrange glassware mentioned in the text Obtain ferroin indicator Introduction: Describe session context and activities Ask the participants to read the handout Glassware Demonstrate and explain salient features of different type of glassware and their correct use. Explain: Temperature specifications meniscus and how to read to deliver and to contain specifications rinsing of pipettes and burettes with solutions to be transferred holding of burette stopcock Analytical balance Explain features of the balance Ask each participant to read the operation manual for the balance and precautions given in the text. Explain the use of the available balance (two pan, single pan-mechanical or electronic) with the help of the suppliers instruction manual Demonstrate how to weigh a light object Discuss results of the exercise Explain balance maintenance Standard solutions Explain the need for heating and cooling of chemicals before weighing purity and different grades of chemicals Preparing standard solutions Describe the steps involved Demonstrate the titration procedure for secondary standard
Time
Tools
2 3
30 min
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Practice: Divide the class in working groups of two persons each. Let each group prepare the standard solutions Wrap up: Clarify doubts
65 min
10 min
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4 Overhead/flipchart masters
OHS format guidelines
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Setting
Arial 30-36, Bold with bottom border line (not: underline) Arial 26, Arial 24, with indent maximum two levels only Sentence case. Avoid full text in UPPERCASE. Use occasionally and in a consistent way
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Required glassware
Volumetric flask Watch glass Glass funnel Reagent bottle Pipette Burette Graduated cylinder Conical flask
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Analytical balance
Capacity: 150 200 g Least count: 0.1 mg Care in operation
- do not exceed capacity - beam arresting mechanism - use tongs / forceps - protect from hot / corrosive substance - cleaning after every use - vibration free table
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x 00417 -------L
1L
122598 g
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025
-------L
1L
98 g
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Standardisation
Standardise Ferrous ammonium sulphate solution against standard potassium dichromate solution, 0.0417M as follows: 1. Dilute 10mL standard K2Cr2O7 to about 100 mL in a conical flask. 2. Add 30 mL conc. H2SO4 and cool. 3. Add (2 to 3 drops) ferroin indicator. 4. Titrate with FAS titrant using 0.10 to 0.15mL 5. Read volume when solution turns red at end point 6. Calculate molarity
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Vol. of 0.0417M K2Cr2O7 solution tritrated, mL Molarity = ------------------------------------------------------------------ x 0.25 Vol. of FAS used in titration, mL
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Exercise
Prepare standard solutions Work in groups of two Time 65 min
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5 Evaluation sheets
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6 Handout
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Required glassware
Volumetric flask Watch glass Glass funnel Reagent bottle Pipette Burette Graduated cylinder Conical flask
Analytical balance
Capacity: 150 200 g Least count: 0.1 mg Care in operation - do not exceed capacity - beam arresting mechanism - use tongs / forceps - protect from hot / corrosive substance - cleaning after every use - vibration free table
Procedure
1. 2. 3. 4. 5. 6.
Chemical dried at 103-105 o C for 2 h & cooled in desiccator. Weigh 12.2598 g in a clean, oven dried, cooled & tared watch glass. Carefully transfer weighed chemical to a funnel placed on 1L volumetric flask. Wash the watch glass with a small amount of water into the funnel. Add about 0.5L distilled water, swirl to dissolve & make up to 1L mark. Store the reagent in a reagent bottle (not in volumetric flask).
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Standardisation
1. 2. 3. 4. 5. 6. Standardise Ferrous ammonium sulphate solution against standard potassium dichromate solution, 0.0417M as follows: Dilute 10mL standard K2Cr2O7 to about 100 mL in a conical flask. Add 30 mL conc. H2SO4 and cool. Add (2 to 3 drops) ferroin indicator. Titrate with FAS titrant using 0.10 to 0.15mL Read volume when solution turns red at end point Calculate molarity
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7 Additional handout
These handouts are distributed during delivery and contain test questions, answers to questions, special worksheets, optional information, and other matters you would not like to be seen in the regular handouts. It is a good practice to pre-punch these additional handouts, so the participants can easily insert them in the main handout folder.
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8 Main text
Page
1. 2. 3. 4. 5. Aim Basic glassware Analytical balance Glassware required for practical exercise Preparation of primary standard, potassium dichromate solution, 0.0417M Preparation of secondary standard, ferrous ammonium sulphate, approximately 0.25M 1 1 1 1
6.
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2.
Basic glassware
Volumetric or graduated flask. Pear shaped flat bottom vessel with a long neck. A line etched around the neck indicates the specified volume it contains when filled to the mark. Used for preparation of standard solutions and exact dilutions. Pipettes. There are two kinds: (1) those which have one mark and deliver a small constant volume; (2) those in which the stem is graduated and deliver various small volumes at the users discretion. Used to transfer exact volumes. Burette. Long cylindrical tube of uniform bore throughout the graduated length, terminating at the lower end in a stopcock. Used to add increasing amounts of solutions in exact volumes as in titration. Graduated cylinder. Graduated cylindrical tubes used to measure and transfer liquid volumes where a high level of accuracy is not required.
3.
Analytical balance
Used for weighing chemicals for the preparation of solutions. Common balances have a weighing capacity of 150 and 200 g and can read down to 0.1 mg. The maximum weighing capacity should not be exceeded. Be certain that the arresting mechanism of the beam is engaged whenever the loading of the balance is being changed and when the balance is not in use. Tongs or forceps should be used to place or remove articles from the pans. Never weigh hot or corrosive substance that is likely to damage the balance. Clean the balance after every use.
4.
Volumetric flask, watch glass, glass funnel, reagent bottle, pipette, burette, graduated cylinder, conical flask.
5.
1. Use primary grade or analytical reagent grade chemical for the preparation of the standard solution. The chemical should be dried at 103-105 o C for 2 h and cooled in a desiccator. 2. Molecular Weight (MW) of K2Cr2O7 = 294g. Therefore weight of reagent for 1L of 0.0417M solution = 294g/mole x 0.0417 mole/L x 1L = 12.2598 g. 3. Weigh the above amount into a clean, oven dried, cooled and tared watch glass. 4. Carefully transfer the weighed chemical to a funnel placed on a 1L volumetric flask. 5. Wash the watch glass with a small amount of water into the funnel. 6. Add about 0.5L distilled water, swirl to dissolve and make up to 1L mark.
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6.
1. Molecular Weight (MW) of Fe(NH4)2(SO4)2.6H2O = 392 g. Therefore weight of reagent for 1L of 0.25M solution = 392 g/mole x 0.25 mole/L x 1L = 98 g. 2. Weigh the above amount into a clean, oven dried, cooled and tared watch glass. 3. Carefully transfer the weighed chemical to a funnel placed on a 1L volumetric flask. 4. Wash the watch glass and the funnel with distilled water into the flask using a wash bottle. 5. Add about 0.5 L distilled water and 20 ml concentrated H2SO4, cool and dilute to 1L mark. 6. Store the reagent in a reagent bottle (not in the volumetric flask). 7. Standardise this solution against standard potassium dichromate solution, 0.0417M as follows: Standardization Dilute 10mL standard K2Cr2O7 to about 100 mL in a conical flask. Add 30 mL conc. H2SO4 and cool. Titrate with FAS titrant using 0.10 to 0.15mL (2 to 3 drops) ferroin indicator. The solution turns red at the end point. Vol. Of 0.041M K2Cr2O7 solution tritrated, mL ------------------------------------------------------------Vol. of FAS used in titration, mL
x 0.25
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