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Writing SOPs for preparation of various solutions

Procedures for preparation of various solutions are a very common task described in production and laboratory Standard Operating Procedures (SOPs).  To help you with this process below we gathered procedures for preparation of several frequently-used solutions:

  • Diluting Concentrated Acids to 1 molar (1M) solutions
  • Indicators
  • Standard Buffer Solutions (According to USP 27)
  • Special Solutions and Reagents

1. Diluting Concentrated Acids to 1 molar (1M) solutions

1.1. General Safety Notes. Wear gloves and protect the eyes with safety goggles or even better, a face shield. Dilution of concentrated acid should always be done in a fume cupboard. Add concentrated acid to water slowly. Never add water to a concentrated acid.
1.2. Hydrochloric acid: 36% HCl → 1M HCl. Add 83.5 mL of 36% hydrochloric acid to about 600 mL of distilled water in a 1 litre measuring cylinder and make up to 1L. 32% HCl → 1M HCl. As above, except use 96 mL of 32% hydrochloric acid.
1.3. Nitric Acid: 70% HNO3 → 1M HNO3. Add 62 mL of 70% nitric acid to about 700 mL of distilled water in a 1 litre measuring cylinder and make up to 1 L.
1.4. Sulfuric acid: 98% H2SO4 → 1M H2SO4. Add 54 mL of 98% concentrated sulfuric acid to about 700 mL of distilled iced water in a 1 litre measuring cylinder and make up to 1 L.
1.5. Acetic acid: 99.5% CH3COOH (Glacial acetic acid) → 1M CH3COOH. Add 57 mL of the Glacial acetic acid to about 600 mL of distilled water in a 1 litre measuring cylinder and make up to 1 L. Glacial acetic acid to about 600 mL of distilled water in a 1 litre measuring cylinder and make up to 1 L.

2. pH Indicators

Indicators are substances which change from one color to another when the hydrogen ion concentration reaches a certain value, different for each indicator (1), and used to determine the specified end-point in a chemical reaction or to indicate that a desired change in pH has been effected (2).
2.1. Litmus: Digest 25g of litmus powder with three successive, 100-mL portions of boiling alcohol, continuing each extraction for about 1 hour. Filter, wash with alcohol, and discard the alcohol filtrate. Macerate the residue with about 25 mL of cold water for 4 hours, filter, and discard the filtrate. Digest the residue with 125 mL of boiling water for 1 hour, cool, and filter (2).
2.2. Methyl orange: Dissolve 1g of methyl orange in 1 liter of water. Filter if necessary (1).
2.3. Methyl red: Dissolve 100 mg of methyl red in 100mL of 95% ethyl alcohol. Filter if necessary (2).
2.4. Phenolphthalein: Dissolve 1g of phenolphthalein in 100mL of 95% ethyl alcohol (2).

color-of-indicators

 

3. Standard Buffer Solutions (According to USP 27)

3.1. Components for Standard Buffer Solutions preparation

3.1.1. Potassium Biphthalate, 0.2 M. Dissolve 40.85g of Potassium Biphthalate [KHC6H4(COO) 2] in water, and dilute with water to 1000 mL.

3.1.2. Potassium Phosphate, Monobasic, 0.2 M. Dissolve 27.22g of Monobasic Potassium Phosphate (KH2PO4) in water, and dilute with water to 1000 mL.

3.1.3. Boric Acid + Potassium Chloride, 0.2 M. Dissolve 12.37g of Boric Acid (H3BO3) and 14.91g of Potassium Chloride (KCl) in water, and dilute with water to 1000 mL.

3.1.4. Potassium Chloride, 0.2 M. Dissolve 14.91g of Potassium Chloride (KCl) in water, and dilute with water to 1000 mL.

3.1.5. Carbon dioxide-free water is distillated water that has been boiled vigorously for not less 5 minutes and allowed to cool without contact with atmosphere.

3.2. Composition of Standard Buffer Solutions (for volume 200 mL). Place 50mL of solution 1 in a 200-mL volumetric flask, add the specified volume of the Solution 2 and add carbon dioxide-free water to volume 200 mL.

composition-of-standard-buffer

4. Special Solutions and Reagents

4.1. Bang’s reagent (for glucose estimation). Dissolve 100 g of K2CO3, 66 g of KCl and 160 g of KHCO3 in the order given in about 700 mL of water at 30ºC. Add 4.4 g of of CuSO4 and dilute to 1 liter after the CO2 is evolved. This solution should be shaken only in such a manner as not allow entry of air. After 24 hours 300 mL are diluted to 1 liter with saturated KCl solution, shaken gently and used after 24 hours; 50 mL equivalent to 10 mg glucose (1).

4.2. Biuret Reagent. Dissolve 1.5 g of cupric sulfat and 6.0g of potassium sodium tartrate in 500 mL of water in a 1000-mL volumetric flask. Add 300 mL of carbonate-free sodium hydroxide solution (1 in 10), dilute with carbonate-free sodium hydroxide solution (1 in 10) to 1000mL and mix (2).

4.3. Bromine Water. Prepare a saturated solution of bromine by agitating 2-3 mL of bromine with 100 mL of cold water in a glass-stoppered bottle (2).

4.4. Chlorine Water. Prepare a saturated solution of chlorine in water (2).

4.5. Denigè’s Reagent. Mix 5 g of yellow mercuric oxide with 40 mL of water, and while stirring slowly add 20 mL of sulfuric acid, then add another 40 mL of water, and stir until completely dissolved (2).

4.6. Lime water. Saturated solution of calcium hydroxide – 1.5 g of Ca (OH) 2 in 1000 mL of water . Use some excess, filter off CaCO3 and protect from CO2 of the air (1).

4.7. Mayer’s Reagent. Dissolve 1.358 g of mercuric chloride in 60 mL of water. Dissolve 5 g of potassium iodide in 10 mL of water. Mix the two solutions, and dilute with water to 100 mL (2).

4.8. Nessler’s Reagent. Dissolve 143 g of sodium hydroxide in 700 mL of water. Dissolve 50 g of red mercuric iodide and 40 g of potassium iodide in 200 mL of water. Pour the iodide solution into hydroxide solution, and dilute with water to 1000 mL. Allow the settle, and use the clear supernatant (2).

4.9. Pasteur’s salt solution. To 1000 mL of distilled water add 2.5 g of potassium phosphate and o.25 g of calcium phosphate(1).

4.10. Saline. Dissolve 9.0 g of sodium chloride in water to make 1000 mL (1).

References
1. CRC Handbook of Chemistry and Physics. 63rd edition. Editor: R.C.Weast. CRC Press.
2. U.S. Pharmacopeia. USP 27 / NF 22.

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