Preparing Saline and Ethanol from Stock Liquids
On Monday morning the lab has two stock liquids: a bottle marked “5% NaCl (w/v)” with no preparation date, and a bottle marked “95% ethanol (v/v)”. You must prepare 2.0 L of 0.9% NaCl solution for a physiology practical and 1.0 L of 70% ethanol for bench disinfection. The only graduated vessels are three 500 mL cylinders, one 2 L volumetric flask, and a 1 L beaker. There is no magnetic stirrer and no autoclaved distilled water; the distilled water carboy was opened last week and kept near the window. The practical starts in 45 minutes. A teacher asks whether the undated NaCl stock can be used. Calculate the required volumes and decide what to do.
1. Calculate the volume of 5% NaCl stock and the volume of 95% ethanol needed. Show the reasoning.
Read the worked answer
Saline: 0.9% w/v means 0.9 g per 100 mL, which is 9 g/L. For 2.0 L you need 18 g NaCl. The stock is 5% w/v = 50 g/L, so volume of stock = 18/50 L = 0.36 L = 360 mL. Put 360 mL stock in the 2 L volumetric flask and add distilled water to the 2 L mark.
Ethanol: C1V1 = C2V2, so V1 = (70% × 1000 mL)/95% = 736.8 mL. Measure about 737 mL of 95% ethanol and add water to 1.0 L. Ethanol-water mixing causes contraction, so make to final volume after mixing. Label both containers.
2. What would you do about the undated NaCl stock and the opened distilled water? Justify your decision.
Read the worked answer
Do not use an undated NaCl stock for a physiology practical. You cannot tell whether it is still 5% or whether water has evaporated; a wrong concentration can harm cells or give false results. Prepare fresh stock from dated solid NaCl, or use a newly prepared and labelled solution.
The opened distilled-water carboy is also questionable for a physiological solution. Use fresh distilled water or water that has been checked or autoclaved. If no fresh water is available, use the old water only for cleaning glassware, not for the practical, and tell the teacher.
3. Give a safe preparation sequence and two labelling details for the two solutions.
Read the worked answer
Wear goggles and gloves, and clean and rinse the 2 L flask. Add about half the water first, then the measured NaCl stock, swirl, and make to the mark. For ethanol, add ethanol to water, not water to ethanol, to reduce heat and splashing.
Use the 2 L volumetric flask for saline. For 1 L ethanol, use two 500 mL cylinders rather than a beaker because a beaker is not accurate for final volume. Label each bottle with the chemical name, concentration, date, preparer, and hazard.