Manual of basic techniques for a health laboratory - libdoc.who.int

362 Manual of basic techniques for a health laboratorySterilize the stock solution by filtering it through a 0.2-mm pore size filter. Ifdisodium hydrogen phosphate dihydrate is not available, the solution can beprepared by dissolving 26.8g of disodium hydrogen phosphate, heptahydrate(Na 2HPO 4·7H 2O) or 35.8g of disodium hydrogen phosphate, dodecahydrate(Na 2HPO 4·12H 2O). Label the volumetric flask “DISODIUM HYDROGENPHOSPHATE STOCK SOLUTION” and write the date. Keep the stock solutionin a refrigerator.3. Mix the two stock solutions in the amounts shown in the table below to obtain100ml of buffered water. The pH should be as indicated in the table. If the pH istoo low, adjust it with sodium hydroxide (NaOH), 0.01mol/l solution (No. 54);if it is too high, adjust it with hydrochloric acid (HCl), 0.01mol/l solution (No.32).pH of working solutionVolume of stock solution (ml)NaH 2PO 4Na 2HPO 4·2H 2O6.4 83.2 16.86.5 75.0 25.06.8 50.8 49.26.9 43.9 56.17.0 39.0 61.07.2 28.0 72.07.4 19.9 81.07.6 13.0 87.07.8 8.5 91.58.0 5.3 94.7Polyvinyl alcohol (PVA) fixative (No. 44)Note: This should be prepared at an intermediate level laboratory, because of thedangerous reagents involved.Modified Schaudinn fixativeMercuric chloride crystals (HgCl 2) 1.5gEthanol, 95%31.0mlGlacial acetic acid5.0mlDissolve the mercuric chloride in the ethanol in a stoppered flask (50 or 125ml) byswirling at intervals. Add the acetic acid, stopper, and mix by swirling. Label theflask “MODIFIED SCHAUDINN FIXATIVE” and write the date.Warning: Mercuric acid is highly poisonous. Glacial acetic acid is highly corrosive.PVA mixtureGlycerol1.5mlPVA powder (low viscosity) 5.0gDistilled water52.5mlIn a small beaker, add the glycerol to the PVA powder and mix thoroughly with aglass rod until all particles appear coated with the glycerol. Scrape the mixture intoa 125-ml flask. Add the distilled water, stopper, and leave at room temperature(20–25°C) for 3 hours or overnight. Label the flask “PVA MIXTURE” and writethe date. Swirl the mixture occasionally to mix.