CHEMISTRY GLOSSARY

volumetric pipette    →   prijenosna pipeta

Volumetric pipettes (transfer or belly pipette) are used in volumetric analysis, when there is a need for taking exact smaller volume of a sample solution or reagent. The upper tube of volumetric pipette has a ringlike marking (mark) which marks its calibrated volume. Pipettes calibrated to deliver (TD or Ex) the indicated volume. By sucking in (with mouth, propipette or a water pump) the liquid is pulled in a little bit above the mark and the opening of the pipet is closed with a forefingertip. Outer wall of pipet is wiped and, with a slight forefinger loosening, the liquid is released until it reaches the mark. Mark must figure as a tangent on a lower edge of the liquid meniscus. A pipette is emptied out by lifting the forefinger off and letting the liquid flow out of the pipette freely. After another 15 s and the tip of the pipette is pulled onto the inner wall of the vessel. It is absolutely forbidden to blow out the contents of the pipette.

Schrotter apparatus for determination of CO2    →   Schrotterova aparatura za određivanje CO2

Schrötter decomposition apparatus (Schrötter's alkalimeter) is used to determining the carbonate content in samples of limestone, gypsum, dolomite, or baking powder by loss of weight. The apparatus is named after the Austrian chemist Anton Schrötter von Kristelli (1802-1875), who devised it in 1871. The size of the filled apparatus (apparatus is 16 cm high) is such that it weights less than 75 g, and can be placed on the pan of an analytical balance.

Procedure: Weigh about 0.5 g of the powdered carbonate sample and introduce it into the decomposition flask C. Pour into the drying tube A 2-3 mL of concentrated sulphuric acid (H₂SO₄), and to the dropping funnel B add about 10-15 mL of hydrochloric acid (w(HCl) = 15 %). Weigh the whole apparatus. Open the upper taps of both parts and allow the hydrochloric acid from B to run slowly down on to the powdered sample. The evolved CO₂ escapes through the strong sulphuric acid and is thus thoroughly dried. When further addition of acid produces no more evolution of CO₂, warm the apparatus up to 80 °C so as to expel the CO₂ from the solution. Connect the upper tap of the drying tube A to a water pump and draw a slow current of air through the apparatus until completely cool. Open the upper taps for a moment to equalize the internal and external pressure and weight the apparatus again. The weight loss is equal to the weight of carbon dioxide liberated from the carbonates.