CHEMISTRY GLOSSARY

Buchner funnel    →   Buchnerov lijevak

Büchner funnel is one device used for pressure assisted filtration. Buchner funnel is a cylindrical porcelain filtering funnel (glass and plastic funnels are also available) that has a perforated plate on which the flat filter paper is placed. A vacuum in the flask underneath the filter allows atmospheric pressure on the sample to force the liquid through the filter paper. It is named after the German chemist Ernst Wilhelm Büchner (1850-1925) who designed this funnel in 1885.

burette    →   bireta

Burette is a graded glass pipe which on its lower side has a glass faucet by which it can drop a precise quantity of liquid. Inner diameter of a burette must be equal in its whole length, because the accuracy of volume measurement depends upon that. Burettes are primarily used in volumetric analysis for titration with standard solution reagent. Most often Schellbach’s burette is used, graded on 50 mL with division of scale on 0.1 mL. Every burette is calibrated on discharge. For serial determining automatic burettes are used.

chlorosity    →   klorocitet

Chlorosity is the quantity determined by volumetric methods and is defined in the same manner as chlorinity except that the sample unit is 1 L of sea water rather than 1 kg of sea water weighed in vacuo.

conductometry    →   konduktometrija

Conductometry is a volumetric analytic method in which the end of titration (equivalent point) is defined by an electric conductivity appliance.

chlorinity    →   klorinitet

Originally chlorinity (symbol Cl) was defined as the weight of chlorine in grams per kilogram of seawater after the bromides and iodides had been replaced by chlorides. To make the definition independent of atomic weights, chlorinity is now defined as 0.3285233 times the weight of silver equivalent to all the halides.

The Mohr-Knudsen titration method served oceanographers for more than 60 years to determine salinity from chlorinity. This modification of the Mohr method uses special volumetric glassware calibrated directly in chlorinity units. The Mohr method uses potassium chromate (K₂CrO₄) as an indicator in the titration of chloride ions chloride (plus a small amount of bromide and iodide) with a silver nitrate (AgNO₃) standard solution.

The other halides present are similarly precipitated.

A problem in the Mohr titration was that silver nitrate is not well suited for a primary standard. The Danish physicist Martin Knudsen (1871-1949) suggested that a standard seawater (Eau de mer Normale or Copenhagen Normal Water) be created and distributed to oceanographic laboratories throughout the world. This water was then used to standardize the silver nitrate solutions. In this way all chlorinity determinations were referred to one and the same standard which gave great internal consistency.

The relationship between chlorinity Cl and salinity S as set forth in Knudsen's tables is

In 1962, however, a better expression for the relationship between total dissolved salts and chlorinity was found to be

complexometry    →   kompleksometrija

Complexometry is a volumetric analytic method which is based on titration of metal ion solutions with a substance that, combined with metal ions yields complex compounds, e.g. EDTA. The stoichiometric ratio of EDTA-metal in complexometric analyses is always 1:1, whatever the valency of the metal

Contat-Gockel’s valve    →   Contat-Gockelov ventil

Contat-Göckel’s valve is used for maintenance of inert atmosphere in a flask. The valve is filled with a saturated solution of sodium bicarbonate (NaHCO₃) so that the end of the tube is covered. Solution inside the valve keeps the flask contents away from the oxygen influence from air. If low pressure is created inside the flask (when the flask is cooled), the solution will penetrate inside it from funnel and in a reaction with acid CO₂ is generated which fills up the flask.

Solution from the funnel will keep penetrating until CO₂ pressure in the flask is equalised with the outer pressure.

graduated pipette    →   graduirana pipeta

Graduated pipettes (Mohr pipette) have a scale divided into units of one and of 1/10th of a millilitre. Because of their wide necks it is less accurate than the volumetric pipette. They are used when taking volume of solutions in which accuracy does not have to be very high. 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 0. A pipette is emptied out by lifting the forefinger off and letting the liquid flow out of the pipette freely.

Kjeldahl’s method    →   Kjeldahlov postupak

Kjeldahl’s method is an analytical method for determination of nitrogen in certain organic compounds. The method was developed by the Danish chemist Johan Kjeldahl (1849-1900).

It involves addition of a small amount of anhydrous potassium sulphate to the test compound, followed by heating the mixture with concentrated sulphuric acid, often with a catalyst such as copper sulphate. As a result ammonia is formed. After alkalyzing the mixture with sodium hydroxyde, the ammonia is separated by distillation, collected in standard acid, and the nitrogen determined by back-titration.

1. Kjeldahl flask for decomposition (500 ml – macro or 100 ml - micro)
2. funnel for alkaline solution
3. Wagner tube (drop catcher)
4. condenser
5. absorption flask with known volume of standard acid

meniscus    →   meniskus

Meniscus is the concave curve of a liquid surface in a graduate or narrow tube. Caused by surface tension. Position of eye at all volumetric vessels must be at the same level as the meniscus to avoid a parallax error.