The glassware used in wastewater laboratories is constructed from specialized borosilicate glass. There are several types of glassware. Each type has a variety of uses based on the level of accuracy required. To properly read the volume of liquid in glassware, the level is measured to the bottom of the meniscus.
Glassware should always be kept clean and is easiest to clean immediately after use. Thorough cleaning with non-phosphate soap and scrubbing followed by several rinses with de-ionized water is the preferred method. The main types of glassware are as follows:
Beakers & Flasks
* Made of Borosilicate glass or plastic and used for mixing, heating, settling and other general procedures.
* Beakers and flasks are not used when accurate measurements of volume are required.
* Made of glass or plastic and used where good accuracy is required.
* Glass graduated cylinders are calibrated “To Deliver” (TD) or “To Contain” (TC). TD cylinders deliver the stated volume, TC cylinders are used for creating specific dilutions.
* Plastic are calibrated TC and TD, because the drops left behind when the contents are poured from the cylinder are accounted for.
* Use a size close to the volume being measured.
* Use Class “B” tolerance or better.
* Made of high-grade borosilicate glass and used where the highest level of accuracy is required, always labeled TC.
* Used to make up primary standards.
* Should be designated Class “A” tolerance.
* Pipettes are used to accurately measure and transfer small amounts of liquids.
* There are two main types of pipets; transfer (volumetric; Class “A”), and measuring (Mohr or serological; Class “B”).
* The accuracy of a pipet is related to its type and the analyst’s technique.
Pipets are classified by their operation:
* Volumetric pipets will deliver the specified or desired volume when drained and “tipped” to the edge of the receiving vessel. This type of pipet should not be “blown out”.
* Mohr pipets are graduated, but not calibrated to the tip. If allowed to drain completely, too much liquid will be delivered. Because of this, Mohr pipets are never “blown out”.
* Serological pipets are graduated, so they can deliver different measured volumes. Serological pipets must be “blown out” to deliver the measured volume and are designated with a frosted band or double lines near the top.
* Burets are used for volumetric titrations where high accuracy is required.
* They are made of borosilicate glass with a glass or Teflon stopper.
* When used properly, burets can deliver Class “A” tolerance.
* Digital titrators are now commonly used in-place of burets.
Accurate temperature measurements are critical to many of the tests that are performed in the wastewater laboratory. Temperature measurements should be made with good mercury thermometers or digital thermometers. Never rely upon the temperature display built into an incubator or drying oven for an accurate indication of the instrument’s temperature. Thermometers located in BOD incubators and drying ovens should be placed in stoppered beakers that contain clean sand, water or mineral oil to protect the thermometer from breakage and mitigate rapid fluctuations in temperature that occur when the unit is opened. For water bath incubators, thermometers should be held upright with their mercury bulb submerged in the water bath itself. The temperature of all operating instruments should be recorded twice a day in an instrument temperature log. Use thermometers that have the sensitivity required for each test. For most tests, use a thermometer with graduations of 0.1º C. To verify the accuracy of laboratory thermometers, each thermometer should be calibrated against a NIST certified thermometer in its working range at least once per year. Once calibrated, laboratory thermometers should be flagged with the date of their last calibration and any correction factor. When a laboratory thermometer is read, the correction factor is included when the temperature is recorded to ensure that the most accurate temperature possible is maintained in the instrument.