How to Laboratory Techniques:
Bottle Labels | Lab Operations | Calibration Volumetric ware | Cleaning Labware | pH Measurements | Temperature Measurements | Preparation Standard Solutions | Solvents for UV Spectro
HOW TO READ LABEL ON CHEMICAL REAGENT BOTTLEWHAT IS A CHEMICAL REAGENT BOTTLE?
A chemical reagent bottle is that container (glass, can or plastic) which contains a chemical, such as sodium carbonate, that you use in your school experiments, lab test procedures or lab analyses. A laboratory worker must understand the chemical, physical and hazardous properties of the chemicals that he/she uses. DO YOU KNOW WHAT ALL THE INFORMATION MEANS ON THE LABEL OF A CHEMICAL REAGENT BOTTLE?
The minimum amount of information that should be printed on a label, is the following:
- Name of manufacturer, or supplier:
- Manufacturer catalogue or product number:
- Batch number:
- Quantity of contents of container/bottle:
- The stated grade (assay or purity type) of reagent:
- Name of chemical and its other names:
- Molecular formula of chemical:
- Molecular mass of chemical (if given):
- Storage conditions (if any):
- Shelf life or expiry date, or date of manufacture:
- S and R phrases:
- Hazard icons if any):
Check the above information on a bottle from your lab!
What is Laboratory Safety? Check out your Lab Safety! Also check for a lesson on safety in the chemical laboratory.
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SOME SIMPLE LABORATORY OPERATIONS ARE:
- taking temperature measurements before sampling of liquids, or noting temperature of water-baths used for conducting tests at specific temperatures such as viscosity and density measurements;
- measuring the density or SG (Specific Gravity) of a liquid using hydrometers to establish the strength or purity of the liquid;
- measuring the refractive index of a liquid using a refractometer to establish the strength or purity of the liquid;
- measuring the turbidity or colour of a liquid using a turbidity meter or colorimeter to establish the strength or purity of the liquid;
- measuring out exact volumes of liquid samples or other liquid chemical reagents using measuring cylinders, pipettes and burettes (titrimetric/volumetric analyses);
- removing solid matter or precipitates (gravimetric analyses) from liquids using filtration procedures;
- measuring the conductivity of liquids and hence calculating the TDS (electrolytes) of waters;
- measuring the acidity or alkalinity of substances using indicator papers, colour indicator solutions or pH meters;
- weighing known amounts of sample or chemicals using balances (mass meters);
- titrations, volumetric work, e.g. acidity test on milk, test for salt, etc.;
- Anti-bumping devices can be glass beads, porcelain chips or small pumice stones or powder; these are used to prevent localised overheating when boiling liquids in a beaker;
- gravimetric work, e.g. precipitations, filtering and weighing.
Check out your Conversion Factors page! for a lesson on reporting test results in different concentration units.
|Temperature t, in degrees C||Volume of 1.0000 gram water, in cm3, at temperature t:|
Laboratory volumetric glassware is usually marked and calibrated in ml at 25°C.
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CLEANING OF LABORATORY WARE:
Good laboratory technique requires clean glassware because the most carefully executed analysis of a sample may give erroneous results if contaminated/dirty glassware is used. Glassware must be physically (visually) and chemically clean without any non-visible detergent residues present. A good indication of cleanliness is uniform wetting of the surface of the glass by distilled water. This is very critical in glassware used for dispensing accurate volumes of liquid such as burettes and pipettes. Grease and other contaminating materials will prevent the glass from becoming uniformly wetted which in turn, will cause liquid globules adhering to the glass surface, thus causing an inaccurate volume of the liquid to be dispensed.
There are many detergents and cleaning agents available, however Chromic Acid although dangerous to handle is still the most efficient glassware cleaning chemical; alternatively one may try alcoholic caustic. The type of cleaning chemical to use is very much dependent upon the type of soiling of the glassware..........there is no such thing as the ultimate clean-all universal agent.
Formulae for cleaning glassware:
- Formula for chromic Acid (HAZARDOUS CHEMICAL): Ideal for inorganic dirt, mineral residues, calcium hardness deposits, etc. 25g sodium dichromate dissolved in 25ml water, and add SLOWLY concentrated sulphuric acid with stirring to final volume of about 1 litre, beware evolution of heat!
- Formula for Alcoholic Caustic (HAZARDOUS CHEMICAL): ideal for organic dirt, fats, oils, foodstuffs, etc. Approximately 56g potassium hydroxide dissolved in minimum water, then add alcohol (methylated spirits) to make up to about 1 litre (i.e. approximate 1N alcoholic KOH). CAUTION: do not soak glass items for longer than 30 minutes in this solution, because caustic will attack glass surfaces.
- Formula for EXTRA effective general detergent: Add about 10ml hydrofluoric acid (HAZARDOUS CHEMICAL) to about 1litre general household dishwasher liquid. Mix well. CAUTION: do not soak glass items for longer than 30 minutes in this solution, because hydrofluoric acid attacks glass surfaces.
- Formula for cleaning sintered glass crucibles: Boil gently in aqua regia (3 parts hydrochloric acid + one part nitric acid) (HAZARDOUS CHEMICAL). Then flush crucible well with water whilst under vacuum filtration.
Formulae for sanitizing bench surfaces, tables, etc:
70% alcohol only; or 75% alcohol + 2% ether + 1% acetone + balance water. Do not apply this to plastics, because acetone will attack plasticware.
Care of Platinum apparatus:
Platinum is a very expensive metal and must be treated with care. Platinum crucibles and other platinium ware, should not be heated above 1000°C unless it is a special alloy that can withstand higher temperatures. Any heating (or ignition in furnace) of the apparatus must be done under oxidising atmosphere. This apparatus must NOT come in contact with iron (e.g. wire gauzes), phosphoric acid, aqua regia, nitric acid, or presence of any nitrates or nitrites. When in doubt, consult analytical chemistry textbooks. Platinum crucibles and dishes should be stored in their original formers to prevent damage.
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CRITICAL MAINTENANCE OF pH METER and ITS ELECTRODES:
The most common fault of all pH meters are dirty or blocked electrodes. The instrument itself (which is basically a millivolt measuring meter) is very robust and requires no maintenance other than to be kept clean and free of liquid splashes. There are many types of pH electrodes available and it is best to purchase one that is robust, accurate and designed for use for the particular type of samples/liquids to be tested. Some problems experienced with pH electrodes (such as slow response, drift, unstable readings and poor slope values) are due to:
- air bubbles in the filling chamber; tap the glass wall of the electrode gently to dislodge the bubbles;
- blocked reference junction/diaphragm (brown AgCl) and bulb contamination with insoluble particulate matter or proteins;
- crystal formation (KCl salt crystals) inside junction or main body of electrode (due to leaving the filling hole open when not in use), may be re-dissolved by heating under running hot water.
- trying to determine the pH of a non-aqueous solution/sample (always use a special pH electrode from supplier for measuring pH in organic solvents and non-aqueous solutions).
Dirty electrodes can be cleaned using various procedures depending upon what type of samples/liquids were tested. For example:
- clean samples, such as water samples, just rinse under hot running water;
- samples of high proteins such as blood, milk, etc., dip into a mixture of 0.1M hydrochloric acid and 5% pepsin, for a few minutes only (but not longer than 30 minutes); or weakly acidic 7% thiourea;
- samples of high organic matter such as foodstuffs, trade effluents, oils, etc., dip into 50% acetone or alcohol for a few minutes only; or, replace the electrode solution with fresh electrode filling solution; do not remove any internal black crystals (AgCl);
- for very dirty electrodes, soak in 0.1M ammonium bifluoride solution for maximum 2 minutes only;
- as a last resort only, take a very fine ladies nail file and gently scrap away any dirt/residue on the top layer of the diaphragm.
Note that after any of the above remedies, always wash electrode thoroughly in clean running water, and then let stand overnight in an electrode storage solution. It is also recommended that on a regular basis the internal electrolyte (solution) should be totally replaced by fresh electrolyte (this is not applicable to gel-type electrodes). Electrodes should not be stored dry when in regular use; rather store in electrode storage solution available from laboratory suppliers, or its electrolyte filling solution (e.g. 3M KCl); or pH 7 for short periods of time. NOTE: always top-up or refill the electrode with the correct strength electrolyte (refilling solution)! If you are not sure which type to use, then check with your electrode supplier. There are various strengths of potassium chloride solutions that are used as electrolytes, such as 3 Molar; 3.3 Molar; 4 Molar (Saturated); also some electrolyte refilling solutions are saturated with silver chloride crystals (AgCl).
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TAKING ACCURATE TEMPERATURE MEASUREMENTS:
What is temperature?
Temperature is a degree of heat (or cold) where energy moves from matter of higher temperature to matter of lower temperature, until temperature equilibrium is reached.
What is a thermometer?
A thermometer is a device capable of measuring the temperature of a substance.
There are basically four types of thermometers:
- dial thermometers e.g. oven or fridge thermometers;
- glass mercury- or alcohol (spirit)- filled thermometers, e.g. laboratory thermometers or the obsolete clinical thermometers. There are basically 3 types of glass thermometers: partial immersion to 76mm, or to 0°C (ring around thermometer); partial immersion up to mercury column; and total immersion of whole thermometer.
- digital thermometers, e.g. portable battery operated meters, that cover a wide temperature range depending upon the type of probe (e.g. Pt-100 for readings below 100°C; or thermocouples for readings above 100°C);
- Infra-Red type temperature measuring instruments; these are usually used for measuring very high temperatures where access is difficult or impossible; e.g. furnaces.
- Dial thermometers are not accurate but are easy to use and are robust if handled carefully;
- Standard mercury-in-glass thermometers are used for accurate work and usually can measure to nearest 0.1°C or better. For accurate readings: the partial immersion 76mm type should be immersed in the liquid up to the 76mm mark or ring on the thermometer; the other partial immersion type is immersed in the liquid up to the same level as the mercury column; whereas the total immersion type is immersed completely in the liquid. Markings on the thermometer will indicate what type of thermometer it is. DO NOT use a glass thermometer as a stirring rod. Always take note of the markings on the back of the thermometer as this gives indication of type of thermometer. ALWAYS CHECK FOR BREAKS IN THE MERCURY COLUMN AND ADJUST READINGS OR RE-CONNECT COLUMN OR DISCARD THERMOMETER!
- Digital thermometers are used extensively in the field and good quality ones are used in various instruments, however their accuracy (not precision) is very much dependent upon CALIBRATIONS and the type and quality of the measuring probe.
- Indicating and Recording (Chart) thermometers are used in factories; e.g. time chart or log recorders.
Mercury glass thermometers:
Separation of the mercury column can occur during transit or mishandling. Some of the techniques of re-connecting the mercury column are:
- gentle heating until the mercury enters the top safety reservoir;
- slow cooling/freezing until the mercury enters the bottom bulb;
- gentle swinging/whirling of the thermometer on end of string;
- gentle jerking of thermometer.
Glass thermometers are subject to changes or irregularities in their capillary glass tubes and volume of the bottom bulb, due to continuous expansion and contraction during use. Note that these errors are not constant over the entire measuring range of the thermometer.
All thermometers must be regularly calibrated against a certified reference thermometer. These calibrations/checks must be done, at least, on three different temperature points preferably at those temperatures most commonly measured in the laboratory.
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Preparation Standard Solutions
|Chemical name||Molecular formula||Molecular mass||Valency of reaction||Mass in grams per 1 liter solution|
|hydrochloric acid (SG 1.16)||HCl||36.4610||1||9.33ml|
|sodium hydroxide solution||NaOH||39.9972||1||3.9997|
|sulphuric acid (SG 1.84)||H2SO4||98.0775||2||4.9039|
|zinc sulphate hydrate||ZnSO4.7H2O||287.5390||2||14.3769|
How to convert mls of titration to concentration of fatty acid? Check out your Conversion Factors page! for acid factors used in the chemical calculations.
How to take correct burette reading:
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|Chemical name||Maximum wavelength (UV Cut-off), in nanometres, above which absorption takes place|