Equipment and Manipulations Associated with weighing

** The mass of many solids changes with humidity because they tend to absorb weighable amounts of moisture.

** This effect is especially pronounced when a large surface area is exposed, as with a reagent chemical or a sample that has been ground to a fine powder. In the first step in a typical analysis, then, the sample is dried so that the results will not be affected by the humidity of the surrounding atmosphere.

** A sample, a precipitate, or a container is brought to constant mass by a cycle of heating (usually for one hour or more) at an appropriate temperature, cooling, and weighing.

** This cycle is repeated as many times as needed to obtain successive masses that agree within 0.2 to 0.3 mg of one another.

** The establishment of constant mass provides some assurance that the chemical or physical processes that occur during the heating (or ignition) are complete.

(1) Weighing Bottles

** Weighing bottles are convenient for drying and storing solids.

** Two common varieties of these handy tools are shown in Figure blow.

** The ground-glass portion of the cap-style bottle shown on the left is on the outside and does not come into contact with the contents.

** This design eliminates the possibility of some of the sample becoming trapped on the ground-glass surface and subsequently being lost.

** Ruggedness is a principal advantage of using plastic weighing bottles rather than glass, but plastic abrades easily and is not as easily cleaned as glass.

(2) Desiccators and Desiccants

** Oven drying is the most common way of removing moisture from solids.

** This approach is not appropriate for substances that decompose or for those from which water is not removed at the temperature of the oven.

** To minimize the uptake of moisture, dried materials are stored in desiccators while they cool.

** Figure shows the components of a typical desiccator.

** The base section contains a chemical drying agent, such as anhydrous calcium chloride, calcium sulfate (Drierite), anhydrous magnesium perchlorate (Anhydrone or Dehydrite), or phosphorus pentoxide.

** The ground-glass surfaces between the top and the base are lightly coated with grease to ensure a good seal when the top is in place.

** A desiccator is a device for drying substances or objects.

** When removing or replacing the lid of a desiccator, use a sliding motion to minimize the likelihood of disturbing the sample.

** An airtight seal is achieved by slight rotation and downward pressure on the positioned lid.

** When placing a heated object in a desiccator, the increase in pressure as the enclosed air is warmed may be sufficient to break the seal between lid and base.

** Conversely, if the seal is not broken, the cooling of heated objects can cause a partial vacuum to develop. Both of these conditions can cause the contents of the desiccator to be physically lost or contaminated.

** Although it defeats the purpose of the desiccator somewhat, allow some cooling to occur before the lid is seated. It is also helpful to break the seal once or twice during cooling to relieve any excessive vacuum that develops.

** Finally, lock the lid in place with your thumbs while moving the desiccator from one place to another.

Note: Very hygroscopic materials should be stored in containers equipped with snug covers, such as weighing bottles. The bottles remain covered while in the desiccator. Most other solids can be safely stored uncovered.

(3) Manipulating Weighing Bottles

** Heating at 105°C to 110°C for 1 hour is sufficient to remove the moisture from the surface of most solids.

** Figure shows the recommended way to dry a sample.

** The weighing bottle is contained in a labeled beaker with a cover glass. This arrangement protects the sample from accidental contamination and also allows for free access of air.

** Crucibles containing a precipitate that can be freed of moisture by simple drying can be treated similarly.

** The beaker holding the weighing bottle or crucible to be dried must be carefully marked for identification.

** Avoid touching dried objects with your fingers because detectable amounts of water or oil from the skin may be transferred to the objects.

** Instead, use tongs, chamois finger cots, clean cotton gloves, or strips of paper to handle dried objects for weighing.

** Figure shows how a weighing bottle is manipulated with tongs and strips of paper.

(4) Weighing by Difference

** Weighing by difference is a simple method for determining a series of sample masses.

** First, the bottle and its contents are weighed. One sample is then transferred from the bottle to a container.

** Gentle tapping of the bottle with its top and slight rotation of the bottle provide control over the amount of sample removed.

** Following transfer, the bottle and its residual contents are weighed.

** The mass of the sample is the difference between the two masses.

** It is essential that all the solid removed from the weighing bottle be transferred without loss to the container.

(5) Weighing Hygroscopic Solids

** Hygroscopic substances rapidly absorb moisture from the atmosphere and, therefore, require special handling.

** You need a weighing bottle for each sample to be weighed.

** Place the approximate amount of sample needed in the individual bottles and heat for an appropriate time.

** When heating is complete, quickly cap the bottles and cool in a desiccator.

** Weigh one of the bottles after opening it momentarily to relieve any vacuum. Quickly empty the contents of the bottle into its receiving vessel, cap immediately, and weigh the bottle again along with any solid that did not get transferred. Repeat for each sample and determine the sample masses by difference.

(6) Weighing Liquids

** The mass of a liquid is always obtained by difference.

** Liquids that are noncorrosive and relatively nonvolatile can be transferred to previously weighed containers with snugly fitting covers (such as weighing bottles). The mass of the container is subtracted from the total mass.

** A volatile or corrosive liquid should be sealed in a weighed glass ampoule.

** The ampoule is heated, and the neck is then immersed in the sample.

** As cooling occurs, the liquid is drawn into the bulb.

** The ampoule is then inverted and the neck sealed off with a small flame.

** The ampoule and its contents, along with any glass removed during sealing, are cooled to room temperature and weighed.

** The ampoule is then transferred to an appropriate container and broken.

** A volume correction for the glass of the ampoule may be needed if the receiving vessel is a volumetric flask.

Reference: Fundamentals of analytical chemistry / Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch. (ninth edition) , 2014 . USA

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