trazas

13

Reagent ChemicalsNinth Edition

ACS SpecificationsOfficial from January 1, 20002000 American Chemical Society

General Directions and Procedures

The general directions and/or procedures for most ACS requirements and testsare described in this section. Where appropriate, the specifications for particularreagent chemicals or standards contain page references to individual test proce-dures described fully in this section. Supplemental information may be found inKenkel, 1994; Kingston and Haswell, 1997; Skoog and others, 1998; Settle, 1997;and Willard and others, 1995.

GRAVIMETRIC METHOD FOR DETERMINING SMALL AMOUNTS OF IMPURITIES

In many of the reagent specifications, it is directed that a precipitate or residue becollected and either dried or ignited so as to provide certain information concern-ing the purity of the reagent. Except where it is directed otherwise in the specificreagent tests, the following directions and precautions will be used in collecting,drying, and igniting precipitates and residues.

General Considerations

It is imperative that the analyst use the best techniques in performing any of theoperations included in this book. Exceptional cleanliness and protection againstaccidental contamination from dirt, fumes, and analytical containers are rigidrequirements for acceptable results. Use of fume or clean air hoods is recom-mended whenever possible. The choice of equipment is generally left to the dis-cretion of the analyst, but it must provide satisfactory accuracy and precision. Allweighings must be determined with an uncertainty of not more than 0.0002 g.(See Moody, 1982; Zief and Mitchell, 1976.)

When the use of a tared container is specified, the container will be carriedthrough a series of operations identical to those used in the procedure, includ-

14 Gravimetric Method for Determining Small Amounts of Impurities Reagent ChemicalsNinth Edition


ing drying, igniting, cooling in a suitable desiccator, and weighing. In general,desiccators should be charged with indicating-type silica gel. Stronger desic-cants may be required for certain applications. The length of the drying orignition and the temperature employed must be the same as specified in theprocedure in which the tared equipment is to be used. Where it is directed todry or ignite to constant weight, two successive weighings may not differ bymore than 0.0002 g, the second weighing following a second drying or igni-tion period.

In those operations wherein a filtering crucible is specified, a fritted-glasscrucible, a porous porcelain crucible, or a crucible having a sponge platinum matmay be used. Certain solutions may attack the filtering vessel; for example, bothfritted glass and porcelain are affected by strongly alkaline solutions. Even plati-num sponge mats are attacked by hydrochloric acid unless the crucible is firstwashed with boiling water to remove oxygen.

Collection of Precipitates and Residues

In many of the tests, the amount of residue or precipitate may be so small as toescape easy detection. Therefore, the absence of a weighable residue or precipitatemust never be assumed. However small, it must be properly collected, washed,and dried or ignited. The size and type of the filter paper to be used is selectedaccording to the amount of precipitate to be collected, not the volume of solutionto be filtered.

Often when barium chloride is added in slight excess to precipitate bariumsulfate, the amount of precipitate produced is so small that it is hard to see andcollect. If a small amount of a suspension of ashless filter paper pulp is addedtoward the end of the period of digestion on the steam bath or hot plate, the floc-culation and collection of the sulfate are facilitated.

Some precipitates have a strong tendency to creep, while others, like mag-nesium ammonium phosphate, stick fast to the walls of the container. The use ofthe rubber-tip policeman is recommended, sometimes supplemented by asmall piece of ashless filter paper. The analyst should guard against the possibilityof any of the precipitate escaping beyond the upper rim of the filter paper.

Precipitates, especially if recently produced, may be partially dissolved unlessproper precautions are taken with both their formation and subsequent washing.In general, precipitates should not be washed with water alone but with watercontaining a small amount of common ion to decrease the solubility of the pre-cipitate. It is much better to wash with several small portions of washing solutionthan with fewer and larger portions. Calcium oxalate precipitates should bewashed with dilute (about 0.1%) ammonium oxalate solution; therefore, mixedprecipitates that may contain calcium oxalate and magnesium ammonium phos-phate should be washed with a 1% ammonia solution containing also 0.1%ammonium oxalate.

Gravimetric Method for Determining Small Amounts of Impurities 15 Reagent ChemicalsNinth Edition


Ignition of PrecipitatesThe proper conditions for obtaining the precipitate are given in the individual testdirections. The precipitate is collected on an ashless filter paper of suitable size andporosity (for example, a paper of 2.5-mm retention for a precipitate made up offine particles), the residue and paper are thoroughly washed with the proper solu-tion, and the moist filter paper is folded about the residue. The filter paper is thenplaced in a suitable tared, preconditioned crucible, and the paper is dried. The dry-ing may be done by using a 105 C oven, by heating on a hot plate, by using aninfrared lamp, or by carefully using a gas microburner in a covered crucible. Whendry, the paper is charred at the lowest possible temperature, the charred paper isdestroyed by gentle ignition, and the crucible is finally ignited at 800 25 C for 15min. The ignited crucible is cooled in a suitable desiccator and weighed.

Insoluble MatterThe intent of these tests is to determine the amount of insoluble foreign matter(for example, filter fibers and dust particles) present under test conditionsdesigned to dissolve completely the substance being tested. After dissolution andprior to filtration, it is a good practice to check visually (using a white back-ground) for insoluble foreign matter.

Procedure. Prepare a solution of the sample as specified in the individual testdirections. Unless otherwise specified, heat to boiling in a covered beaker, anddigest on a low-temperature (100 C) hot plate (or a steam bath) for 1 h. Filterthe hot solution through a suitable tared, medium-porosity (1015-mm) filteringcrucible. Unless otherwise specified, wash the beaker and filter thoroughly withhot water, dry at 105 C, cool in a desiccator, and weigh.

Residue after EvaporationThis test is designed to determine the amount of any higher-boiling impurities ornonvolatile dissolved material that may be present in a reagent chemical. It is usedchiefly in testing organic solvents and some acids.

The preferred container for the evaporation of almost all reagents is a plati-num dish. However, in many cases other containers (for example, dishes of porce-lain, silica, or aluminum) may be found suitable. In a few cases, such as whenstrong oxidants are evaporated, platinum should not be used.

A low-temperature (100 C) hot plate is usually specified in the tests in thisbook for evaporations. A steam bath may be used, if the steam is generated fromdeionized or distilled water in a properly maintained apparatus.

The residue is not dried to constant weight in this test because continuedheating may slowly volatilize some of the high-boiling impurities. The drying

16 Gravimetric Method for Determining Small Amounts of Impurities Reagent ChemicalsNinth Edition


conditions specified will, however, yield reproducible and reliable results so thatdrying to constant weight is not necessary.

Procedure. Place the quantity of reagent specified in the individual testdescription in a suitable tared container (see General Considerations, page 13).Evaporate the liquid gently so that boiling does not occur, and do so in a hood,protected from any possibility of contamination. Unless otherwise specified, drythe residue in an oven at 105 C for 30 min. Cool the container in a suitable desic-cator, and weigh. Calculate the percent residue from the weight of the residue andthe weight of the sample.

Loss on Ignition

This test is intended to determine the amount of volatile material due to occludedwater or process contamination. The procedure follows directives in the Residueafter Ignition test except that the addition of sulfuric acid is eliminated. Specificvariances are detailed in the individual monographs. Calculate the percentage ofmaterial lost on ignition.

Residue after Ignition

These tests are designed and intended to determine the amount of nonvolatileinorganic material that may be present in a reagent. They are applied to thoseinorganic reagents that can be sublimed or volatilized without decomposition;among these are reagents such as ammonium salts, mercury salts, and some inor-ganic acids. Various organic reagents are also tested against this requirement.The interpretation of the instruction to ignite has varied widely. After consider-able study of the various residues involved, a temperature of 800 25 C for 15min has been adopted. This should assure constant conditions for conversion ofthe residues to the desired composition without causing appreciable loss of theimpurities themselves. The procedure cited in the following paragraph is notinflexible. In particular, certain organic reagents are difficult to volatilize, andslight variations in the test have been suggested where appropriate. Reagents witha high water content should be dried before ignition to prevent loss of sample. Itis left to the discretion of the analyst as to the method of heating to be used, suchas a hot plate, gas microburner, or infrared lamp. The final ignition at highertemperature should be done under oxidizing conditions, preferably in a mufflefurnace.

Procedure. Ignite the quantity of reagent specified in the individual testdescription in a tared preconditioned crucible or dish in a well-ventilated hood,protected from air currents. Heating should be gentle and slow at first and should

Gravimetric Method for Determining Small Amounts of Impurities 17 Reagent ChemicalsNinth Edition


continue at a rate such that 1 to 2 h is required to volatilize inorganic samplescompletely or to char organic samples thoroughly. If the sample is a liquid, evap-orate completely by heating gently without boiling. Cool the crucible, andmoisten the residue with 0.5 mL of sulfuric acid, unless specified otherwise. Ignitethe crucible until white fumes of sulfur trioxide cease to evolve. Then, finally,ignite at 800 25 C for 15 min. Cool in a suitable desiccator, weigh, and calculatethe percentage of residue.

Loss on Drying

This test is designed to determine the amount of volatile material present in asample due to process contamination, inherent affinity for volatile impurities, ordesignated moles of hydration. The volatile content must be removable at rela-tively low temperatures.

Procedure. Accurately weigh the sample in a conditioned and tared vesselcompatible with the material being tested (see General Considerations, page 13).Unless otherwise noted, dry the material to constant weight in an oven at 105 C;care must be taken to avoid sample loss as a result of spattering or crystal frac-tionation. Cool sample in a desiccator and weigh. Calculate the percentage ofmaterial lost on drying.

Substances Not Precipitated by [...]

These procedures are designed and intended to determine those soluble impuri-ties that remain after the principal constituent has been removed by precipitation,as prescribed in the reagent specification.

In these tests, a solution of the reagent in question is treated with a particularprecipitating agent, the precipitate is removed by filtration, and a specified vol-ume of the filtrate is used for the determination. The aliquot portion is trans-ferred to a tared dish on a low-temperature (100 C) hot plate (or a steam bath),and the liquid is evaporated without boiling or spattering. A moist residue, oftenan ammonium salt, may remain. When volatilizing this residue, great care mustbe taken to avoid spattering, which may cause a loss of some of the residue. Afterthe residue has been volatilized, it is ignited as directed in the particular testdescription, cooled in a suitable desiccator, and weighed.

Weights of Precipitates and Residues

With few exceptions, the weight of the sample used in the test will ensure that atleast 0.001 g of the impurity to be determined will be present if the sample failsthe test.

trazas

Documents