Handbook Analytical Chemistry

Handbook Analytical Chemistry

(Parte 1 de 13)

SECTION 1

Table 1.6Concentrations of Solutions of H2SO4,NaOH,and CaCl2Giving Specified Vapor Pressures and Percent Humidities at 25°C1.16

Table 1.9Chemical Resistance of a Hygroscopic lon-Exchange Membrane1.20 1.5.8 Critical-Point Drying 1.20

Table 1.10Transitional and Intermediate Fluids for Critical-Point Drying1.21 1.5.9Karl Fischer Method for Moisture Measurement1.21 1.6THE ANALYTICAL BALANCE AND WEIGHTS1.2 1.6.1 Introduction 1.2

1.2SECTION ONE

1.1.1Handling the Sample in the Laboratory

Each sample should be completely identified, tagged, or labeled so that no question as to its origin or source can arise. Some of the information that may be on the sample is as follows:

1.The number of the sample. 2.The notebook experiment-identification number. 3.The date and time of day the sample was received.

Table 1.17Maximum Amounts of Combustible Material Recommended for Various Bombs1.36

Table 1.23General Properties of Filter Papers and Glass Microfibers1.4 Table 1.24Membrane Filters1.47 Table 1.25Membrane Selection Guide1.47 Table 1.26Hollow-Fiber Ultrafiltration Cartridge Selection Guide1.48 Table 1.27Porosities of Fritted Glassware1.49 Table 1.28Cleaning Solutions for Fritted Glassware1.49 1.8.3 Filtering Accessories 1.49 1.8.4Manipulations Associated with the Filtration Process1.50 1.8.5 Vacuum Filtration 1.51 1.9SPECIFICATIONS FOR VOLUMETRIC WARE1.52 1.9.1 Volumetric Flasks 1.52

Table 1.31Tolerances of Micropipettes (Eppendorf)1.53 1.9.4 Burettes 1.54 Table 1.32Burette Accuracy Tolerances1.54

A computerized laboratory data management system is the solution for these problems. Information as to samples expected, tests to be performed, people and instruments to be used, calculations to be performed, and results required are entered and stored directly in such a system. The raw experimental data from all tests can be collected by the computer automatically or can be entered manually. Status reports as to the tests completed, work in progress, priority work lists, statistical trends, and so on are always available automatically on schedule and on demand.

The sampling of the material that is to be analyzed is almost always a matter of importance, and not infrequently it is a more important operation than the analysis itself. The object is to get a representative sample for the determination that is to be made. This is not the place to enter into a discussion on the selection of the bulk sample from its original site, be it quarry, rock face, stockpile, production line, and so on. This problem has been outlined elsewhere.1–5In practice, one of the prime factors that tends to govern the bulk sampling method used is that of cost. It cannot be too strongly stressed that a determination is only as good as the sample preparation that precedes it. The gross sample of the lot being analyzed is supposed to be a miniature replica in composition and in particle-size distribution. If it does not truly represent the entire lot, all further work to reduce it to a suitable laboratory size and all laboratory procedures are a waste of time. The methods of sampling must necessarily vary considerably and are of all degrees of complexity.

No perfectly general treatment of the theory of sampling is possible. The technique of sampling varies according to the substance being analyzed and its physical characteristics. The methods of sampling commercially important materials are generally very well prescribed by various societies interested in the particular material involved, in particular, the factual material in the multivolume publications of the American Society for Testing Materials, now known simply as ASTM, its former acronym. These procedures are the result of extensive experience and exhaustive tests and are generally so definite as to leave little to individual judgment. Lacking a known method, the analyst can do pretty well by keeping in mind the general principles and the chief sources of trouble, as discussed subsequently. If moisture in the original material is to be determined, a separate sample must usually be taken.

1.1.2.1Basic Sampling Rules.No perfectly general treatment of the theory of sampling is possible. The technique of sampling varies according to the substance being analyzed and its physical characteristics. The methods of sampling commercially important materials are generally very well prescribed by various societies interested in the particular material involved: water and sewage by the American Public Health Association, metallurgical products, petroleum, and materials of construction by the ASTM, road building materials by the American Association of State Highway Officials, agricultural materials by the Association of Official Analytical Chemists (AOAC), and so on.

A large sample is usually obtained, which must then be reduced to a laboratory sample. The size of the sample must be adequate, depending upon what is being measured, the type of measurement being made, and the level of contaminants. Even starting with a well-gathered sample, errors can

PRELIMINARY OPERATIONS OF ANALYSIS1.3G. M. Brown, in Methods in Geochemistry, A. A. Smales and L. R. Wager, eds., Interscience, New York, 1960, p. 4.D. J. Ottley, Min. Miner. Eng.2:390 (1966).C. L. Wilson and D. W. Wilson, Comprehensive Analytical Chemistry, Elsevier, London, 1960; Vol. 1A, p. 36.C. A. Bicking, “Principles and Methods of Sampling,” Chap. 6, in Treatise on Analytical Chemistry, I. M. Kolthoff and P. J. Elving, eds., Part 1, Vol. 1, 2d ed., Wiley-Interscience, New York, 1978; p. 299–359.G. M. Brown, in Methods in Geochemistry, A. A. Smales and L. R. Wager, eds., Interscience, New York, 1960, p. 4.

occur in two distinct ways. First, errors in splitting the sample can result in bias with concentration of one or more of the components in either the laboratory sample or the discard material. Second, the process of attrition used in reducing particle sizes will almost certainly create contamination of the sample. By disregarding experimental errors, analytical results obtained from a sample of nitems will be distributed about m with a standard devitation

In general, sandm are not known, but scan be used as an estimate of s, and the average of analytical results as an estimate of m.The number of samples is made as small as compatible with the desired accuracy.

If a standard deviation of 0.5% is assigned as a goal for the sampling process, and data obtained from previous manufacturing lots indicate a value for sthat is 2.0%, then the latter serves as an estimate of s. By substituting in Eq. (1.1), andn=16, number of samples that should be selected in a random manner from the total sample submitted.

To include the effect of analytical error on the sampling problem requires the use of variances.

The variance of the analysis is added to the variance of the sampling step. Assuming that the analytical method has a standard deviation of 1.0%, then

where the numerator represents the variance of the sampling step plus the variance of the analysis. Thus

andn=20, the number of samples required. The above discussion is a rather simple treatment of the problem of sampling.

1.1.2.2Sampling Gases.6Instruments today are uniquely qualified or disqualified by the Environmental Protection Agency. For a large number of chemical species there are as yet no approved methods.

Thesize of the gross sample required for gases can be relatively small because any inhomogeneity occurs at the molecular level. Relatively small samples contain tremendous quantities of molecules. The major problem is that the sample must be representative of the entire lot. This requires the taking of samples with a “sample thief” at various locations of the lot, and then combining the various samples into one gross sample.

Gas samples are collected in tubes [250 to 1000 milliliter (mL) capacity] that have stopcocks at both ends. The tubes are either evacuated or filled with water, or a syringe bulb attachment may be used to displace the air in the bottle by the sample. For sampling by the static method, the sampling bottle is evacuated and then filled with the gas from the source being sampled, perhaps a cylinder. These steps arerepeated a number of times to obtain the desired sampling accuracy. For sampling by the dynamic method, the gas is allowed to flow through the sampling container at a slow, steady rate. The container is flushed out and the gas reaches equilibrium with the walls of the sampling lines and container with respect to moisture. When equilibrium has been reached, the stopcocks on the sampling container are s n s n s

1.4SECTION ONEJ. P. Lodge, Jr., ed., Methods of Air Sampling and Analysis, 3d ed., Lewis, Chelsea, Michigan, 1989. Manual of methods adopted by an intersociety committee.

closed—the exit end first followed by the entrance end. The sampling of flowing gases must be made by a device that will give the correct proportion of the gases in each annular increment.

Glass containers are excellent for inert gases such as oxygen, nitrogen, methane, carbon monoxide, and carbon dioxide. Stainless-steel containers and plastic bags are also suitable for the collection of inert gases. Entry into the bags is by a fitting seated in and connected to the bag to form an integral part of the bag. Reactive gases, such as hydrogen sulfide, oxides of nitrogen, and sulfur dioxide, are not recommended for direct collection and storage. However, TedlarTMbags are especially resistant to wall losses for many reactive gases.

In most cases of atmospheric sampling, large volumes of air are passed through the sampling apparatus. Solids are removed by filters; liquids and gases are either adsorbed or reacted with liquids or solids in the sampling apparatus. A flowmeter or other device determines the total volume of air that is represented by the collected sample. A manual pump that delivers a definite volume of air with each stroke is used in some sampling devices.

1.1.2.3Sampling Liquids.For bottle samplinga suitable glass bottle of about 1-L capacity, with a 1.9-centimeter (cm) opening fitted with a stopper, is suspended by clean cotton twine and weighted with a 560-gram (g) lead or steel weight. The stopper is fitted with another length of twine. At the appropriate level or position, the stopper is removed with a sharp jerk and the bottle permitted to fill completely before raising. A cap is applied to the sample bottle after the sample is withdrawn.

Inthief samplinga thief of proprietary design is used to obtain samples from within about 1.25 cm of the tank bottom. When a projecting stem strikes the bottom, the thief opens and the sample enters at the bottom of the unit and air is expelled from the top. The valves close automatically as the thief is withdrawn. A core thiefis lowered to the bottom with valves open to allow flushing of the interior. The valves shut as the thief hits the tank bottom.

When liquids are pumped through pipes, a number of samples can be collected at various times and combined to provide the gross sample. Care should be taken that the samples represent a constant fraction of the total amount pumped and that all portions of the pumped liquid are sampled.

Liquid solutions can be sampled relatively easily provided that the material can be mixed thoroughly by means of agitators or mixing paddles. Homogeneity should never be assumed. After adequate mixing, samples can be taken from the top and bottom and combined into one sample that is thoroughly mixed again; from this the final sample is taken for analysis.

For sampling liquids in drums, carboys, or bottles, an open-ended tube of sufficient length to reach within 3 m of the bottom of the container and of sufficient diameter to contain from 0.5 to 1.0 L may be used. For separate samples at selected levels, insert the tube with a thumb over the top end until the desired level is reached. The top hole is covered with a thumb upon withdrawing the tube. Alternatively the sample may be pumped into a sample container.

Specially designed sampling syringes are used to sample microquantities of air-sensitive materials. For suspended solids, zone sampling is very important. A proprietary zone sampler is advantageous. When liquids are pumped through pipes, a number of samples can be collected at various times and combined to provide the gross sample. Take care that the samples represent a constant fraction of the total amount pumped and that all portions of the pumped liquid are sampled.

1.1.2.4Sampling Compact Solids.In sampling solids particle size introduces a variable. The size/weight ratio bcan be used as a criterion of sample size. This ratio is expressed as

A value of 0.2 is suggested for b; however, for economy and accuracy in sampling, the value of b should be determined by experiment.

The task of obtaining a representative sample from nonhomogeneous solids requires that one proceeds as follows. A gross sample is taken. The gross sample must be at least 1000 pounds (lb) if the pieces are greater than 1 inch (in) (2.54 cm), and must be subdivided to 0.75 in (1.90 cm) before reduction to 500 lb (227 kg), to 0.5 in (1.27 cm) before reduction to 250 lb (113 kg), and so on, down weight of sample

PRELIMINARY OPERATIONS OF ANALYSIS1.5 to the 15-lb (6.8-kg) sample, which is sent to the laboratory. Mechanical sampling machines are used extensively because they are more accurate and faster than hand-sampling methods described below. One type removes part of a moving steam of material all of the time. A second type diverts all of stream of material at regular intervals.

For natural deposits or semisoft solids in barrels, cases, bags, or cake form, an auger sampler of post-hole digger is turned into the material and then pulled straight out. Core drilling is done with special equipment; the driving head should be of hardened steel and the barrel should be at least 46 cm long. Diamond drilling is the most effective way to take trivial samples of large rock masses.

For bales, boxes, and similar containers, a split-tube thief is used. The thief is a tube with a slot running the entire length of the tube and sharpened to a cutting edge. The tube is inserted into the center of the container with sufficient rotation to cut a core of the material.

For sampling from conveyors or chutes, a hand scoop is used to take a cross-sectional sample of material while in motion. A gravity-flow auger consists of a rotating slotted tube in a flowing mass. The material is carried out of the tube by a worm screw.

1.1.2.5Sampling Metals.Metals can be sampled by drilling the piece to be sampled at regular intervals from all sides, being certain that each drill hole extends beyond the halfway point. Additional samples can be obtained by sawing through the metal and collecting the “sawdust.” Surface chips alone will not be representative of the entire mass of a metallic material because of differences in the melting points of the constituents. This operation should be carried out dry whenever possible. If lubrication is necessary, wash the sample carefully with benzene and ether to remove oil and grease.

For molten metals the sample is withdrawn into a glass holder by a sample gun. When the sample cools, the glass is broken to obtain the sample. In another design the sampler is constructed of two concentric slotted brass tubes that are inserted into a molten or powdered mass. The outer tube is rotated to secure a representative solid core.

1.2MIXING AND REDUCTION OF SAMPLE VOLUME

1.2.1 Introduction

The sample is first crushed to a reasonable size and a portion is taken by quartering or similar procedures. The selected portion is then crushed to a somewhat smaller size and again divided. The operations are repeated until a sample is obtained that is large enough for the analyses to be made but not so large as to cause needless work in its final preparation. This final portion must be crushed to a size that will minimize errors in sampling at the balance yet is fine enough for the dissolution method that is contemplated.

Every individual sample presents different problems in regard to splitting the sample and grinding or crushing the sample. If the sample is homogeneous and hard, the splitting procedure will present no problems but grinding will be difficult. If the sample is heterogeneous and soft, grinding will be easy but care will be required in splitting. When the sample is heterogeneous both in composition and hardness, the interactions between the problems of splitting and grinding can be formidable.

(Parte 1 de 13)

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