Section: 8 | Standards for Laboratory Weights |

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STANDARDS FOR LABORATORY WEIGHTS

Thomas J. Bruno and Paris D. N. Svoronos

Requirements and Applications

The following tables provide a summary of the requirements for metric weights and mass standards commonly used in chemical analysis (Refs. 1–3). The actual specifications are under the jurisdiction of ASTM Committee E-41 on General Laboratory Apparatus and are the direct responsibility of subcommittee E-41.06 that deals with weighing devices. These standards do not generally refer to instruments used in commerce. Weights are classified according to Type (Design), either Type I or Type II (Table 1); Grade (Physical Property) S, O, P, or Q (Table 2); and Tolerance (Deviation) 1, 2, 3, 4, 5, or 6 (Table 3). Details about each classification are included in the tables.

Information on the applications for various mass standards (Table 4) is presented to allow the user to make appropriate choices when using analytical weights for the calibration of electronic analytical balances, for making large-scale mass measurements (such as those involving gas cylinders), and in the use of dead-weight pressure balances. Some historical context can be found in Ref. 4.

Weight and Mass

The terms weight and mass are often used interchangeably, which is acceptable on the Earth’s surface. Mass is the amount of substance (an inertial property whose value is equal to the sum of the mass of all electrons, protons, and neutrons) in an object. Weight is the gravitational interaction between objects that have mass; on the Earth’s surface, that is the gravitational interaction between the object and the Earth. A (stationary) electronic balance measures the gravitational force of an object (i.e., its weight) by use of a load cell and converts the measured weight to mass using the relation weight (W) = mass (m) × gravitation (g), W = mg, where g is roughly 9.81 N kg^-1.

Some highly precise measurements require the measurement of g locally to obtain a correction factor due to gravitational variation on the Earth’s surface. For most measurements in analytical chemistry, however, the conversion used by electronic balances are adequate. Buoyancy corrections were commonly required when two pan analytical balances were in common use. For electronic balances, this consideration is typically required only for a calibration.

References

Annual Book of ASTM Standards, ANSI/ASTM E617-18 Standard Specification for Laboratory Weights and Precision Mass Standards, Book of Standards Vol. 14.04, 2021.
Battino, R., and Williamson, A. G., J. Chem. Educ. 61, 51, 1984. [https://doi.org/10.1021/ed061p51]
Bruno, T. J., and Svoronos, P. D. N., CRC Handbook of Basic Tables for Chemical Analysis – Data-Driven Methods and Interpretation, Fourth Edition, CRC Press/Taylor and Francis, Boca Raton, FL, 2021. [https://doi.org/10.1201/b22281]
NBS Circular 547, Precision Laboratory Standards of Mass and Laboratory Weights, Washington, DC, 1954 (reprinted as NIST-IR 78-1476, Washington, DC, 1978)

TABLE 1. Type — Classification by Design

Type I	One-piece construction; contains no added adjusting material; used for highest accuracy work
Type II	Can be of any appropriate and convenient design, incorporating plugs, knobs, rings, etc.; adjusting material can be added if it is contained so that it cannot become separated from the weight

TABLE 2. Grade — Classification by Physical Property

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Table 1. Grade — Classification by Physical Property


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Grade S	Density:	7.7 g cm^–3 to 8.1 g cm^–3 (for 50 mg and larger)
	Surface area:	not to exceed that of a cylinder of equal height and diameter
	Surface finish:	highly polished
	Surface protection:	none permitted
	Magnetic properties:	no more magnetic than 300 series stainless steels
	Corrosion resistance:	same as 303 stainless steel
	Hardness:	at least as hard as brass
Grade O	Density:	7.7 g cm^–3 to 9.1 g cm^–3 (for 1 g and larger)
	Surface area:	same as grade S
	Surface finish:	same as grade S
	Surface protection:	may be plated with suitable material such as platinum or rhodium
	Magnetic properties:	same as grade S
	Corrosion resistance:	same as grade S
	Hardness:	at least as hard as brass when coated; smaller weights at least as hard as aluminum
Grade P	Density:	7.2 g cm^–3 to 10 g cm^–3 (for 1 g or larger)
	Surface area:	no restriction
	Surface finish:	smooth, no irregularities
	Surface protection:	may be plated or lacquered
	Magnetic properties:	same as grades S and O

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STANDARDS FOR LABORATORY WEIGHTS

Requirements and Applications

Weight and Mass

References

TABLE 1. Type — Classification by Design

TABLE 2. Grade — Classification by Physical Property

Table 1. Grade — Classification by Physical Property

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