admin January 9, 2019 No Comments

All solid or liquid laboratory chemicals necessarily adsorb and possibly absorb and react with the components of the atmosphere inside the container, which is exchanged for the external atmosphere upon any withdrawal of the product, unless suitable procedures are adopted. Since interactions of the chemicals with atmospheric components generally degrade their quality, this paper explains how to minimize this loss and preserve quality. Part 1 considers the theoretical aspects of the topic, and Part 2 will examine its practical facets.

Introduction

A chemical (short for “chemical compound,” “chemical substance” or “pure substance) is characterized by composition and propertiesoften available from the literature that do not change with time. However, this is the concept, or virtual, or symbolic chemical, represented by a single formula (generally conventional) that allows the symbolic handling of the chemical, like writing possible chemical reactions and carrying out the relative zero-order stoichiometric calculations.

Image result for lab chemicals

The real chemical product (i.e., chemical or product) is different because 1) its composition, generally shown on the label of the container, only approximates that of its conventional formula, since it necessarily contains more or less small fractions of identified (such information is labeled “chemical speciation”) or unidentified diverse compounds; 2) it has time-dependent properties due to the fact that it is subjected to several time-dependent interactions, both of the field and contact type. Even the most important properties—elemental and molecular composition—may change from those shown on the long-lasting container label, which thus can potentially provide misleading indications. Worse, composition changes, if unnoticed, may be dangerous, and can result in errors and increased dispersion of data from related measurements.

A chemical is designated as “degraded” when the values of the properties of interest differ from those acceptable for the programmed use. In many cases, the concentration of particular substances must be below critical, very low, limits, attainable in solids by adsorption (or physisorption) only.

The successful use of a chemical requires knowledge of its current composition to establish whether it is compatible with the intended operation. To avoid the time-consuming and potentially useless analysis of each chemical prior to each use, it is possible to circumvent or adjust, sometimes drastically, some of the interactions listed above, and update the label to show reliable information.

Laboratory chemicals

The time dependence of the composition of ordinary, liquid or solid laboratory chemicals, packed in a container with impermeable walls and reversible closure, is taken into account by defining for each a characteristic life cycle that can span several orders of magnitude, from days to indefinite. This consists of 1) a storage time, running from the purchase or preparation to the first opening of the container to use the chemical; 2) a service time, from first opening of the container to end of use of the contents and 3) postuse time.

Quality preservation in the former interval rests mainly on the expertise of the vendor, sometimes adjustable on request, for instance, of packing under inert gas. The normally limited usefulness of chemicals in time led to their classification in terms of shelf life, which (under appropriate conditions) can be excellent (>5 years), good (3–5 years), fair (1–3 years), poor (<1 year) and the consequent expiration date.13

During the service period, parts of the contents are taken out several times, using procedures that are dependent on the sensitivity of the product to the atmosphere. Here we concentrate on the common handling of the product in the atmosphere, referring to published information for the handling of the most sensitive, frequently very dangerous, products.

Presently an enduring protective medium that permits (trained) handling in the atmosphere is provided by vendors only in the cases of substances that are very sensitive to atmospheric components. Examples are Li, Na, K (kept under mineral oil) and yellow P (kept under water). To reduce the introduction of atmospheric moisture, but not oxygen, containers may be kept in dessiccators. Recently a proprietary packing claimed similar results. The concentration of water is kept low by the addition of insoluble dessiccants such as an appropriate molecular sieve sometimes enclosed in a gas-permeable container attached to the cap, known as a water trap.

The effects of atmospheric oxygen are generally kept to a minimum by the addition of stabilizers (hydroquinone, H3PO2, 2,6-di-t-butyl-4 methyl phenol or BTU). Contamination by atmospheric components is prevented in closed, dedicated systems for high-purity liquids. However, the only published device and procedure usable for opening and closing the container of liquids in the atmosphere that is very sensitive to atmospheric components. For normal chemicals, we found that simpler practices, indicated below, meet most needs, provided adequate attention is given to the cap and the volume of the containers.

The “postuse time” basically consists of checking and/or purifying for a new storage period, or disposing of the remaining product. In the laboratory, this phase can easily be minimized.

Analysis and model of the system during the service stage

During the service stage, most of the time the substances rest on the lab shelf (idle time, 1 month‒5 years) and their container is repeatedly but briefly opened to withdraw part of the contents (working time, e.g., 20 times for 30 seconds each).

In the cumulative idle time, the chemical is enclosed in a container provided with a temporary gas-tight port with impermeable, rigid and thermally conducting walls and subjected to both contact and field interactions. The contact interactions take place with stabilizers, inner atmosphere, container walls, inlet/outlet port and cosmic rays. The field interactions occur with gravity, electric and magnetic fields from the environment or the chemical itself, and from the scalar temperature and pressure fields from the surrounding atmosphere. With the exception of thermostated spaces (e.g., within refrigerators) lab temperature and pressure change during the day/night and season sequences, providing a continuous drive toward cyclic equilibrium conditions.

In addition to the easily avoidable photochemical effects of UV-VIS radiation and the unavoidable gravitational setting, the relevant processes involving the chemical are:

  • Adsorption (involving mainly surfaces, inclusive of pores)
  • Absorption (involving the bulk of the material, controlled by diffusion)
  • Mixing
  • When possible, reactions of the substance with the components of the atmosphere (or gas phase) internal to the container.

Most of these processes appear to have a characteristic time that is short with respect to the idle time. Thus, the approximate description of the product as a thermodynamic closed system consisting of a chemical and a gas phase internal to the container should be a useful guide.

Any chemical in a condensed phase contains as adsorbates, solutes or reaction products the components of the internal atmosphere, because their initially zero concentration in the substance implies (in the usual absence of impermeable or semipermeable boundaries) a very high propensity (initial ∆mix – ∞) to reach a finite concentration.

In the working phases, the normally closed and (assumed) gas-tight closure of the container (its most problematic part; see next section) is temporarily opened to remove a portion of the chemical. During such operation the system becomes an open system, exchanging intentionally part of the chemical and, unintentionally, by pressure difference, gravity and diffusion, more or less of the internal atmosphere with the external atmosphere, which contains the necessarily soluble and reactive components oxygen, water and carbon dioxide. If the chemical is usually removed by pouring, each use will replace the entire (heavier or lighter) internal atmosphere plus the volume of the chemical removed, with the laboratory atmosphere.

Old, leaky caps and especially pouring are the main causes of emission into the lab atmosphere, leading to alteration of solid or liquid chemicals. The pervasive presence of pouring rings and dripless pour lips on the bottlenecks of the containers is detrimental. The oxygen entering at each withdrawal produces aldehydes or ketones from alcohols, explosive peroxides from ketones, ethers and olefines; phosgene from chloroform and oxides on the surface of metals. Water reacts with many compounds, inorganic—such as hydrides: LiH, NaH; oxides: Na2O, K2O, P4O10; carbides: CaC2, Al4C3; sulfides: Al2S3, P2S3; halides: AlCl3, SnCl4, TiCl4); AlP; organometallics such as carbonyls and organic substances such as anhydrides, acyl halides, orthoesters and esters (with the evolution of CO2 in the case of di-tert-butyldicarbonate), and must be substantially absent in many chemical procedures. Carbon dioxide, especially in the presence of water vapor, reacts with most basic substances giving various carbonates. Even very stable compounds like squaric acid show surface discoloration unless the atmosphere of the container is made inert; “activated” zinc deactivates in contact with air. In thermodynamic terms, such openings introduce uncontrolled composition variables, sometimes called chemical noise.

A recent paper analyzes in detail the changes of phase diagrams of pure substances in the presence of air, but considers air as a single component rather than a mixture of variable composition, and also inert, that is, insoluble and unreactive toward the substance of interest. Such results support the present goals, but are not useful to their attainment.

References/notes

  1. IUPAC’s Gold Book (1997). Online corrected version 2006–; accessed May 30, 2014.
  2. Meaning “relevant properties” and referring to given environmental conditions. Relevant properties are those needed for the selective specification of thermodynamics systems opposed to the exhaustive specification of mechanical systems). Density is a relevant property; the distance from the moon is not.
  3. Most chemicals are ensembles of several microscopic entities, some of which differ from the analytical formula. For instance, liquid acetic acid contains molecules CH3COOH (CH3COOH)2, ions CH3COO and CH3COOH2+in concentrations dependent on temperature.
  4. http://www.cmu.edu/homepage/environment/2011/fall/chapter_04.pdf ; accessed 11.06.2014.
  5. Current literature on “time sensitive chemicals” seems to neglect the effects of time on reactions and measurements, limiting to the effects on safety; see, for example http://education.qld.gov.au/health/pdfs/healthsafety/time-sensitive-chemicals.pdf; accessed 27.01.2015.
  6. As acquired from research on adducts in solution, see, for instance, Lunelli, B.; Francesconi, R. et al. Faraday Trans., 199793, 2527‒32.
  7. The most common is water, as evident, for example, from the commercial availability of anhydrous-grade solvents.
  8. Even the moisture adsorbed on borosilicate glass must be eliminated when using air-sensitive chemicals, see, for example, Aldrich Technical Bulletin AL-134 (Handling Air-Sensitive Reagents).
  9. Gases do not interact with the atmosphere internal to the container because they occupy all the available volume.
  10. http://clark.cleanharbors.com/tt/sl.ashx?z=776bb3d6&dataid=1605.

Posted by Oluwakemi Adi

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08060874724.