In the Laboratory
Properties of Zeolite A Obtained from Powdered Laundry Detergent An Undergraduate Experiment Alison L. Smoot and David A. Lindquist Department of Chemistry, University of Arkansas at Little Rock, Little Rock, AR 72204 Until recently, sodium tripolyphosphate (Na5P 3O10) was the preferred builder in laundry detergents. Since phosphates stimulate growth of algae in wastewater they have been replaced in most laundry detergents by the sodium form of zeolite A (Na-A) (1). Zeolite A acts as a water softener by cation exchange of its sodium ions for calcium and magnesium ions. The utility of zeolites extends far beyond water softening. Zeolites are used in immense quantities as catalysts for petroleum cracking and isomerization reactions (2). Other applications include their use as desiccants, selective adsorbents, and ion exchange materials. In this experiment students are introduced to the myriad properties of zeolites using Na-A from detergent. The appeal of this experiment is the study of a useful and versatile material obtained from a common household product. As an extension to the experiments described here one may synthesize Na-A to compare its properties with those of the commercial material (3). Na-A has a high ion exchange capacity because it is an aluminum-rich zeolite containing approximately equal numbers of aluminum, silicon, and sodium atoms with an empirical formula of NaAlSiO4 . The composition of zeolite A can be varied considerably to form silicon-rich material by using lesser quantities of aluminum during synthesis. Silicon-rich compositions have lower ion exchange capacity, so they are not used in detergents. The framework of zeolites is based on a simple motif of tetrahedrally coordinated aluminum and silicon atoms connected by bridging two coordinate oxygen atoms. Figure 1 is an illustration of the cubic unit cell of zeolite A (4). Each vertex in the figure represents an aluminum or silicon atom and each line represents an Al–O–Si or Si–O–Si linkage. There are several types of zeolites, but all have the common feature of a network of internal pores. The size and geometry of the pores in a specific zeolite are defined by the crystal structure. The diameter of the eight-member ring entrance Figure 1. Zeolite A unit cell. to the pores in zeolite A is 4.1 Å (2). The zeolite framework is neutral in charge if composed solely of silicon and oxygen. However, the zeolite framework adopts one negative charge for each aluminum incorporated into the structure, since aluminum has a formal charge of 3+ whereas silicon is 4+. Additional cations are needed to accommodate the increasing charge with increasing aluminum content. In the case of Na-A, these additional cations are sodium ions residing in the pores. Sodium ions can be readily exchanged for other cations small enough to fit into the pores (i.e.,
