letters The Reaction of Copper(l1)with lhiocyanate Ions
Identifying Plastics by Density To the Editor: Kenneth E. Kolb and Doris K. Kolb [1991,68,3481present a method based on density for separating and identifying plastics with specific reference to the problem of recycling. The Society of the Plastics Industry has adopted a voluntary coding system for plastic containers that identifies the plastic in many cases thus simplifying the identification. The code consists of a number, 1through 7, and the recycle symbol (a broken triangle of arrows).
Arecent article by R. J. Tykodi (1)provides us with a lot of interesting and useful descriptive chemistry However, one of the reactions described, namely the reaction of Cu2+(aq)with SCN-(aq), deserves a further comment. Tykodi states that CU(NCS)~ is unstable and decomposes quickly to CuSCN according to reaction 1
Plastic
Code number 1
Polyethylene terephthalate (PETor PETE)
2
High density polyethylene (HDPE)
3 4
Polyvinyl chloride (PVC)Ninyl(V) Low density polyethylene (LDPE)
5
Polypropylene (PP)
6
Polystyrene (PS)
7
Mixed plasticpnot generally recyclable (MIXED) or (OTHER)
A quick check of the shelves of a local mini-market revealed that about 75% of the plastic containers were coded by ,umber and most of these also were identified by the initials given in parenthesis. The most common plastic found was high density polyethylene. Alfred S. Levinson Portland State University Portland, OR 972074751
To the Editor: We are quite aware of the fact that many plastic objects (bottles, cups, etc.) are now being stamped with code numbers for recycle purposes. However, this practice hardly eliminates the usefulness of methods for identifying unknown plastic materials. In the fust place, there are still many plastic products that are not being stamped with recycle codes. Furthermore, plastic pellets, rods, and sheets do not have identifling code numbers. Nor do the older plastic objects lying around the house or garage or laboratory. Three of the eight plastics that we mentioned are not among the high volume six and, therefore, do not even have assigned code numbers. One of the points made in the article is that density can be used as a basis for separating plastic materials as well as identifying them. Kenneth E. Kolb Doris K. Kolb Bradley University Peoria, IL 61625 174
To the Editor:
Journal of Chemical Education
Nevertheless, it has been reported that Cu(NCSX can be obtained as a black or dark-brown solid by adding thiocyanate solution to copper(I1) salt solution (2, 3).The precipitate decomposes on standing to the white cuprous salt, the reduction being accelerated by heating the solution (2). Dry CU(NCS)~, on the other hand, is relatively stable and can be stored for several months without substantial changes (3).In order to select the proper conditions for a demonstration, I have mixed 1M solutions of CuS04 and KSCN. At room temperature, a black precipitate of CU(NCS)~ appeared inmediately, and reduction to CuSCN was so slow that most of the solid remained unchanged aRer standing at room temperature for 24 h. When the reaction mixture was heated in a bath of boiling water, the black precipitate inmediately transformed to white CuSCN with vigorous gas evolution. The product isolated from reaction 1is not only temperature- but also concentration-dependent. When 0.25 M solutions of CuSOl and KSCN were mixed, no Cu(SCN)z, but, rather, white CuSCN, precipitated after a few minutes at room temperature. Another interesting aspect of reaction 1is the nature of the gas evolved. Unlike the highly toxic cyanogen, formed by reaction of Cu2+(aq)with CN-(aq), thiocyanogeu is not a gas under ordinary conditions. Colorless crystals of (SCNh melt below 0 'C and polymerize readily to give red solid parathiocyanogen (3, 4). Aqueous solutions of thiocyanogen decompose according to reaction 2 (3): 3(SCNI2+ 4H20+ SHNCS + H2S04+ HCN
(2)
Therefore, the gas evolved by reaction of Cu2+(aq!with SCN-(aq) is most probably HCN and the proper precautions should be taken when performing the experiment. Literature Cited 1. hkodi,R. J. J. Chen. Edm. 1881,68.106109. 2. Maasey,A.G. InComprehPnsil~InorganiiC&miiny;Tmtman-I)I&-m,A Pergamon:Ckfad, 1973:Vol. 3, Chapter 27.
F.:Ed.,
3. Co1ub.A M.; Kohler, H.;Skopenko, V. V Che-hy ofPeu& holide@;Elaevier:Amsterdam.. 1986:. Cha~ter 5. . 4. Halliday, A. K; Hughes, G ; Walker, 8. M. In Campmhendw Inorganic Chamistry: Potman-Dick-ensrm, A. F., Ed.; Pergamon:Oxford, 1973;Vol 1,Chapter 13.
Davld Tudela ~epartarnento de Quimica (C-VIM) Universidad Aut6noma de Madrid 28049-Madrid,Spain
