filtrates a residues
JAMES O. SCHRECK
University of Northern Colorado Gresley,CO 80639
Gravimetric-Gasometric Determination of Zinc on Galvanized Nails Albert W. Burgstahler The University of Kansas, Lawrence, KS 66045 A common stoichiometric exercise in general chemistry is the calculation of the mass of a n active metal, such a s zinc, magnesium, or aluminum, from reactions involving dissolution of the metal in aqueous acid. Recently, a laboratory experiment of this type for determining the mass of the zinc coating on small pieces of galvanized sheet metal or other objects by either a direct mavimetric or a n EDTA titration was described& these pages.' As part of a n effort to develop rapid low-cost chemistry experiments for use in schools with small science budgets, we have devised a simple acid-dissolution method for a combined gravimetric-gasometrie determination of the amount of the zinc metal coating on hoedipped galvanized nails by the reaction: Galvanized nails are readily available from building supply and hardware stores and are widely used because of their resistance to corrosion. The experiment presented here requires only a few minutes to perform, and it gives students a n opportunity to compare results from direct weighings with those obtained from measurements of gas volumes. Procedure When weighing i s possible only to the nearest 0.1 g, about 9-10 e of ~alvanizednails of any convenient size and type arc weighed and placed flat in thb hottorn of a narrowneck flask fitted with a gas-tiaht one-hole stopper containor rigid plastic tubing. (Alternaing a short piece of tively, a filtering flask and a solid stopper can be used.) A 500-mL graduated cylinder filled to the brim with water a t roam temperature is capped tightly with the palm of one hand and quickly inverted into water a t the same temperature i n a deep trough, a large beaker, a small pail, or other suitable container.' To the glass (or plastic) tube in the one-hole stopper (or to the side arm of the filtering
Figure 1. Apparatus assembly for collecting and measuring the volume of Hz in an inverted graduated cylinder.
flask) is attached a short piece of flexible tubing connected to a small U-tube inserted into the open end of the inverted maduated cylinder (see Fig. 1). - sufficient-6 M aqueous H C is ~ added to cover the nails, the stoooer is auicklv inserted. and the raoidlv evolved hvdepedding , on the akount of z i k drogen'(240 t i 5 0 0 6 ~ oresent) is collected bv disolacement of water in the inberted cylinder:' K A U ~ O NHydrogen : is flammable and hiehlv cxolosive in air. This ex~erimentshould be cond&&ed with adequate vent%ation and proper eye protection and with no flames or sparks nearby.) When gas evolution is practically complete (3-5 m i d , the volume of Hzis measured after adjusting the position of the eraduated cvlinder so that the water level in it is the same a s in the trough. (If necessary, water may be added to the t r o u ~ h . The ) molar auantitv of H?. which is theoretically equarG that of the zinc, is c"alcu1ated using the ideal gas law aRer subtracting the vapor pressure of the water from the barometric (atmospheric) p r e s ~ u r eThe . ~ solution
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'Gillum and Herrmann (1). Note that the calculation by these authors gives the total mass of zinc per square centimeter (g/cmZ)on both the top and bonom surfacesof the galvanized sheet metal. For a single surface the result must be divided by two. 21f a graduated cylinder is not available, a comparable, uncalibrated, large-bore, transparent cylinder closed at one end can be used instead. The final position of the water level after collecting the hydrogen is marked on the side of the cylinder, and the volume of H, is determined afterward by measuring the amount of water required to refill to this level with the cylinder in an upright position. An eauallv satisfacto~.alternative method for determininothe vol5 to meas~rethe amom1 of water asp aced fro&a f as< ume of Into a bea6er (see F g 21 as aescr beo n many general chemtstry Prlor to laboratory man.als for determm ng tne mo ar voldme of 02. the reaction water is blown from the flask into the beakerto fill the delivery tube completely with water. With the pinch clamp still open the beaker is raised so that the water levels in the beaker and the flask are equal. The pinch clamp is then closed and the beaker emptied. At the start of the reaction. and at the same time acid is added to me na~lsand the stopper nserleo, the p ncn camp 1s opened wltn tne end of 1ne del very 1Loe a1 the same eve1 as me top of me water n tne f ask (St~oents w flnd I easler to work n pars for tnese operations.)At the end of the reaction the water levels are again equilibrated, the pinch clamp is closed, and the volume of water displaced by Hz into the beaker is measured. 3When the barometric pressure is below 725 torr, as in locations at hiaher altitudes. the auantitv of nails. if the zinc content is hiah. mav nave to oe reaied bj abolt 10 5. to avo 0 generating more;; 'ha; the cal bratw capaclty of the cyl noer. In the aosence of a mercJry barometer, stanoaro taoes can oe Jsea to corren readily available sea level-adjusted aneroid or local weather sewice "barometricpressure" readings to the unreduced barometric pressure needed for this experiment. Values for making these corrections (here in an inverse sense) for elevation. latitude. and outside temoerature are found. for examble, in Lange (4.
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is then decanted and discarded, and the nails are rinsed with water, wiped dry, and reweighed. In a typical experiment, 9.8 g of galvanized nails furnished 0.325 L of Hz measured at 745 tom and 21 "C (294 K). From handbook tables the vapor pressure of water at 21 O C is 19 ton: Then, according to the ideal gas law: pv nH =-=
[w]
atm [ y ) L = 0.0129 mol = nz,
mol .K The calculated mass of zinc dissolved is therefore: gz,, = (0.0129 mo1)(65.39 g/mol) = 0.84 g, or 8.6% of the original total mass of nails. In this experiment, the nails after removal of the zinc weighed 8.9 g, corresponding to a directly determined mass loss of 0.9 glor 9%.5For most nails the mass loss is 7-13% of the original mass of the nails and, if the experiment is done carefully, is very close to that calculated from the volume of hydrogen collected, thus indicating that the amount of oxide or other nonreducing material in the zinc coating is trivial. Additional mass loss by further contact of thenails with the acid for another 5 min is negligible and amounts at most to only a few milligrams per nail6 Ifweighing can be done to the nearest 0.01-0.001 g, then the scale of the experiment can be reduced. In this case, two threepenny (3d) galvanized box nails (total mass = 1.80-2.00 g) are placed in a 20 x 150-mm (or similar size) test tube (or sidearm test tube) instead of a flask. ARer addition of acid. the hvdroeen (50-100 mL) is collected in an inverted, water-filled 1 6 0 - m ~ graduated ~ylindef?.~ (or a eas-measuring ., buret. if available). and the rest of the experiment is completfd'as described abnve. In dace of 6 M HCI. 3 M sulfuric acid eives eouallv satisfactory results. sliihtly more dilute & dsi aiso can he used, but the reaction is slower. Nitric acid cannot be used,
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4With water already saturated with displaceable air, the slight solubility of Hp in water (1.91 mU100 g at 298 K and 1 atm) can be ne-
olected. " - - -~
5~ fh the more accurate weighings actually used nere, this mass aifferencewas 9 78 g - 8.93 g = 0.85 g, or 8.7%of the original mass
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6Atho~gn a small amomt (0.14.2 g) of antimony potassium tartrate (lanar emetic) or otner source ot ~onicantlmony can oe added to the reaction to suppress attadn by acid on the exposed iron, this expedient is neither necessary nor recommended. Highly toxic stib ine gas (SbH,, detectable by the Marsh mirror test) is liberated into the exiting hydrogen, and an annoying thin black deposit of metallic antimony must then be wiped off the nails before they are weighed. 'The author is grateful to Clark E. Bricker for advice and enwuragement and to Umashanker Sampath for the computer-generated line drawings.
Figure 2. Alternative apparatus assembly (see note 2) for measuring the volume of H, by displacement of water into a beaker. however, since it reacts rapidly with the exposed iron aRer the zinc has dissolved. Moreover, the reaction of HN08 with zinc and iron releases copious amounts of toxic nitro: gen oxides instead of hydrogen. Because zinc is amphoteric, it also dissolves in strong aqueous base, i.e., Zn(s) + 2NaOH(aq)+ 2H20(1)+ NazZn(OH),(aq) + H2(g), but this reaction is unsatisfactory because it is very slow with galvanized nails, even with heating. Discussion Besides providing a visually impressive demonstration of the quantitative evolution ofhydrogen by the action of acid &zinc (in contrast to the much slower reaction with iron), this experiment shows that hot-dip zinc coatings are not uniform and that they differ considerably even among nails from the same lot. This variation is instructive because it affords students an o~oortunitvto comoare the range of their experimental r&ults a n i to calklate the standard deviation from the mean. Moreover, even though the results are variable and cannot be anticipated beforehand. the close ameement between the mavimetric and gasohetric determinations furnishes good evidence for their accuracy. Finally, this experiment offers an excellent occasion for discussing the chemistry of iron and steel corrosion and how metals like magnesium and zinc are useful as sacrificial anodes to protect against corrosion, as described in most general chemistry textbooks.' Literature Cited 1. Gill-, 0.6.;H e m s n n , M . 8. J. Chem.E d v c ISSO,67,62. 2. Lange, N. A . Handbook of Chemistry, 13th ed. J. A Dean, Ed.; Mffiraw-Hill: Ymk,1985,P 2-67.
Experiments, laboratory exercises, lecture demonstrations, and other descrptons of the use of chemicals, aDparatJs, and instruments are presented in this Journal as illustrative of new, novel, or improved ioeas or concepts in chemistry instructionand are directed at qualified teachers. Although every effort is made to assure and encourage safe practices and safe use of chemicals, the Journal of Chemical Education cannot assume responsibility for uses made of its published materials. We strongly urge all those planning to use materials from our pages to make choices and to develop procedures for laboratory and classroom safety in accordance with local needs and situations.
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