joint. In doing so the class tube may b~ l~ahtlytapped on a ruhher surface roarhieve a firmer parking. After filling thc glass t u h its ~ other end is also provided with a wad of ~trttonwu01, SO thaT the material cannot drop out,
in aminimum amount of HN03 (-1% ml of 15M). A magnetic stirrer is helpful but not essential. Then add solid NaC2Ha02(-2 g), with stirring, until the AgaAs04reprecipitates. The addition of nitric acid, followed by sodium acetate, can he repeated several times.
Experlments
Procedure B-Silver Phosphate Dissolve 1.9 g (5 mmoles) Na3POa.12HzO in 200ml Hz0 and proceed as in Procedure A. Silver ohosnhate is lemon vellow and will he less easilv seen in a lame leetuie hail than will silver arsenate. How~, ever, it hszardow uasw dispusal prrsmu a problem, thm phwphau' is the sysrem of choice. A magnetic stirrer is helpful for rliswlving s i h r phosphate in nitric a r d Allow 1'2 2 ml of 15 .\I HNOIand vigorous stirring.
Separation of a Pentanel Hexane Mixture. Material: the apparatus, 1hypodermic syringe (entire volume 1-2 ml), pentane and hexane. Procedure: The hydrogen flow rate is regulated at the reduction valve within the range 150-200 mllmin. The hydrogen is lit at the outlet of the capillary. The flame usually reaches a height of 11, in. A hypodermic syringe filled with 0.1 ml pentanelherane mixture (1:4 by vol.) is injected into the thick rubber tube, so that the needle punctures the tube close to the opening of the glass tube. The efflux of pentane and hexane after completed separation enlarges and intensifies the flame. The flame can be seen particularly well in a darkened room. Separation of a Methylene ChloridelChloroformlCarbon Tetrachloride Mixture Material: the apparatus, 1 hypodermic syringe (5 ml by vol) 1 copper wire 1%in. long, 1nail, methylene chloride, chloroform, and carbon tetrachloride. Procedure: The hydrogen flow is regulated within the range 200-220 mllmin. A short copper spiral is made by coiling a copper wire round a nail. The copper spiral is set over We end of the capillary. The hydrogen is lit and the end of the copper wire is bent into the flame and serves to identify chlorine-containing compounds in the flame. A 5-ml hypodermic syringe is filled from the vapor over the liquids (not the liquids themselves!) with 0.3 ml methylene chloride gas, 1 ml chloroform gas, and 1.5 ml carbon tetrachloride gas. The gas mixture is then injected into the vacuum tube. Theeffluxof the components is recognizable by the green coloring of the flame.
Equilibria in the Silver Arsenate-Arsenic Acid and Silver Phosphate-Phosphoric Acid Systems
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Hazards and Disposal: Arsenate-containing compounds are highly poisonous and should n o t b e ingested n o r allowed to come i n contact w i t h mucous membranes o r skin. The arsenate mixture from Procedure A should he disoosed of accordine to the hazardous waste procedures used at the reader's institution. Donot disposeof it down thcdrain! If noacceptable method of disposing t ~ arsmutes f is availal~lr,use Procedure R. The mixturr resultine from Procedure H ran he flushed down the drain with plen& of water.
Discussion This demonstration can be used to illustrate relative strengths of acids, p H effects on solubility, and, t o some extent, periodic relationships. Chemical equations for the arsenate and phosphate systems are as follows: X=As,P
+
3Ag+(aq) HzxO~-(aq)= AgaXO,(s)
-
+ 2Hf(aq)
AgaXOds) +excess H+(aq) HaXOr(aq) + 3Agc(aq) 3Ag+(aq)+ HsXOdaq) + excess NaCzHaOz(s) Ag&Oa(s) + 3CzHaOzH(aq) + Na+(aq) CzH30z'(aq). Pertinent equilibrium constants for the two systems are displayed below:
-
+
SIBUrnED 8":
Patricia L. Samuel Boston University Boston, MA 02215
&
Substance
K1 s.7.52 X K2 16.23 X
HsPOI'
Ks
C m m sr:
= 2.2
X
lo-" 10-19
George Virtes Ward Melville High Schaol Setaukel, NY A qualitative test for arsenate ion' provides the basis for a colorful demonstration of the principles of relative acid strength, solubility product, and effect of p H on solubility. Silver arsenate, a chocolate brown solid, is formed by mixing aqueous solutions of silver nitrate and potassium arsenate. T h e ~reciuitawis then dissolved with nitric acid. Addition of solidsodLm acetate causes silver arsenate to reprecipitate. Phosphate can be substituted for =senate;= lemon-yellow s i l ~ e ; ~ h o s ~ h aist ethe precipitate. Materials 1-1Erlenmeyer flask HNOs 15 M AgN03,O.l M KH2As04,or other arsenate salt (Procedure A) NaaPOp.IZHz0, or other phosphate salt (Procedure B) NaCeH201. - " -. solid medicine dropper or disposable pipette spatula magnetie stirrer (optional) Procedure A-Sliver Arsenate Uirrrrlve 0.0 6 I3 mmoles~KH2As04in ?UO ml distilled w e e r in a 1-1 Erlenmeyer flask. To this solut~onadd 20 ml of u.1 M AgN03. Chwdate brown Ac3ArOr will precipitate. 1)issolre thp precipitate
~.
A&ASO~"
K,
= 1.08 X
'"Handbook of Chemistry and Physics". 6lDted.. CRC Press. 1980-81. Llnke. N. F. and Seidell. A,. "Solubilitiesof lnaganic and ~etal-organic Compaunde." 4m ed., american Chemical Sociew,1958.
Silver phosphate and silver arsenate are typical salts of weak acids and thus dissolve in acidic solution. Since acetate ion is a stronger base for protons than either dihydrogen phosphate or dihydrogen amenate, addition of sodium acetate to these solutions results in the formation of acetic acid: HsXOdaq) + CzH30z-(aq) * CzHaOzH(aq) + HzXOh-(aq) The conjugate bases of phosphoric and arsenic acids dissociate to phosphate and arsenate, allowing the reprecipitation of the silver salts. Periodic relationships between structure and acid strength
Presented in part at the Forum on General Chemisby. Colorado School of Mines. June 5 and 6. 1961. Sorum. C. H. and Lagowski, J. J.. "introduction to Semimicro Qualitative Analysis," 5th ed.. Prentice-Hall, Inc.. 1977. Hufstedler. R. S., Virginia Wesleyan College, Norfolk, VA, private communication. ~
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Volume 59
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Number 12 December 1982
1043
can he illustrated by comparing the two systems. Phosphoric acid is stronger than arsenic acid because phosphorousin able to polarize the 0-H bond to a greater extent than is arsenic. -TheK, values for silver phosphate and silver arsenate are very nearly the same. The slightly higher value for silver
1044
Journal of Chemical Education
phosphate could he attributed to greater hvdration enerev. . . since phosphate ion is a stronger protonic bask than is arsenik and wuuld beextensively hydrolvzpd. While the latticeenerm of silver phosphate is expected he higher than that of silv& arsenate, apparently the hydration energy predominates.
