Fred A. Snavely and Claude H. Yoder Franklin and Marshall Colleae Lancaster. Pennsvlvania 17604
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The Preparation and Analysis of Some Double Salts An introductory experiment
Ihe introduction of analytical techniques into the general chemistry laboratory has too often meant the addition of routine of 'Lunknoms." Chemists are generally more concerned with the identification and charact,erization of their own preparations, and thus it would seem desirable to adopt the same approach in the student laboratory. Students often develop a sort of "rapport" with their creation which stimulates additional initiative and enthusiasm. Compounds suitable for this approach a t the beginning level should be easy to prepare and isolate in the pure state and readily amenable to analysis by simple techniques. The double salts of potassium or ammonium sulfate with the sulfabes of the divalent met,als, manganese through zinc, fulfill these requirements. They can he prepared by mixing aqueous solutions of either potassium or ammonium sulfate with equimolar quantities of the divalent metal sulfates and are readily isolated as pure compounds with a definite amount of water of crystallization.' Pre~arationof the Double Salts Potassium Salts. Weigh out accurately into a. 250-ml beaker 0.05 mole of potassium sulfate, and into a 50-ml beaker 0.05 mole of the divalent metsl sulfate. Add 20 ml of distilled water to the divalent metsl sulfate, and heat with stirring until the salt is dissolved. Gravity filter the hot solution into a clean 250-ml beaker. Add 65 ml of distilled water to the potassium sulfste, dissolve as with the divalent metal sulfate, and gravity filter through the same funnel into the divalent metal sulfate solution. Cool with intermittent stirring to room temperature and then externally with ice. Suction filter the product and then air-dry for one day on a large watch glass. Store the product in a screwcap bottle and record the yield. The vrevaration of the ~otassium-manertnesedouble salt requires only 10 ml of water for the dissolution of 0.05 mole of MnSOd ,4H20. The mixed sulfates solution must he evaporated to about 50 ml before cooling. Ammonium Salts. The eopper(II), nickel(II), cobalt(II), and zinc(I1) derivatives are obtained according to the procedure above exceut that the ammonium sulfate is dissolved in 20 ml of hot water. The preparation of the ammonium-manganese double salt requires only 10 ml of water for 0.05 mole of ammonium sulfate and 15 ml of water for 0.06 mole of MnSOI.4H20. The ammonium-iron(I1) salt is prepared as follows: Dissolve 0.05 mole of FeSOl .7H10 in 50 ml of hot wrtter to which has been added 0.5 ml of conc. HISOI. Add 0.05 mole of ammonium sulfate and aravitv filter the solution into a clean beaker cantaininz a clean iron naic Slowly evaporate to crystallization (about hkf volume), and cool with intermittent stirring to room tempera-
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'CAYEN,R. M., AND MITCHELL,T . C., J . Chem. Soe., 1428 W., J . Chem. Soc., 2628 (1924); CAVEN,R. M., AND JOHNSTON, W., J . Chem. Soc., 2358 (1926); CATEN,R. hl., A N D JOHNSTON, (1927).
ture. Suction filter the product and air-dry the salt avoiding effervescence. Sodium Salt. Add 0.14 male of CuSO4.5H20 and 0.24 m d e of sodium sulfate to a 25C~mlbeaker. Add 75 ml of distilled water and digest with stirring at moderate temperatures (50-6n°C) for 15-20 min. Cool to room temperature and suction filter. Trmsfer the crystals to a watchglass and air-dry overnight.
-..-.,-.,The components of each compound can be determined by wet qualitative methods. Water of hydration can be determined by heating weighed samples in an oven a t the proper temperature to constant weight. Table 1 presents temperatures Table 1. -ComuoundMU) - M(II) NH, Mn NHI Fe NIL Co NHI Ni NH, Cu NA.
M(1)~SO~~M(II)SOa.nH20 TemTemperature perature Moles DSC endothermsd ovendried HnO ("C) ("C) lost 72 and 100 110 6 Multipeaked 80 487 110 6 110 160 6 4b 80 59 and 121 RR iin R
" Heating the cam ound to higher temperatures results in extensive oxidation. f h e other two moles of water cannot be obtained in this way. Heating the compound to 150°C releases two more moles of water. No additional water is lost at higher temperatures; the DSC curve shows no more water present. d Temperatures recorded are those a t the beginning of the endo- - ~ ~ ~
a t which endothermic loss of water occurs as determined by differential scanning calorimetry as well as appropriate oven temperatures and the number of moles of water lost at these temperatures. For several compounds the water of hydration is not completely removed a t a single temperature. For example, the two copper double salts (potassium and ammonium) show two distinct DSC endotherms which are far enough apart to suggest the possibility of obtaining a lower hydrate by heating a t 80". Subsequent heating to constant weight in an SO0 oven shows that four moles of water are lost to give the dihydrate. The other two moles of water are removed by heating to 150". I n addition, endotherms and exotherms not associated with loss of water were observed a t relatively low temperatures for those metals that are readily oxidized-iron, cobalt, and manganese. Volume 48, Number 9, September 1971
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Table 2.
Data for Colorimetry
Double salt
A(w)
Conc. (M)
K-Cu NH4-Cu K-Ni NH,-Ni K-Co
850 850 720 720 50.5 505
0.01-0.024 0.01-0.024 0.07-0.14 0.07-0.14 0.03-0.10 0.03-0.10
NH.-Co
The salts can be analyzed for sulfate by the classical gravimetric barium sulfate procedure. Quantitative analysis of the colored cations, divalent copper, nickel, and cobalt, can be accomplished by colorimetric analysis of aqueous solutions of the salts using a Spectronic 20. Suitable wavelengths and concentration regions in which Beer's law holds are given in Table 2. Manganese can be determined by colorimetry after oxidation to permanganate. The gravimetric zinc pyrophosphate procedure has been reasonably success-
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ful for zinc. There are, of course, a number of standard procedures for volumetric and colorimetric analysis of iron. The ammonium content can be obtained by the Kjeldahl procedure. Alternatively, potassium and ammonium can be determined by difference. Extensions
There are a number of areas where the student can be encouraged to continue on his own. The type compound could be extended to other monovalent and divalent cations: a few double salts of sodium have been reported2 and we have prepared N%SO4.CuS04. 2H20. This preparation is more difficult and the compound is isolated a t room temperature (not from a 1: 1 ratio of the two salts in solution). The vanadyl, magnesium, and cadmium divalent cations form other possibilities. CAVEN,R. M., (1928).
AND
JOHNSTON, W., J. Chem.
Sac., 2506
