Preparation and Spectrophotometric Analysis of Hexaamminenickel(ll] Chloride A General Chemistry Experiment Grace M. Wieder Brooklyn College of the City University of New York, Brooklyn, NY 11210 first dissolving weighed 0.2-g samples in 35 mL of standardExperiments for the preparation and analysis of coordinaized 0.2 N HCI delivered from a buret. The acid must he tion compounds have heen described1-' fur the general present in excess to convert virtually all of the NH3 to NH4+ c h e m i s t r ~ l a h ~ r a t oSuch r ~ . a project is usually assigned near ion. After adding 20 mL of water this solution is titrated with the end of the first year because i t combines synthetic and 0.1 N NaOH until the oink color of methvl red standardized analvtical technioues., illustrates comolex ion chemistw. ?. indicator fades to give a nearly colorless solution. ~ o t e n t i o and encourages students to refresh their stoichiometric calmetric titrations show that the endpoint occurs at a p H of 5.0 culation skills. At Brooklyn College the preparation and due to the hydrolysis of the NH4+ ion. The NaOH solution is ammonia analysis of an amminenickel(I1) chloride suggestpreviously prepared by the students from concentrated base ed hy Nordmann' has heen modified and extended to inand standardized against the HCI standard provided. clude a spectrophotometric analysis for nickel. Thus, a furThe spectrophotometric nickel analysis is performed in ther advantage is offered hv introducing freshman to a quandilute acid in order to measure the absorbance of the green titative instr;mental technique based on Beer's law. hexaaquanickel(I1) ion at 395 nm. This wavelength is the Three 3-hour laboratory are scheduled for this . periods . well-defined peak maximum discussed by Pacer5 in the visiexperiment: one for the relatively simple synthesis, one for ble absorption spectrum of aqueous NiSO&H20. Solutions the determination of ammonia by an acid-baie back-ritraof NiC19.6H?0 show the same isolated band in the 390-395 tion. and one for the nickel determination with a Hausch & nm region. students prepare a standard stock solution by ~ o m hSpectronic 20 or 21. One spectrophotometer is adeweighing a 2-2.5-g sample of NiClz6H20 in a beaker, disouate for everv 6 8 students. From their results students solving the sample in 1N H2SOaand transferring the solucompute the ammonialnickel ratio and the empirical formution quantitatively with acid rinsinc to a 100-mL volumetric la for the complex cation. The chloride-nickel ratio is deflask..^ solution df the synthesizedcoordination compound duced from the electroneutrality of the compound. As an is prepared in the same manner by dissolving a 1-1.5-g samalternative a fourth laboratory period might be assigned to ple in the sulfuric acid. Two additional Ni standards are analyze for chloride either gravimetrically by directly adding made in 25-mL flasks by diluting pipetted 10- and 20-mL acidic AeNO* - - or volumetricallv bv a Mohr titration after portions, respectively, of the standard stock solution with passing a sample solution through an acidic ion exchange the 1 N H2SOa. Students then measure the absorbance a t column to remove Ni2+ ions. It is suggested that students 395 nm of each of the four solutions in cuvettes against the 1 consider the following as possible compound formulas: INi(NHd41C12, lNi(NH~)&lICl, I N ~ ( N H ~ ) s ( H ~ O ) I C ~N~H~2 S 0 4blank. They are then able to determine the concentration of nickel in the comoound solution from a linear plot [Ni(NH&]C12, and [Ni(NH&]C12.H20. of absorbance versus m g & ~Ni in the standard solutions, Although the labile complex amminenickel cations readily either by interpolating from the graph or from a leastexchange water for the ammonia ligands, the following synsquares fit. The student average linear correlation coeffithesis recipe gives a product in reasonahle yield that is stable cient of 0.998 with a standard deviation of 0.003 suggests in a stoppered vial for several months. Students working that Beer's law holds quite well in the concentration range with safety goggles under a hood add 30 mL of 15MNHs to a above. warm solution of 12.0 g of NiC126H20 dissolved in 15 mL of The tahle summarizes the results of the 24 students who water. The mixture is stirred for several minutes, cooled in performed this experiment in several different laboratory an ice bath, and suction-filtered. After the violet crystalline sections. Three-fourths of this erouo deduced the correct product is washed twice with 10-mL portions of concentratformula for the compound. The error in the table is exed ammonia, strong suction is applied for 15 min. The cryspressed as the 95% confidence level, N = 24. tals are further dried for 1 h spread in a folded packet of The student average result for the ammonia content was double filter paper hefore storage in a dry stoppered vial. In low by about 4%. Since analyses of the samples by both the erouo - . of 24 students who have tried the exoeriment. the Kjeldahl and Dumas methods gave 25.8 f 0.2 mmol NHdg, average yield was 59% with a standard deviation of 7%. If the synthesis produces the hexaammine compound in readesired. alareer vield can he obtained bv the more expensive sonably high purity. Potentiometric acid-base titrations perproced&e ofgdding cold 95% ethanol to the crystaliization formed to investigate for possible Ni2+ ion interference mixture as suggested by Schlessinger4. vielded a normal shaoed curve. 25.6 mmol N H h . and agreeThe synthesized compound is analyzed for ammonia by Lent between the 'potentiometric and vis&i'endp&ts. Therefore. it can be concluded that the low student averaee result was caused mainly by a tendency for some students to Nordmann. J.; Kuljian, E. S. "Quantitative Freshman Chemistry": Burgess: Minneapolis. 1966; pp 106-109. Bramwell, F. B.: Dillard. C. R.; Shahani, C. J.; Wieder. G. M. Summary of Student Results of Analysls ot [Nl(NHs)s]Clr "Investigations in General Chemistry": Burgess: Minneapolis, 1977; pp 146-153. NH, (mmollg) Ni2+ (mmollg) NHs/Ni2+ ratio Olmsted. J.. ill J. Chem. Educ. 1984, 61.1098. Schlessinger, G. G. "Inorganic Laboratory Preparations"; Chemi6 25.9 4.31 actual cal Publishing: New York. 1962; p 194. 4.36 i 0.11 5.71 + 0.13 student average 24.9 + 0.6 Pacer, R . A. J. Chem. Educ. 1984, 61,467.
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overtitrate the methyl red endpoint. The error, however, is not large for freshman chemistry and careful students still arrive at the correct integral ratio of ammonia to metal ion. The nickel results, on the other hand, were excellent. Half of the students determined the Ni content to within 1%of the correct value and 62% of them achieved results within 2%. The smaller group who did not perform as well either used poor quantitative technique when preparing the solu-
tions or misread the spectrophotometer scale. Most of the students found the experiment stimulating because of the variety of techniques employed. They especially enjoyed working with the spectrophotometer. In summary, this experiment provides freshmen with the opportunity to prepare a beautiful coordination compound and to review classical volumetric analysis while learning the newer instrumental spectrophotometric technique.
Volume 63
Number 11
November 1986
880
