Determination of Iron in a Bar of Soap M. A. Grompone Facultad d e Quimica. Avda. Gral. Flores 2124. Uruguay Spectrophotometric determinations are well established as Dart of undermaduate laboratow on instru"exneriments . mental analysis. The literature is full of instances where visible snectroscovv .- has been e m.~ l o.v e das a tool for identification, specification of purity, or assignment of structures to compounds. But it is of interest to find experiments illustrat& real situations. Most of the suggested experiments aive the students an erroneous feeling of simplicity and do not show them the problems they 2 1 1 encounter in their future professional work. Iron(II1) forms a colored compound with thiocyanate in acidic solution. This reaction is probably the most widely used for its colorimetric determination (1-5). The purpose of this paper is to apply this technique to determine the iron(111)content in a bar of laundrv or toilet soap. The basic reaction in soapmaking is quite~simple.It consistsof reacting fat withan alkali to producesoapand glycerol:
-
C,H,(O,CR), fat
+ 3NoOH-
3RC0,Na + C,H,(OH), sodium coap glycerol
After saponification the soap undergoes a wries of phase changes for removing impurities, recovering glycerol, and reducing moisture runtent t u n relatively low level. Some ut'rhe newer manufacturing processes use the fatty acid route, where fatsare first converted into fatty acids and glycerol by a high-pressure fat-splitting proceus. The crude fatry acids are then distilled and neutralized:
During manufacture heavy metals are introduced and are an impo;tant cause of rancidity, spotting, and discoloration of soap. The metals that degrade thr soap by oxidation are t h o s e i f the transition type, including iron, copper, and cobalt. Traces of metal catalyze oxidation by enhancing the formation of free radicals or by acting as prime oxidizing agents, usually in the highestvalence state (6).The soap may become contaminated by iron during the soapmaking process. If iron is to he entirely eliminated, then the soap manufacturer must use extreme caution in raw material storage and in manufacturing operations. Therefore, the determination of iron(II1) in laundry or toilet soap is of considerable importance in the practice of industrial soapmaking. The iron usually found in soaps is normally present present in only a few parts per million, so the ordinary gravimetric and volumetric methods are unsatisfactory for their quantitative estimation. The method described here involves measurement of the visible absorntion of the comnlex comnound formed hetween iron(li1) and thiocyanate with a spectrophotometer suitable for making measurements in the spectral region between 400 nm and 800 nm. T h e reaction between iron(II1) and thiocvanate cannot he done directlv in the aqueous soap solution since the soap reacts with the mineral a&:
-
+
RCOaa + HH+ RC02H Naf T h e fatty acids produced are not soluble in water and form
an upper organic layer which usually is solid a t room temperature. Iron(II1) remains dissolved in the aqueous layer and is determined with thiocyanate. Experimental
Apparatus The Shimadzu CV-Vir Recording Spectroph,~tometerUV-lfi0 o r equivalrnr, with a range of 400-XMl nm, equrpped wrth Iong.path rrctdngular 5-cm glass ahrorptiun cell can he used. The 1IV-IfiOla s microcomputer-controlled double-beam recording spectrophotometer. It can alsouse calorimeters withealor filters and for eolorimetric work of low accuracy, visual comparison methods are satisfactory (Nesslerglasses, tintometers, comparators). Reagents Reagent-grade chemicals shall be used in all tests: concentrated hydrorhlurir acid (sp. p.1.191 cmrentrated nitric acid isp. gr. 1.38) iron, standard rulution (I mL = 10ug Fe") ammonium thiocyanate (8%) (While ammonium thiocyanate is specified throughout, there is no reason why the corresponding sodium or potassium salts cannot be used). Procedure Follow the manufacturer's instructions to set up, operate, maintain and calibrate the spectrophotometer. None of the usual constituents of soaps cause interference in this method, unless the coloring agent of the soap is soluble in water. Standardsolution. Prepare a working standard solution (1mL = 10 pg Fe3+)by one of the published procedures (14). Absorption spectra. Place 15mL of the iron standard solution in a 100-mL volumetric flask. Add 4 mL of concentrated HN08 and 10 mL of 8%NHlSCNsolution.Dilute to the mark with distilled water and mix well. Carry a blank of 15 mL of distilled water through the same procedure. Read the absorbance versus a blank (5-cmahsorption glass cell) at 400-800 nm. Record the wavelength at which there is maximum absorhance. Calibration euroe. Add to each 100-mL volumetric flask, respectively, 5,7,10,20,and 30 mL of iron standard solution. Add 4 mL of concentrated HN03 and 10 mL of 8% NHISCN solution to each flask. Dilute to the mark with distilled water at room temperature, and mix well. Prepare a hlank of 5 mL of distilled water in the same wav. For each solution. read and record in a table the absorbance versus a hlank at the waveleneth of maximum absorbance. Plot the phoromrrric wading tor r l w sol~~tion as absrissa against the pans per million of iron 89 ordinnte or linrnr graph paper. If this kystmn follows the Lamhert-Beer law, a straight line will be obtained. In such a case calculate its parameters by means of the least-squares approximation. Iron content of a soap bar. Dissolve 20 f 0.01 go of the sample in 150 mL of hot distilled water contained in a 250-mL Erlenmever flask. When the solution is complete, add 15 mL of concentrated HCl. Heat the flask until the fatty acids separate as a clear layer. Cool the solution in an ice bath until the fatty acids solidify. Decant the liquid through a filter into a 250-mL volumetric flask. Add 30 mL of hot distilled water and 1 mL of concentrated HCI to the Erlenmeyer flask. Heat until the fatty acids melt, and then carefully boil themixtureafew mintues. Caoland filter into thesame 250-mL volumetric flask. Repeat the wash with distilled water and HCI. Filter into the flask with the previously decanted liquid and dilute to the mark with distilled water. Mix well. Measure an 80-mL aliquot from the flask, and place in a 100-mL,glass-stoppered,graduated cylinder. Add 4 mL of concentrated HN03 and 10 mL of 8% ~~
~~~
Volume 64
~~~
-
~~~~~~
~~
~~~~
~
Number 12
December 1987
1057
NHBCN solution. Dilute to the mark with distilled water and mix well. Preparea hlankof 8OmLof distilled water in thesamemanner. Read the absorbance against a blank (5-cm absomtion elass cell) at the wavelength of max&um absorbance. ~eterminet i e parts per million of Fe3+in the soap by using the calibration curve. It may he necessary to vary the size of the sample or aliquot until a suitable reding is obtained. Calculate the iron content of the sample, in parts per million (ppm) as follows: S X 25000 iron, ppm = AXW
where S = micrograms of iron equivalent to the absorbance of the solution obtained from the standard curve, A = aliquot of the sam. ple solution, in milliliters, and W = weight of sample, in grams. Results ln a typical determination on a bar of soap purchased in the local market, t h e wavelength of maximum absorbance
1058
Journal of Chemical Education
was 477 nm. T h e linear working curve calibration was calculated according t o the calibration curve procedures above, a s
+
iron, ppm = 1.453(ahsorbance) 0.012 T h e sample had a n absorbance = 0.350and iron = 0.52 ppm. From this value, iron content of t h e soap bar was calculated (8 P P ~ ) . Literature Clled
1. Voaei,~.J,Quimicoanaliti~a~uontitoti~o;~~~~lua~:~~~nos~ire~,1960;~~1.2, 2. Char1nt.G. Chimi~onoly~iquaquon~ifofivo;Masaon:Paris, 1974; VOI. 8. 3. Kreshkou,A.:Yarosl6vt.ou.A.CursodeQuimieoAnaiirico.Andlisiacunafifotiuo:MIR: Masc6.198S; p 299. 4. Meloan, C. E.: Kiser, R. W. Problemas y erperimonfos en onllislr inrturnenla$ R~vo+M ~.
S", .".",w ,971 ~
~
... ~
~
S. Skow.D. A.: West.D. M. Andiisis inrtr~msnfoliIntersmeriesna: Merim, 1 9 M p 189. fi. Swern, D., Ed. BoileyBIndurfriol Oil ondfot &ducts: Wiiey: New York, 1979: Vol. 1,
P 147.
