V O L U M E 2 7 , NO. 11, N O V E M B E R 195.5 ACI S
Apparatus. T h e absorbance Eeasurenients were made with a Beckman Model DU spectrophotometer equipped mith an ultraviolet accessory set and 1.000-cm. silica absorption cells. T h e reference cells contained a reagent blank solution unless otherxise stated. T h e hydrogen discharge lamp was used for measurements from 220 to 400 mp and the tungsten lamp for the 400- to TOO-mp region. T h e pH measurements were made with a Beckman Model H pH meter equipped n i t h a glass electrode. Solutions. THIOCYAKATE ~IETHOD Standard . Cobaltous Sulfate. Dissolve 0.3350 gram of cohaltous ammonium sulfate hexahydrate in redistilled m t e r and dilute to 500 ml. One milliliter of this solution contains 0.100 mg. of cobalt. Ammonium Thiocyanate (33%). Dissolve 125 grams of reagent grade ammonium thiocyanate in 250 ml. of redistilled water. Isoamyl Alcohol (ACS specifications). Saturate n-ith ammonium thiocyanate and remove ewess reagent by filtering through a fritted-glass funnel. Diverse ions. Prepared from reagent grade chemicals. ~T I EHTOHLO DAmmonium . citrate. ~-XITRO~O-~-~~PH Dissolve 260 grams of citric acid in 60 ml. of water and neutralize n ith 250 nil. of ammonium hydrouide (specific gravity 0.9) 1-Xitroso-2-naphthol (Eastman, practical grade). Prepare b y boiling 1 gram of the reagent in 200 ml. of water containing 10
nil. of 5 N sodium hydroxide. Cool solution, filter, and dilute to 1 liter. Hydrochloric Acid, 5.V. Prepare using redistilled water and reagent grade hydrochloric acid. THIOCYANATE METHOD
One of the classical colorimetric methods for determining cobalt has been the thiocjanate method. Although this color reaction of cobalt with thiocyanate was known before 1-ogel’s time, he is generally given credit for its discovery. Since 1900 there have been numerous papers related t o the use of this reaction in analytical chemistry. There are two common variations in the procedure used for determining cobalt colorimetricall>- by the thiocyanate method. According to one procedure acetone is added to prevent dissociation of the cobalt thiocyanate complex, thus serving to intensify the color normally obtained in aqueous solution. The other procedure involves extraction of the colmlt thiocyanate complex n i t h isoamyl alcohol. The blue color of the cobalt thiocyanate complex has been measured spectiophotometrically a t its absorbance maximum a t about 620 mu. Thiq part of the investigation was concerned rvith studying the ultraviolet absorption spectrum of the cobalt thiocyanate complex and the effect of certain solution vaiinhles npon the
General Experimental Procedure. A definite ainount of the standard cobaltous sulfate solution was transferred to a 50-1111. volumetric flask. After the desired amount of 3376 ammonium thiocyanate solution was added, the solution \vas diluted t o 60 ml. with redistilled vater. The complexed cobalt was extracted from the water solution with an isoamyl alcohol solution ~ h i c h had been previously saturated m-ith ammonium thiocyanate. T h e extracts were added t o a 50-ml. volumetric flask. The extract was diluted to the graduation mark with isoamyl alcohol-thiocyanate reagent and the contents of the flask were thoroughly mixed. T h e absorbance measurements Tvvere made immediately using the extractant in the reference cell. The solutions were sufficiently stable to permit the necessary absorbance measurements t o be made. During the study of the effect of diverse ions, all ions were added before complexation. Effect of Cobalt Concentration. The absorption spectrum of the cohalt thiocyanate complex for various concentrations of
ANALYTICAL CHEMISTRY
1732 cobalt was studied. Figure 1 shows the comparison of the ultraviolet absorbance maximum a t 312 mp with the small absorhance maximum a t 620 mp. The concentration of cobalt TVW 6 p.p.m. Conformity to Beer's law was found a t 312 mp for concentrations from 0.2 to 10 p,p.m. of cobalt. Effect of Thiocyanate Concentration. The effect of variablc amounts of ammonium thiocyanate \vas determined using ci p.p.m. of cobalt. The absorbance of each solution was measured a t 312 mp after extraction. An>-amount of thiocyanate solutioli added exceeding 40 ml. did not appreciably change the absorhnnce. I n order to eliminate :Idding such a large volume of reagent it is recommended that t,he concentration of ammoninrn thiocyanate be increased to 44%. I t was found that 25 ml. of a 44% ammonium thiocyanate reagent gave maximum absorbance measurements. Effect of Acidity. The effect of acidity using 6 p.p.m. of cobalt was studied by varying the pH of the final aqueous solution from 1.6 to 7.0. A dilute solution of perchloric acid was used to decrease the pH and a dilute solution of ammonium hydroxide was used t o increase the pH. The pH was read before the extraction with isoamyl alcohol. llaximum absorbance was ohtained for solutions having a pH of 1.6 to 1.9. Because absorb:>nee readings a t this pH were not very reproducible and because the absorbance decreases such a sm:rll amount from the range 1.6 to 1.9 to the range 3.0 t o *5.:%!thir latter p H range is recommended. The use of hydrorhloric :wid and sulfuric acid does not give as reproducible results, possihly due to traces of iron(II1 i, and is not recommended. Effect of Diverse Ions. The effect of diverse ions was studied using 1000 p.p.m. of the ion and 6 p.p.m. of cobalt, complexing, adjusting the pH t o 5.0, extracting, and reading the absorbance at a wave length of 312 mp. Successively smaller amounts of the diverse ion were added until a deviation of 3% or less w a s recorded. Ions which were expected to interfere were added ill lesser amounts for the original reading. T h e reference cell cont,ained the extractant. A negligihle error was obtained with 1000 p.p.m. of acetate, arsenate, ammonium, cadmium, chloride, calcium, magnesium, nitrate, potassium, sodium, and sulfate. Table I list those ions which were found to interfere. Extraction. The cobalt thiocyanate complex was extracted with isoamyl alcohol saturated with ammonium thiocyanate. Without extraction the cobalt complex ion does not exhibit an absorbance maximum in the ultraviolet region of the spectrum. The isoamyl alcohol was saturated to ensure complete color formation and to increase the stability of the extracted complex. This effect was studied by comparing the absorbance nieasurements of solutions extracted with isoamyl alcohol and with isoamyl alcohol saturated with ammonium thiocyanate. In order to ensure constant ahsorhnnce for the ext,ractant, it ifi necessary to filter the excess aninionium thiocyanate from the isoamyl alcohol. The effect of the numher of extraction.? T V ~ Q
Table I. Ion VOa MOO2
Effect of Diverse Ions
Added, Error, Added A s P.P.31. % (Tliiorsanate nietlirid) 10 1000
10
-
1 7 3.8
10
34.0 23.5 63.3
I 000
1.G 9.3
10
10
100 100 10
10
10 50
100 100
I00 100 20
3.5 8.9
34.5 4.4 a9 7 1.2
Permissible Amount, P.P.3I. 10
100 0
0 0 10
100 75 25 0 5 0
50
0
-10.6 1.9
100
2.9
100
-14.6
- 8.2
0 0
OR.
--L
283
.
.
.
.
I
360
4 40
520
600
680
WAVELENGTH, rn)
Figure 1. Absorption s p e c t r u m of cobalt t h i o c y a n a t e complex 6 p.p.m. of cobalt
-tidied and it was found that two 20-ml extractions were just satisfactor! as were four 10-ml. extractions, or two IO-ml and one 20-ml. extractions. It ifi recommended that all isoamyl alcohol used for extracting purposes, regardless of manufacturer or purity, be tested for 100% transmittance a t 312 mp with Lmter in the reference cell. Stability. The stability of the extracted solutions was studied by treating a solution containing 4 p.p.m. of cobalt in the usual manner. An absorbance reading a t 312 mp was taken immediately after extraction. Other readings were taken after i n creasing intervals of time. The last measurement was read after 24 hours and this absorbance value agreed with the originaL reading within the range of experimental error. I t was concluded that the extracted solution \$as stahle long enough for ordinan absorbance measurements. RS
1-NITHOSO-LNAPHTHOL METHOI)
Col)altour; ions in a slightly acidic citrate-buffered ~oliition when treat,ed with 1-nitroso-2-naphthol solution give :t prrcipitate of cobalt-1-nitroso-2-naphtholate. This complex is . soluble in chloroform, producing a yellow orange solution (6'). A procedure for determining cobalt photometrically using the colored extract of a filtered and dried cobalt-1-nitroso-2-naphtholate precipitate was developed h y Valdbaiier and K a r d ( 4 ). A 'tiidy of the ultraviolet spectrum of the extracted compound revealed an a1)sorbance maximum in the ultraviolet region The possibility of extracting the precipitate from the solution without filtering and the elimination of the interference of iron was : h o investigated. General Experimental Procedure. d definite amount of the standard cobalt solution was added to 10 ml. of the ammonium citrate rcagent and 10 ml. of water in a conical flask. The mixture WN':heat,ed to hoiling and 10 ml. of the 1-nitroso-2-naphthol reagent were added. The solut,ion was cooled and allowed to stand for 2 hours. The precipitate was extracted three times with 30 ml. of cahloroforni. The final volume was adjusted with chloroform to 100 nil. and t'he absorbance measurements were made on thc chloroform extract in the ultraviolet region. Effect of Cobalt Concentration. The absorption qiectra for yarioua roncentrations of cobalt were determined and conformity t o Beer's law was found a t 317 mp in concentrations from 0.2 t o 2 p.p.m. of cobalt. Figure 2 s h o m the characteristic absorhnnce rnnxima obtained. The discontinuity of the curve in the 365 to 100 mp region is due to the high ahsorbance of the hlank and the limited sensitivity of the spectrophotometer.
V O L U M E 2 7 , NO. 11, N O V E M B E R 1 9 5 5
1733 Effect of Diverse Ions. The effect of diverse ions was studied using 1 p.p.m. of cobalt. Absorbance readings were taken a t 317 mp. A change of 3% or lesp mas considered negligible. A negligible error was obtained with 500 p.p.m. of chloride, nitrate, perchlorate, sulfate, and tungsten. A negligible error was also obtained with 200 p.p.ni. of aluminum, cadmium, chromium, molybdenum, and zinc. T:thle I1 lists the interfering ions.
Table 11.
Element
0
Figure 2.
Absorption spectrum of tris-(-nitrosonaphtho1o)-cobalt(II1) 2 p.p.m. of cobalt
Effect of Acid Concentration. The effect of various concentrat,ions of hydrochloric acid mas determined using 2 p.p.ni. of vobalt, 10 ml, of ammonium citrate, and 10 ml. of water with 0, I , 2, and 5 ml. of 5 N hydrochloric acid and 10 ml. of precipitant. I t was found that the absorbance of the extracted coniplex decreased rapidly with increased acidity. -4maximum in stisorbitnce was obtained when no acid was added, or the solution had a p H of about 5.1. The pH should be adjusted within the mnge 4.0 t o #5.5. Effect of Digestion Time on Precipitate Formation. .liter the precipitat,e was formed it was allowed t o stand for :30 niinutes, 1 hour, 2 hours, and 3 hours, respectively, before extraction with chloroform. It was found that the absorbance increased with longer periods of digestion before extraction until the dige3tioii time of 2 hours was used. Increasing the time of digestion from 2 t o 3 hours increased the absorbance inappreciably. Therefore, the 2-hour standing period was adopted as a antisfactory period for digestion of the precipitate. Stability of Complex. The absorbance u-as meawurcd OVCI' :I 24-hour period and wa3 found t o be constant.
Interfering Ions
(I-nitroso-2-naphthol method) .\mount Added, Error, Added a c P.P.M. %
Permisaible Amount, P.P.M.
Removal of Ferric Ions Prior to Spectrophotometric Determination of Cobalt. Inasmuch as 5 p.p.m. of ferric ions produced an error of 45% in the ultraviolet spectrophotometric determination of 1 p.p.m. of cobalt an effort' was made to circumvent this interference. Lundell and Hoffman have shown that ferric ions can be removed bl- extraction with ether (1). I t was fourid that appreciable amounts of iron coiild 1~ removed using the following procedure. Place the sample containing cobaltous and ferric ions iii a Reparatory funnel. -4dd an equal volume of an ether solution which was prepared by shaking 100 ml. of diethyl ether twice with 100 ml. of 6 N hydrochloric acid. After shaking, the aqueous layer is allowed t o separate. The aqueous layer is extracted again with the ether solution. The aqueous layer is boiled to remove traces of ether. The precipitation of the tris-(-l-nitroso-2iiaphtholo)cobalt(111) is performed using the general experimental procedure. One part per million of coluilr rvas determined in samples iwntaining 5, 10, and 20 p.p.m. of ferric ions with a negligible wror (less than 2.5700). Thus, :I preliminary extraction with chther is effective in removing the iron from the cobalt. LITEH4TURE CITED (1)
Hoffman, J. l., and Lulldeli. G.I I . , IXD.EXG.CHEY..ASAL. ED., 14, 727 (1942). KI.CTII.EDfor review Aumi.r lli, 1'134. A w r ~ i t e dJuly 30, 1B55. Di\-iaion of l n d y t i c a l Chemistry. 1 2 5 t h J I w t i r i x A C ~ S , Kansas City, N o . , 1!l54.
