COMPARISON OF RESULTS
The average results (Table I) obtained by manual and machine operation compare favorably, but the precision as shown by the probable error favors machine operation. Although the difference between the average results for a low temperature char is slightly larger than permitted by ASTM methods, the lomer value by machine operation indicatrs less possibility of mechanical losses. LOW TEMPERATURE CHAR
Pittsburgh Consolidation Coal Co. has devoted a considerable amount of research to low temperature carbonization. The result has been a heavy demand on the analytical laboratory
for volatile matter determinations on chars both for control operations and for material balance data. The automatic unit described was developed t o reduce errors, because as chars are nonagglomerating they are highly susceptible to sparking and are exceedingly difficult to run (6). DISCUSSION
While this unit was designed specifically for volatile matter determinations on coal and char as outlined by the American Society for Testing Materials, it has sufficient flexibility t o warrant its use for other operations requiring controlled time and temperature measurements. The unit has been used t o determine carbon residue on tars and
pitches. On several occasions the automatic unit has been used to make laboratory studies designed t o simulate specific carbonization conditions. LITERATURE CITED
(1) Am. SOC.Testing Materials, “ASTM Standards on Coal and Coke,” D271-48, 17-19 (1954).
(21 Ibid.. D. 36.
Invest. 3168, 1-2 (1932). (5) Selvig, W. A, Ibid., 3739 (1943). RECEIVED for review September 10, 1956. Accepted October 24, 1957. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1966.
Determination of Zinc and Separation from Ashed Biological Material J. A.
STEWART and J. C. BARTLET
Laboratories o f the Food and Drug Directorafe, Department of National Health and Welfare, Ottawa, Canada
b A method utilizing 4-chlororesorcinol is proposed for the colorimetric determination of zinc. The zinc-4-chlororesorcinol reaction obeys Beer’s law over the range of 0.1 to 5 p.p.m. o f zinc. Interference from cations is comparable to that with other reagents currently used. Use of a single reagent, sodium diethyldithiocarbamate, simplifies separation of zinc from other cations in the ash of biological material. Zinc and other cations which form diethyldithiocarbamates are extracted from buffered aqueous solution at pH 8.5 into chloroform. The zinc i s re-extracted with aqueous 0.1 6M hydrochloric acid, and determined colorimetrically with 4-chlororesorcinol or Zincon. With 4-chloresorcinol and standard zinc solution, the average recovery at the 15-7 level is loo%, and at the 50-7 level, 97%; the coefficients of variation are =k 1.4 and 12.2%, respectively. The mean recovery of zinc added to ash from agar-agar i s 95.470. Factors leading to variable blanks were investigated and means of reducing blank variation devised.
I
THE PRESEXCE of ammonium hydroxide, resorcinol reacts with zinc to produce a n indicator which is blue in alkaline and red in acid solution (4, 6). This reaction is the basis of a quantitative method for the coloripi
404
ANALYTICAL CHEMISTRY
metric determination of zinc (11, 14, and has been used for the estimation of zinc in milk (18). Resorcinol also reacts with cadmium, calcium, copper, cobalt, manganese, and nickel (14)-no more cations than reported for dithizone (17) and a new reagent, Zincon (Z-carboxy-2’-hydroxy5’-sulfoformazylbenaene) (16). Because of the lack of specific reactions for the microdetermination of zinc, the nature of the resorcinol-ammonium hydroxide-zinc system rvas investigated. The colored dye from the reaction was isolated, and analyzed by infrared spectroscopy. The wave lengths of the absorption bands indicated that the dye contained benzene rings substituted in the meta positions. I n view of this, it was decided to examine the possibility of using other substituted phenols as a reagent for zinc in place of resorcinol. Particular attention mas paid to phenols with two hydroxy groups meta to each other. EXPERIMENTAL
A number of hydroxyaryl compounds were investigated t o establish their reactivity toward zinc in the presence of ammonia. Monohydroxy- and o-dihydroxybenzene compounds, and hydroxynaphthalene compounds reacted very slowly or not a t all. Mono-substituted resorcinols with the meta position relative t o each hydroxyl group occupied, such as orcinol and phloro-
glucinol, were nonreactive; those with the meta position unoccupied gave colored reaction products. These reactiye m-dihydroxylaryl compounds are listed in Table I, in order of decreasing rate of reactivity, with the absorbance data for the dye produced in the presence of zinc and aqueous ammonia. The compounds considered most promising as analytical reagents for zinc were 4chlororesorcinol resorcinol, 4-bromoresorcinol, 2,4-dihydroxyacetophenone, 2,4-dihydroxybenzaldehyde, and 2,4-dihydroxybenzoic acid, because in each instance the absorbance peaks of the blank and the test solution occur at different wave lengths. Bey and Faillebin (4) discovered that the reaction of resorcinol with certain cations in the presence of ammonium hydroxide required oxygen. I n the present investigation the effect of oxygen absorption from the atmosphere was controlled by use of reaction vessels of uniform shape containing a fixed volume of solution. S o attempt was made to prevent loss of ammonia from the reaction, because changes in ammonium hydroside concentration within defined limits do not affect the reaction. COMPARISON OF 4-CHLORORESORCINOL, 4-BROMORESORCINOL, AND RESORCINOL
The t h e e n i x t reactive compounds reported in Table I-resorcinol, 4bromoresorcinol. and khlororesorcinol --nere compared, to determine the
RESORCINOL
m 0.6 r
4- CHLORORESORCINOL
0.2
I
I 2
I
I
C
3
ML. 5% REAGENT
Figure 1. Effect of reagent concentration on the absorbance produced by 9 y of zinc Reaction Time, Hours 2 1 2 1
-X-
O -
x----
o----
Ammonium Hydroxide Added, M e q .
7.5 7.5 3.75 3.75
Final volume 5 ml., 1-cm. fight path, resorcinol absorbance measured a t 690 mp, 4-bromoresorclnol a t 6 3 0 mp, and 4-chlororesorcinol a t 6 4 0 mp.
most effective reagent for the quantitative determination of zinc. Apparatus. All glassware was thoroughly cleaned with chromic-sulfuric acid cleaning solution and rinsed several times with ion-free water.
A Beckman Model B spectrophotometer equipped with a red-sensitive phototube was used for the absorbance measurements. Reagents. The reagents used for the developmental work are described below; 5% solutions of resorcinol, 4-bromoresorcinol, and 4-chlororesorcinol in ion-free water were used. The first two compounds were recrystallized in the same manner as 4-ch1ororesorcino1, except t h a t benzene was used as a solvent instead of carbon tetrachloride. The resorcinol and 4-chlororesorcinol were purchased and the 4bromoresorcinol was synthesized (6). Procedure. T o obtain data for comparison of the resorcinol, 4-bromoresorcinol, and 4-chlororesorcinol reagents, known amounts of zinc, ammonium hydroxide, and the reagent were placed in graduated centrifuge tubes, and the volume was made to 5 ml. with ion-free water. Absorbance was measured in the spectrophotometer relative t o a reagent blank a t a predetermined wave length after the times specified in Figures 1,2, and 3. Results. Figure 1 s h o w the effect of reagent concentration in the presence of 9 y of zinc at two concentrations of ammonium hydroxide after 1- and 2hour reaction. I n the presence of 1 ml. of 5y0 4-chlororesorcinol reagent there were only minor changes in absorbance with variations in reaction time and ammonia concentration. I n the case of resorcinol and 4-bromoresorcinol, wider variations exist, regardless of reagent concentration. Typical calibration curves for zinc employing resorcinol and 4chlororesorcinol after various reaction times, in the presence of different ammonia concentrations, are plotted in Figure 2. Because of scattered results the data for 4bromoresorcinol were not included.
Table I. Asymmetrically Substituted Resorcinols with Positive Reaction toward Zinc Ion in Presence of Ammonium Hydroxide
Wave Length of Maximum Absorbance, l l l p Blank With zinc 590 640 600 600-695 580 630
Compounds 4Chlororesorcinol Resorcinol &Bromoresorcinol 2,PDihydroxyacetophenone 550 2,PDihydroxybenzophenone 610 B-Resorcylaldoxime 650 2,PDihfdroxybenzaldehvde 550 2,4-Dih$droxybenzoic acid 550 (2,4Dihvdroxy. phenyfazo)benzene sulfonic acid 490 Arranged in order of reactivity.
620 610 650 610 570 490 decreasing
The color reaction of zinc with 4-chlororesorcinol is superior to that with resorcinol, as the former follows Beer's law over twice the range of zinc concentration. From 3.75 to 7.5 meq. of added ammonium hydroxide has no effect on the calibration curve for 4chlororesorcinol, other than to extend the range of linearity from approximately 20 to 25 y of zinc. Figure 3 shows the effect of ammonium hydroxide concentration and reaction time upon the zinc4chlororesorcinol color reaction. Essentially constant sensitivity is obtained over the range of 3.75 to 8 meq. of ammonium hydroxide and reaction times of 1 t o 2 hours. I n the resorcinol and 4-bromoresorcinol systems, constant sensitivity exists only over a narrow range of
4- CHLORORESORCINOL
0.8
Y
0.4
2m
0
5
a2
,
I
I
10
15
20
Y
ZINC
Figure 2. Calibration curves for resorcinol and chlororesorcinol
-X
Reaction Time, Hours 2
-0
1
x----
25
4-
Ammonium Hydroxide added, M e q . 7.5 (5.25 in case of resorcinol) 7.5 (5.25 in case of resorcinol)
2 3.75 1 ml. 676, 4-chlororesorcinal, final volume 5 ml., 1-cm. light path, absorbance measured a t 640 mp. 1 ml. 10% resorcinol, final volume 5 ml., 1-cm. light path, absorbonce measured at 6 9 5 mp,
0.2
s'
I'
I
I
I
I
I
2
4
6
8
IO
MEQ. A M M O N I U M HYDROXIDE
Figure 3. Effect of ammonium hydroxide concentration on 4-chlororesorcinol system for 9 y of zinc Reaction Time, Min.
X 00-
120 90 60
1 ml. 5% 4-chlororesorclnol, flnal volume 5 ml., 1-cm. light path, absorbance measured a t 640 mp.
VOL. 30, NO. 3, MARCH 1 9 5 8
a
465
Table II.
Interference Coefficients for Cations upon Reaction with 4-Chlororesorcinol a t 25-7Level
Metal Zinc Nickel Magnesium Aluminum Cobalt
Coefficient of Interference X 100 at 640 M p
Copper
Cadmium Calcium Manganese Iron (ic) Lead Iron (ow)
100
121 93 77
69 58 57 51 47 25
23 11
ammonium hydroxide concentration, after 90 minutes. A major drawback to the use of resorcinol as a colorimetric reagent for zinc is that the wave length of maximum absorbance shifts from 630 to 695 mp as the reaction proceeds. When the absorbance peak lies a t 695 mp, the reaction is complete. This shift was not found in similar studies using 4chlororesorcinol. Interferences. The cation interferences for 4-chlororesorcinol and 4-bromoresorcinol are very similar t o those reported for resorcinol (14). Because of the superior analytical properties of 4-chlororesorcinol as a reagent for zinc, the extent of these interferences is given only for this compound expressed as an interference coefficient, defined as: absorbance for 25 y of interfering cation measured a t 640 mp divided by the absorbance for 25 y of zinc measured a t 640 mp. The reaction time was 2 hours and a blank correction was made in each instance. Table I1 includes only metals found to have appreciable reaction a t the 1000-y level. It also contains information pertaining to the wave lengths of maximum absorbance for these metal ions, the relative width of the absorbance spectra, and the sensitivity a t the wave length of maximum absorbance. Metals which did not react include sodium, potassium, silver, bismuth, and mercury. The relative magnitude of the sensitivity values a t the wave length of maximum absorbance shows that the color reactions with nickel, magnesium, aluminum, cobalt, copper, and manganese were mQre intense than with cadmium, calcium, iron, and lead. Of the farmer group, nickel has the greatest interference, as its maximum absorbance is only 5 mp from that for zinc. Although the remaining metals in the first group react nearly as strongly as nickel, their interference is not so pronounced, because their peak absorbances are all a t a shorter wave length than that for zinc. Because zinc exhibits appreciable ab-
406
ANALYTICAL CHEMISTRY
Max. Absorbance, Mr 640 (broad) 645 (broad) 630 (broad) 605 (broad) 580 (sharp) 620 (sharp) 640 (broad) 670 (broad) 620 (sham) 560 (broa'dj 600 (broad) 570 (broad)
Sensitivity at Peak Absorbance, y/Ml./Cm. of Light Path 0.29 0 32
0 26
0 27 0 46 0 32 0 14
0 14 0 19 0.12 0 04 0 06
14-
fm
1
550
SEPARATION OF ZINC WITH SODIUM ETHYLDITHIOCARBAMATE
\
/
,/
\
600
650
dependent on the quantity of zinc present, another reaction unrelated to zinc concentration continues to produce coloration with a peak absorbance a t 590 mp. For this reason, it is necesary to measure the absorbance of the sample and the blank after the same reaction time. That cations are complexed in some fashion is indicated by the fact that the mnsimum absorbance for the cations listed in Table I1 occurred a t different wave lengths and the contours of their spectra varied markedly. The formation of a complex between the cation and the dye produced is consistent with the findings of Bey (4) for resorcinol. Bey also mentions that, in the case of resorcinol the blue dye formed in the presence of different cations is the same, thereby postulating a catalytic oxidation mechanism for the metal. However, if the reaction %-as one of catalytic oxidation alone, all cations would create a dye with the same absorbance spectrum, and the rate of dye formation, not the amount, would be dependent upon the concentration of zinc,
700
MP
Figure 4. Absorption spectrum of zinc4-chlororesorcinol-ammonium hydroxide system
sorption a t wave lengths up to 700 mp, the absorption due to zinc could be measured in this region and interference due to the manganese, cobalt, and copper with sharp absorbance spectra a t 620, 580, and 620 mp, respectively, would be cronsiderably reduced. However, such a change would produce a loss in sensitivity. Absorption Spectra of 4-Chlororesorcinol System. The absorption apectrum for the 4-chlororesorcinolammonia-zinc system is shown in Figure 4. The wave length of maximum absorbance is 590 mp in the absence of zinc and 640 mp in its presence. This shift in absorption peak, and the linear relationship between the zinc concentration and the absorbance a t 640 mp (Figure 2), indicate the existence of a zinc-dye complex. Although the absorbance reaches a value
DI-
The 4-chlororesorcinol reagent described for the colorimetric determination of zinc requires a procedure that will isolate zinc from certain interfering cations, solutions of high ionic strength, and oxidizing or reducing agents. As samples received in this laboratory are usually of biological origin, a method was developed for the isolation of zinc from the ash of such materials. Two procedures were considered for the separation of zinc from the ash of biological materials: ion exchange and extraction of aqueous solutions of zinc with organic solvents in conjunction with one or more complexing agents. In the ion eschange procedures, zinc is usually absorbed onto an anion resin as the chloride complex, and then eluted with very dilute hydrochloric acid, nitric acid, and/or water (10, 16, 16). The more diikult separation of zinc from cadmium may be accomplished by absorption of the cadmium iodide complex in the presence of sulfate on an anion resin @, IO). Experiments in this laboratory using anion exchange resins for the separation of zinc from the ash of biological material gave poor recoveries. Therefore, this work was discontinued in f$vm Q$ an extraction procedure. The extraction methods which use dithizone as a reagent for the separation or colorimetric determination of zinc, or both, are numerous. Many of them are described by Sandell (17). In the early dithizone extraction procedures thiosulfate or cyanide was used to complex the interfering cations. C o w h g and Miller (8) suggested the use of
-
sodium diethyldithiocarbamate as a complexing agent for zinc in the dithizone method. Their method successfully separates zinc from most metals except cadmium, but recovers only approximately 80% of zinc. The diethylammonium salt of diethyldithiocarbamate has also been used as a reagent for the isolation of zinc from most cations except bismuth ( 1 ) . Heinnen and Benne (9) simplified the Cowling and Miller procedure, but blanks are T-ariable (3). Recently, the procedure has been further modified by Verdier, Eteyn, and Eve (29). Cholak, Hubbard, and Berkey ( 7 ) replaced dithizone with its homolog, di-2-naphthylthiocarbazone and found that the zinc is 100% recoverable, even \Then diethyldithiocarbamate is used as the complexing agent. Martin (23) has shown that either di-2naplithylthiocarbazone or diethyldithiocarbamate is effective in separation of zinc from cobalt and nickel; the work of La Coste, Earing, and Wiberley ( l a ) suggests diethyldithiocarbamate as capable of separating zinc from a still greater number of cations. CRITERIA
FOR
SELECTING DIETHYLDITHIOCARBAMATE
llurtin ( I S ) reported the use of dietliyldithiocarbamate or di-2-naphthyltliiocarbazone for separating zinc from cobalt and nickel. Thus, it seemed that the method of Cholak, Hubbard and Berkey ( 7 ) , which employs both these reagents] could be simplified to use only one reagent. To test this hypothesis, experiments were carried out to determine the distribution of metals during extraction. An aqueous solution containing the metals listed in Table I11 was buffered with ammonium citrate and ammonium hydroxide a t pH 9.8; the final solution contained approximately 1% citrate. The solution was then repeatedly extracted with 10-ml. portions of di-2naphthylthiocarbazone in chloroform until the chloroform layer no longer changed color. The combined chloroform extracts were extracted several times with 0.16M hydrochloric acid. The metals retained in the initial aqueous buffered solution, the chloroform extract solution, and the final hydrochloric acid extract were determined qualitatively by spectrographic analysis. The effectiveness of diethyldithiocarbamate in isolating zinc from other cations was also investigated in a similar fashion, except that the aqueous phase m-as buffered t o p H 8.5. Table I11 shows that the carbamates of zinc, cadmium, lead, bismuth, cobalt, thallium, antimony, and silver were completely extracted from the aqueous layer into chloroform a t pH 8.5. Several other metal carbamates were partially extrncted under these conditions. Only zinc and cadmium were reextracted by shaking the chloroform
Table 111. Spectrographic Analysis of Residues and Extracts from Diethyldithiocarbamate and Di-2-naphthylthiocarbazone Methods
Metals in Buffered hqueous Solution Zn Cd Pb Bi co T1 Sb Cr Fe Mg
A1
Ca Ni
cu
Na K
Extraction by CHCla Containing Di-2-naphthylDDTC thiocarbazone Complete Complete Complete Complete Complete Complete Complete Complete Complete Complete Complete Not extracted Complete Not extracted Partial Complete Nearly complete Extracted Kearly complete Extracted Partial Extracted Partial Extracted Partial Partial Partial Partial Partial Partial Xot extracted Not extracted Not extracted Not extracted
solution with 0.16M hydrochloric acid. With di-2-naphthylthiocarbazone zinc, cadmium, lead, bismuth, and cobalt were completely extracted by chloroform; zinc, cadmium, and lead were completely re-extracted by 0.16M hydrochloric acid, while bismuth was partially extracted. As diethyldithiocarbamate is commercially available to a higher degree of purity and separates zinc from more of the cations ordinarily found in the ash of biological material than does di-2naphthylthiocarbazone, it was selected for the isolation of zinc. Factors Affecting Reagent Blank. By employing 4-chlororesorcinol as the colorimetric reagent for zinc it was discovered that impurities in stopcock grease and redistilled chloroform contributed t o variable blanks. During the extraction of zinc diethyldithiocarbamate into chloroform from aqueous solution, some of the stopcock grease with its attendant impurities dissolved in the chloroform. Of the greases tested, only Apiezon hI was reasonably satisfactory. By stepwise elimination, it was found that impurities present in redistilled chloroform also contributed to variable blank values. Blank values were determined on O.16M hydrochloric acid extracts which had been shaken with samples of chloroform previously purified in different ways. The results showed that the impurities in redistilled chloroform, which produces coloration in the 4-chlororesorcinol blanks, may be removed by extracting the chloroform with 0.16M hydrochloric acid prior t o use. This series of tests was carried out in hlojonnier flasks, to eliminate possible contamination from stopcock lubricants. EXTRACTION AND DETERMINATION OF ZINC
Reagents.
Store all reagents in
Re-extraction by 0.16M Hydrochloric Acid from CHCl, Extract Containing Di-2-naphthylDDTC thiocarbaxone Complete Complete Complete Complete Complete Not extracted Partial Not extracted Not extracted Not extracted Not extracted Not extracted Not extracted Not extracted Xot extracted S o t extracted Not extracted Kot extracted Not extracted Not extracted Kot extracted Not extracted Not extracted Not extracted Not extracted Kot extracted S o t extracted Kot extracted Not extracted Xot extracted Not extracted Not extracted Not extracted Not extracted
polyethylene containers to avoid contamination from traces of metals. IOK-FREE WATER. Pass distilled water through a borosilcate glass column (3 X 75 cm.) containing Amberlite MB-3, analytical grade, mixed-bed exchange resin, a t the rate of 30 to 60 drops per minute. HYDROCHLORIC ACID, 0.16M. Prepare zinc-free 0.16M hydrochloric acid by diluting constant boiling hydrochloric acid with ion-free water. CHLOROFORM. Dry reagent grade chloroform over a mixture of anhydrous calcium chloride and calcium oxide for a t least 3 hours. Filter and distill in a suitable fractionation unit. Collect the distillate between 61.0' to 61.5" c. Add 1% absolute ethyl alcohol and store in a dark bottle in a cool place. Prior to use, wash the redistilled chloroform four to five times with 0.16M hydrochloric acid, and once with ion-free water, using 1 part of acid or water to 5 parts of chloroform. Perform this latter operation in a Mojonnier flask, to eliminate stopcock lubricants, and prevent the chloroform from becoming contaminated with grease. STOPCOCK LUBRICANT.Use Apiezon M or other high quality petroleum-base grease to lubricate the separatory funnel stopcocks. Do not employ rubber-base or silicone greases. If high blanks are obtained because of impure grease, purify the grease by dissolving it in chloroform and mash the solution three to four times with 0.1611f hydrochloric acid, and twice with ion-free water. Discard the washings, and recover the lubricant by removing the chloroform under vacuum. SODIUM DIETHYLDITHIOCARBhhLTE (DDTC), 2y0. Dissolve approximately 2 grams of Eastman Kodak reagent in 70 to 80 ml. of ion-free water. Filter the solution through Whatman No. 42 paper] and make the volume up to 100 ml. Prepare this reagent fresh daily. L4hIMONIUM CITRATE BUFFERSOLUTIONS, 5y0AND 2%. Dissolve 25 grams of reagent grade citric acid monohydrate in 300 ml. of distilled water. Adjust VOL. 30, NO. 3, MARCH 1958
407
Table IV.
Recovery
of Zinc from Solutions Containing Other Metal Ions
(Micrograms of metal ions added) Error, Sample 1 2 3 4 5
Ni
Fe
...
1000
. I .
... 200 200
...
6 7
...
Go
...
...
... 300 1000 600 300 300
100
200 200 100 100
cu
... ...
200 100 300 200
200
Ag
Mg
Ca
Sn
A1
...
. . 300
... 1000 500
... ... 100 ...
... ...
...
100 ... 100
100 100
...
, . .
300
300 300
500
500
500
...
100
100
200 ... 500 200 200
Zinc, y Added Found 0 0.9 0 0.3 5 6.6 10 11.1 20 23.2 30 30.0 50 50.7
Y
Zinc 0.9 0.3 1.6 1.1
3.2 0.0
0.7
COLORIMETRIC DETERMINATION OF
Table V.
Recovery of Added Zinc
Agar-ilgar Sample No. 1
2 3
4 5
Added ... 14.9 ... 25.3
...
37.3 62.9
...
from Agar-Agar Ash Samples Containing Zinc
Zinc, P.P.M.
Recovery,
Found 15.4 30.4 17.4 39.4 14.1 51.8 16.1 74.7 14.4 91.4
70
100:7
... 87.0 100: 1
ZINC. Transfer 1 to 2 ml. of the acid extract to a 15-ml. graduated borosilicate glass centrifuge tube with a conical bottom. Add 1ml. of 1 to 1 ammonium hydroxide and 1 ml. of 4-chlororesorcinol reagent, and dilute to 5 ml. with ion-free water. After 2 hours, measure the absorbance a t 640 mp, in 1-cm. cells relative to a blank obtained by carrying ion-free water through the complete procedure.
...
93.2
...
RESULTS AND DISCUSSION
The procedure for extraction and determination of zinc was tested with standard zinc solutions, solutions containing many metal ions, and solutions of the ash of agar-agar. solution containing not more than the pH to 8.5 k 0.1 with 1 to 1 ammoWith standard zinc solutions, the 60 y of zinc in a 100-ml. beaker. Add nium hydroxide, and dilute t o 500 ml. recovery was investigated a t the 15- and a few drops of concentrated sulfuric This reagent is usually free of zinc 50-7 levels. The average recovery n‘as acid and 5 to 10 ml. of ion-free water. and does not require purification. Bring the solution to a boil on a hot 15.0 y, with a coefficient of variation of If zinc is present, it may be removed plate, and cool. Transfer the solution as follows: Prior to use, for each sample, 3 ~ 1 . 4 %and ~ 48.5 y, with a coefficient of to a 125-ml. separatory funnel (Squibb dilute 10 ml. of the 5% citrate buffer variation of =!~2.2%~respectively. pear-shaped), using 5 to 10 ml. of ionsolution to 25 ml., add 1 mi. of 2% Table IV shows the recovery of zinc free water to rinse beaker. Finally, diethyldithiocarbamate, wash twice with by the extraction procedure carried out neutralize the acidic solution to phenol10 to 15 ml. of purified chloroform, disin the presence of 100 to 1000 y of a phthalein “pink” with 1 to 1 ammonium card the chloroform, and retain the number of metal ions. The solutions hydroxide and make the volume to 25 citrate solution. contained larger amounts of nickel, iron, nil. with ion-free water. AMMONIUM HYDROXIDE,1 to 1. EXTRACTION OF METALCARBAMATES. cobalt, copper, silver, magnesium, calDilute C.P. reagent grade ammonium Add 10 ml. of 5% citrate buffer or 25 hydroxide with an equal volume of cium, tin, and aluminum than would ml. of purified 2% citrate buffer and ion-free water. normally be encountered in the ash of 2 ml. of 2y0 diethyldithiocarbamate to STANDARDZINC SOLUTIONS.Dry biological samples. Cadmium, which the separatory funnel containing the zinc sulfate monohydrate overnight a t interferes with the procedure a8 desample, and make the final volume to 135’ C. Dissolve 1.3724 grams in veloped, was not included as it does not approximately 50 ml. Extract this ion-free water containing a few drops occur in interfering amounts in this type aqueous solution three times with 5-ml. of dilute hydrochloric acid, and dilute of sample. The acid extract containing portions of purified chloroform; shake to 1 liter. This stock solution contains zinc was washed only once with chlorofor a t least 1 minute and allow ample 500 y of zinc per ml. Prepare other form; however, a second chloroform time for the layers to separate. Transstandards by dilution as required. fer the chloroform extracts to a second wash reduces the possibility of high 4-CHLORORESORCINOL16% W./V. The 125-ml. separatory funnel. Discard shelf life of the dry chemical is limited results. the aqueous solution in the first separato a few months unless purified as folThe procedure was also tested on the tory funnel, rinse the funnel thoroughly lows. Dissolve 10 grams of 4-chloroash of a number of agar-agar samples, with ion-free water, and retain this resorcinol in 1 liter of boiling carbon which contain significant amounts of funnel for use in the next step. Wash tetrachloride. Decolorize the solution zinc. Samples were analyzed with and the combined chloroform extracts once with SO-mesh activated coconut charwithout added zinc (Table V). Over with 3 ml. of ion-free water, transfer coal, and filter hot through Whatman the range of 15 to 80 y of added zinc the the chloroform to the first separatory No. 41 H filter paper. Crystallization procedure gave recoveries ranging from funnel, which should be dry, and discard occurs spontaneously upon cooling; the wash water. 87 to 101% with an average of 95.4%. when complete, remove the solvent by EXTRACTION OF ZINC. Add exactly decantation. Air-dry the pure white As Zincon has been reported recently 3 ml. of 0.16M hydrochloric acid to the crystals. Dissolve 6 grams of the as a colorimetric reagent for zinc (16), funnel containing the chloroform exrecrystallized product in ion-free water tests were carried out using the diethyltracts, shake for 1 minute, and allow and dilute to 100 ml. dithiocarbamate extraction procedure, Procedure. TRANSFER OF SAMPLES. the layers to separate. Remove the followed by zinc estimation with Zincon. chloroform, and wash the acid layer Digest a sample of biological material Table VI shows the recoveries of zinc once with 10 ml. of purified chloroform, with nitric, sulfuric, and perchloric from standard solutions and from the retaining the 3 ml. of 0.16M hydroacids t o destroy organic matter. ash of agar-agar with and withoutladded chloric acid. Place a known quantity of the sample 80.3
Av.
408
ANALYTICAL CHEMISTRY
95.9 95.4
Table VI. Recovery of Zinc from Standard Solutions and Agar-Agar Ash with Zincon after Diethyldithiocarbamate Extraction
Zinc, Recovery, Added Found Standard solutions
41 50 200 250 300 500
42.7 50.3 190 250 283 470
104.0 100.6 95.0 100.0 94.2 94.0
Agar-agar
...
30 127.5 32 126.5
...
100
...
100
97.5
...
94.5
form extract is highly colored, the 0.16M hydrochloric acid extract should be washed a t least twice with chloroform t o reduce contamination. ACKNOWLEDGMENT
The authors wish to thank J. H. Mahon and R. A. Chapman of this laboratory for their kind encouragement and criticism of this work. LITERATURE CITED
Analyst 73, 304 (1948). Baggott, E. R., Willcocks, R. G. W., Ibid., 80, 53 (1955). Benne, E. J., J . Assoc. OBc. Agr. Chemists 38, 403 (1955). Bey, L., Faillebin, M., Compt. rend. 188, 1679 (1929).
zinc. Recoveries ranged from 94 to 104% with an average recovery of 97%. For optimum results, the system should be adjusted so that the first chloroform extract of the aqueous layer removes all the colored carbamates; the concentration of citrate should not exceed 1% (13, 20) and, if the chloro-
Blatt, A. H., “Organic Synthesis,” Coll. Vol. 11. D. 100. Rilev. *, New York, 1943. Cerdan, A. C., Puente, J., Anales A
SOC.
espa6,fis. qutm. 1 1 , 98 (1913).
Cholak, J., Hubbard, D. M., Burkey, R. E., IND.ENG.CHEM.,ANAL. ED.15,754 (1943). Cowling, H., Miller, E. J., Ibid., 13, 145 (1941).
Heinnen, E. J., Benne, E. J., J . Assoc.
Ofi.Agr. Chemists 35, 397 (1952).
Hunter. J. A.. Miller. C. C.. Analvst 81, 79 (1956). Jones, O., Jones, T. IT7., “Canning Practice and Control,” p. 92, Chapman & Hall, London, 1937. La Coste, J. R., Earing, M. H., Wiberley, S. E., ANAL.CHEM.23, 871 (1951).
Martm, A. E., Ibid., 25, 1853 (1953). Mellan, I., “Organic Reagents in Inorganic Analysis,” p. 620, Blakiston, Philadelphia, 1941. Miller, C. C., Hunter, J. A., Analyst 79,483 (1954).
Rush, R. M., Yoe, J. H., ANAL. CHEM.26, 1345 (1954). Sandell, E. B., “Colorimetric Dete:; mination of Traces of Metals, Interscience, Kew York, 1944. Streichowa, E., Roczniki Palistwowego Zakladu Hig. I, 205 (1950). Verdier, E. T., Steyn, W. J. A., Eve, D. J., J . Agr. Food Chem. 5,354 (1957).
Walkley, A., private communication, to A. E. Martin. RECEIVEDfor review April 29, 1957. Accepted December 3, 1957. Presented in part at Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 1956.
Rapid and Precise Carbon-Hydrogen Determination Auto ma tic Macrocombustion Apparatus T. T. WHITE, V. A. CAMPANILE, E. J. AGAZZI, L. D. TeSELLE, P. C. TAIT, F. R. BROOKS, and E. D. PETERS Shell Development Co.,Emeryville, &/if.
,An automatic combustion apparatus has been developed to permit rapid and precise carbon-hydrogen determination with a minimum of operator training and attention. The sample is alternately heated with a bare resistance wire heater and cooled with an air blast, to maintain the desired vaporization rate, while a stream of air flows over the sample. The heater and air blast are controlled by a mercury manometer that monitors pressure changes in the combustion tube. The sample is vaporized rapidly; the combustion cycle is completed in about 20 minutes and up to 24 analyses can be completed in a single d a y with a dual combustion unit. Standard deviation values of 0.03% carbon and 0.02% hydrogen were obtained on a variety of materials. A number of innovations contribute to ease and reliability.
0
numerous analytical methods employed in a petroleum and chemicals research laboratory, the determination of carbon and hydrogen ranks among those in continuous and high demand. Early attempts in this F THE
laboratory to improve existing methods for the macrodetermination of carbon and hydrogen culminated in the development of a dual, unitized apparatus and procedure capable of producing results of high precision and accuracy (12). Significant advances have since been made in both apparatus and technique and this report summarizes the work at its present stage. The development work has proceeded along lines intended t o reduce maintenance problems, time of analysis, and operator training requirements, and increase range of applicability. One of the shortcomings of the apparatus (12) was that close attention of a highly skilled operator was required for adequate control of the combustion rate. When the sample was vaporized too rapidly, an explosion sometimes shattered the combustion tube. Even if there was no explosion, the momentary depletion of the excess oxygen supply in the catalyst section of the combustion tube resulted in incomplete oxidation of the carbon. The sample vaporization rate was controlled by two means, both fallible. One involved observation of
the rate of reduction, by sample vapors of a heated strip of oxidized copper gauze placed in the combustion tube ahead of the filling; the other, measurement of the excess oxygen by a rotameter as exit gas emerged from the absorber train. I n the first case when samples contained metals, sulfur, or halogens the copper oxide gauze became discolored and reduction could no longer be observed. In the second case the time lag in the gas flow through the tube filling and absorbers was such that the combustion could be out of control before the excess oxygen flow dropped below the prescribed limit. Indeed, a momentary rise in the flow rate due to expansion of the sample vapors usually added to the unreliability of this means of control. These difficulties have been eliminated by modifications in the tube packing and the mode of introducing excess oxygen into the tube. A roll of copper gauze is placed in the section of the combustion tube formerly occupied by the indicator strip and the tube introducing a secondary supply of oxygen is extended into this roll through an opening in its center. The copper is heated VOL. 30, NO. 3, MARCH 1958
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