Zr+4, borate, Br-, C W , CNS-, citrate, F-, oxalate, PO4+, SiOs-l, sod-’, and tartrate. The ions which interfereand should be absent may be classified as: (1) reducing ions, which reduce selenious acid to elemental selenium (thwe include Fe+2, Sn+2,S-2, s203-2,S03-*, I-, As+3,Sb+3, and ascorbic acid) ar!d (2) oxidizing ions, which oxidize the reagent (these include Cr04-+, IOs-, C103-, c104-, Mn04-, &os-, hf0Ci3-, VOa-, and Fe+3). Hg+ and Ag+ precipitate as their chlorides. PtCI6-2, F’e+3, A u + ~ ,and inetavanadate form precipitates with 1-naphthylamine. The presence of Cu+2 causes low results, presumably owing to its catalysis of the decomposition of the diazonium salt. C O +and ~ Cr+3 interfere when more than 40 p.p.m. is iresent in the solution because of their own absorbance a t 520 inp. Selenium(V1) appears to react like selenium(IV), only perhaps more slowly. Tellurium(V1) does not interfere, even when a 100-fold excess is present. A 50-fold excess of tellurium(1V) can be tolerated. The limiting value of the concentration of foreign ion was taken as that which caused an error of 30/, in the absorbance in the deterxination of 0.4 p.I).m. (10 pg.) of selenium. This value corresponds to an error of 0.009 absorbance unit (three times the standard deviation). The liniiting concentrations of some of the most) important interfering ions are shown in Table II. Elimination of Interferences. The
Table 111. Determination of Selenium in Mixed Salt Solutions
Determination of Selenium in Presence of Tellurium Se(IV) Te(1V) Se(1V) Error, found, present, added, rg. 5 10 15 20 25
lrg.
rg.
7%
200 200 600 400 800
4.9 9.9 14.6 20.4 25.2
-2.0 -1.8 -2.7 +2.0 +0.8
Determination of Selenium in Mixed Salt Solut.ions Added, Found,
Ion
a.
cu+*
1000 2000 100 2000 1000 100 1000 1000 4000 200 1000 2000 500 300
c u +a Fe +3 Se +4 c u +z Fe +3 Te + * Se +4 c u +2
Fe + 3 Te;,“ Se
rg.
(37 100
205
ducing agent. It was not possible to eliminate the interference of iron and copper with complexing agents in the highly acid medium used. Satisfactory results were obtained, however, when iron and copper were extracted into chloroform as their cupferron complexes from hydrochloric acid solution (4). Tellurium(1V) interferes when a large excess is present, but a concentration of 20 p.p.m. can be tolerated. Results for the determination of selenium in synthetic solutions containing tellurium and selenium, and in solutions containing iron, copper, and tellurium after extraction of iron and copper as their cupferron complexes, are shown in Table 111.
-3.0
0
+2.5
LITERATURE CITED
(1) Blorn, J., Ber. 59, 121 (1926). (2) Boltz, D. F., “Colorimetric Determination of Nonmetals,” Interscience, New York, 1958. (3) Feigl, F., Demant, V., Mikrochzm. Acta 1, 134 (1937). (4) Furman, N. F., Mason, W. B., Pekola, J. S., ANAL. C n m . 21, 1325 (19491 \ - - - - I
304
+1.3
principal intcrferences, from a practical viewpoint, are iron and copper. Iron(II1) interferes more seriously than iron(I1); attempts to eliminate the interference of small amounts of iron(II1) by reduction to iron(I1) were unsuccessful, however, because of the difficulty of removing excess re-
(5) Griess, P., Ber. 12, 427 (1879). (6) Hahn, F. L., Jaeger, G., Ibid., 58, 2335 (1925). (7) Kolthoff, I. M., Elving, P. J., “Treatise on Analvtical Chemistrv.” Part 11.’ T’d. 7. Interscience. New Y6;k. 1961.
tkrmination of Traces of Metals,” 3rd ed., Interscience, New York, 1959. RECEIVED for review December 17, 1962. Accepted March 8, 1963. ~
Cation Exchange Separation of Lead JAMES S. FRITZ and I K H A R D G. GREENE Institute for Atomic Research and Department o f Chemistry, Iowa State University, Ames, Iowa
b Lead(l1) can b e separated from most metal cations by elution from a hydrogen-form cation exchange column with 0.6M hydrobromic acid. Precipitation of lead bromide i s prevented b y use of a heated column. Interference from bismiJth(l1l) and cadmium(ll) can b e avoided b y prior elution with dilute hydrochloric acid; tin (IV) is removed by elution with dilute hydrofluoric acid or by, volatilization.
S
ION EXCHANQE methods for the separation of lead have been published during the 3ast few years. Each method has its advantages and its limitations. Lead is eluted from an anion exchange column by hydrochloric acid eluents, 8M or greater. Lead, however, is retained a t lower hydroEVERAL
chloric acid concentrations ( I O ) . Many specific applications of this separation have been published. Nelson, Rush, and Kraus (11) also studied the anion exchange behavior of lead and other elements with hydrochloric acid-hydrofluoric acid mixtures. Minami and Ishimoi (9) separated lead from barium by elution from an ammonium-form cation exchange column with ammonium acetate a t pH 6. Tsintsevich and n’azarova ( I S ) separated lead from copper, iron, cadmium, and gallium by elution with ammonium acetate from a hydrogen-form cation exchange column. Lead has been separated from cobalt and manganese by elution from a cation exchange column with an eluent containing acetone, hydrochloric acid, and water in the ratio 93:1:6 ( 2 ) . Fritz and Rettig
(6) studied the elution of inany elements from cation exchange columns with aqueous hydrochloric acetone eluents, but did not include lead in their elution schemes. Klement and Sandmann ( 7 ) separated lead from gallium by elution with 0.8~Mhydrochloric acid from a cation exchange column. They also eluted antimony and indium nith 0.2-11 and 0.4M hydrochloric acid, respectively, before elution of the lead. Fritz and Garralda (4) separated mercury(II), bismuth(III), and cadmium(I1) from other metal cations by elution from a cation exchange column with 0.1 to 0 . M hydrobromic acid. Their work indicated that lead(I1) is eluted from the column with 0.6dl hydrobromic acid. We have found that lead can be separated from a large number of metallic elements by elution VOL. 35,
NO. 7,JUNE 1963
81 1
of lead(I1) from a hydrogen-form cation exchange column with 0.6M hydrobromic acid. Clogging of the ion exchange column, because of lead bromide precipitation, is avoided by prior precipitation of part of the lead, or by carrying out the separation on a hcated column. EXPERIMENTAL
Apparatus and Reagents. Conventional 12-mm. i.d. glass columns with coarse glass frits were used for the ion exchange separations. Plastic columns with an i.d. of 17 mm. were used for ion exchange separations in which
Table 1. Elutiono f Metal Ions with 0.6M Hydrobromic Acid from a 1.2- X 16-cm. Cation Exchange Column
Reagent or
indicator used for detection NAS
&!eta1 ion
Al(II1) Ba(I1) Bi( 111)
("&SO4
Xylenol orange Erio. black T NAS NAS H,O,-
Ca(I1) Cd(I1) Co(1I) Cr(II1)
NOH
NAS Arsenazo Splenol orange I'AS I'AS
Cu(I1) Dy(II1) Fe(111) Ga(II1) HdII) In(II1) Mg(I1)
XAS
Erio. black T NAS
Mn(I1) Ni( 11) Pb(I1) Sc(II1) Sn(1V) Sr(I1)
N9S
NAS Arsenazo NAS Methyl thymol blue Ti(IV) Cupferron UOZ( VI) Arsenazo VO(1V) NAS Xylenol Y(II1) orange Zn(I1) NAS Zr(1V) Xylenol orange
Elution
Breakthrough ml.
corn-
plete, ml.
...
>300 >300
..
>300
*..
0- 20 300-320 >300
... ...
0- 20
60
50
... ... ... ... 40 ...
240-250
>300
>300
>300 0- 20 50- 60 240-260
...
>300
290-310 50- 70 >300 20- 40 200-220
... ...
150
...
>75 ...
100-150 230-250 185 300
... ... ...
220-240 300
...
...
Table II. Elution o f Lead with 0.6M HBr from a 1.2- X 16-cm. Cation Exchange Column
Quantity
of lead on
column, mmoles
0.25 (52 mg.) 0.50 0.75 1 .oo 1.50 2.00
812
Elution Elution complete complete with heated with column unheated (50"-65" C.), column, ml. ml. 140-160 170-200 >200 >200 >300 >300
ANALYTICAL CHEMISTRY
140 140 160 185 215 225
hydrofluoric acid was the eluent. Dyne1 wool, obtained from Union Carbide Development Co., Division of Union Carbide Corp., New York 17, K.Y., was used to support the resin in the plastic columns. The heated column consisted of a conventional glass column wound with electrical heating tape. The temperature was controlled with a Variac. Dowex 50W-X8, 100- and 200-mesh resin in the hydrogen form, was used for all separations. The resin was prepared by placing approximately one pound of it in a large column and backwashing it until most of the fine particles had been removed. Three liters of 10% ammonium citrate, followed by 3 liters of HC1, were then passed through the resin. Finally, the resin was washed with water until a negative chloride test was obtained with a silver nitrate. The resin was poured into the ion exchange column as a slurry of resin in water until a resin height of 16 cm. was obtained. Except for titanium(IV), which was made up in sulfuric acid solution, 0.05M metal ion solutions were made up from either the nitrate or perchloric salts. A vanadium(V) solution in nitric acid was used; it was reduced to vanadium (IV) prior to being put on the resin. Procedures. E D T A solutions were standardized by titration of a standard zinc solution using Naphthyl Azoxine S (NAS) as the indicator as described by Fritz, Abbink, and Payne (3). Zinc(II), cadmium(II), cobalt(II), iron(II1) , nickel(I1), lead(I1), vanadium(IV), dysprosium(III), yttrium (111), gallium(III), copper(II), titanium(IV), and aluminum(II1) were all analyzed by titration with EDTA using XAS indicator ( 3 ) . Bismuth(III), tin(IV), and zirconium(1V) were analyzed by titration with EDTA using Xylenol Orange indicator, as described by Korbl and Pribil (8). Calcium was determined by back-titration of excess EDTA with magnesium using Eriochrome Black T indicator, as described by Barnard, Broad, and Flaschka (1). Strontium was determined by titration with EDTA using metalphthalein indicator as described by the same authors ( I ) . The analysis of uranium was carried out by the oxidation-reduction method of Sill and Peterson (12). The titration of lead with EDTA presents a problem because of the insolubility of lead bromide. Precipitation could be avoided by eluting the lead into approximately 0.1M ammonium acetate, or into about 200 ml. of water containing a measured excess of EDTA. The excess EDTA was then titrated with copper(I1) between pH 5 and 6 with NAS indicator. RESULTS
Elution of Lead. SEPARATION FROM OTHERMETALS. The elution behavior of 0.25 mmole of various metal ions with 0.6M hydrobromic acid, using a cation exchange column 1.2 X 16 cm., is described in Table I. The flow rate is 2 ml. per minute. Few inter-
ferences would be expected in the lead separation because most elements do not break through until after the elution of lead is complete. Bismuth (111) and cadmium(I1) can be eluted with dilute hydrochloric acid prior to the elution of lead with hydrobromic acid (4). Frequently, lead bromide precipitates on the ion exchange column during elution. This slows down the flom rate and also requires a larger volume than usual of hydrobromic eluent. With samples containing 0.25 mmole, or less, of lead, this difficulty can be largely avoided by diluting the sample to 50 ml. with 0.2~11hydrobromic eluent before adding it to the ion exchange column. Prior precipitation of lead in the sample as lead bromide reduces the lead concentration sufficiently so that no precipitate forms on the column. The precipitate is filtered and combined with the column effluent containing lead. This method is usually successful because there is very little coprecipitation of other metal ions with lead bromide. Column separations of lead from several other elements have been successfully carried out using the prior precipitation method. Samples containing 0.25 mmole of lead and 0.25 mmole of another metal ion have been separated for the following elements: aluminum(III), cobalt (11), copper(II), dysprosium(III), iron(III), gallium (111) , manganese(11), nickel(11), strontium(II), yttrium(III), zinc(II), and zirconium(1V). The average recovery of metal ion for the separation and analysis of 16 individual samples (32 individual determinations) gave an average recovery of 99.8% with a standard deviation of =t0.42. Lead chloride and lead bromide are much more soluble in hot water than they are a t room temperature. For this reason, the ion exchange separation was attempted using a heated column. The results a t a temperature of 50" to 65' C. were very successful. There is no precipitate whatsoever in samples containing 0.25 mmole of lead. Samples containing 0.25 to 1.5 mmole of lead had little, if any, precipitation. There is some difficulty with precipitate formation when the amount of lead exrceds 1.5 mmole. Data for the elution of various amounts of lead from a 16 cm. column is presented in Table 11. A heated column has little effect on the elution of other metal cations tested. Break-through of some metal ions is delayed a t the higher temperature, resulting in a more advantageous separation of lead from that element. For example, the breakthrough of U02+2 (0.5 mmole) occurred a t 230 ml. at room temperature and a t 300 ml. using a heated column. The breakthrough of zinc(I1) also comes slightly
1:tter on a heated coluliin. Quantitative data for the separation of lead from various other metal i m s are presented in Table 111. Separation of Bismuth, Cadmium, and Tin from Lead. Before elution of lead from the ion exchange column with O.6M hydrobromic acid, bismuth (111) and cadmium(lI), can be eluted with 0.5M hydrochloric acid. Since a miniinum of 100 ml. of hydrochloric acid is required in actual separations, 120 to 130 ml. is used t o elute the bismuth plus cadmium. A heated column is advantageous here also to avoid clogging the column with a precipitate of lead chloride. When elution with hydrochloric acid was attempted, tin(1V) behaved erratically. In some c:tses, tin could be removed completely with hydrochloric acid, but other times removal was very incomplete. Probably, hydrolysis of tin occurs. Became of this, other methods are recommended for the removal of tin. Tin can be volatilized from hydrobromic-perchloric acid solutions. By this method, fairly large amounts of tin can be removed. Our work shows that the recovery of lead was slightly low but the error was constant as the amount of tin increased. Another method that can be used to separate tin(1V) from lead(1I:i is by elution of the tin from a cation exchange column (hydrogen-form) with dilute hydrofluoric acid (6). To elute 0.25 mmole of tin from a 1.2- ;< 16-cm. column required 130 to 150 ni!. of 0.2M hydrofluoric acid. Analysis of Lead in Standard Alloys. Two Kational Bureau of Standards samples and a synthi.tic sample, made b y mixing known :mounts of lead, cadmium, and copper metal., were analyzed for lead. The tin and antimony were volatilized off as bromides, or, alternatc,ly, were separated by elution with hydrofluoric acid from a cation exchange colunin. The phosphor bronze alloy (9.35% Pb, 9.78% Sn, 77.96% Cu, 0.54% Sb, 0.47% Fe, 0.74y0 Zn) was analyzed according to the first method by taking a 1-gram sample, di+solving it with a mixture containing 9 ml. of 48% hydrobromic acid and 1 ml. of liquid bromine, adding 10 ml. of ptrrhloric acid and volatilizing the tin and antimony off, and filtering the residue. The resulting solution was diluted with water to 200 ml. in a volumetric flask. Then 20-ml. aliquot portions of 1his solution were diluted with water to approximately 50 ml. and put on the heated ion exchange columns. The solution has to be diluted when it, is out on the column to lower its acidity. After the sample was on the column, t i e lead b a s eluted with 0.6M hydrobromic acid, leaving the other metals on the resin. From
Table 111.
Separations Using 1.2-
Metal ions separated Pb(I1) Ba(1I) Pb(I1) Ca(I1) Pb(I1)
Cr(II1) Pb( I1j hTg(I1) Pb( I1j Sr(I1) Ph(1I) C(V1) Pb(I1) TT(VI) I'h( I1j V(IY) Pb(I1) Zn(I1) Pb(I1) Zn(I1) Pb(I1) Zn(I1) Pb(I1) Zn(I1)
X 16-cm. Cation Exchange Columns Heated to 50" to 65" C.
Eluent Type 0.6M HBr
ml.
Theorv
Actual
Differcnce, ml.
160
5.04 50 9 94 4 9 98
5.05
+0.01
9 92 49. 20 5.06
-0 02 -0 79 $ 0 02
...
. . 0 6AI HBr 3.0M HC1 0 6AI HBr . .
175 300
0 6Al HBr 3 0Jf HC1 0 6-11 HBr 3 O X HCl 0 6 M HBr 3 O M HCl 0 G J I HBr 3 0 A f HCI 0 631HBr 3 O J f HCl 0.6Jf HBr 3. 0JI HC1 0 6 M HBr 3 O M HC1 0 6.M HBr 3 O M HC1
160 200 160 200 200 I50 200 150 160 100 1 100 240 75 17.5
0.6M HBr 3 . O M HCl
17.5
0.05M EDTA, ml.
...
...
-1.3
Cr
/a
4 97 30 12 4 07 19 60 I 9 92 51 9:a 39.34 69.27" 4.97 9.68 2.98 29.61 39.34 3.97 14.76 29.72 19.67
19.83
4 . !J7
30.24 4 98 19.74 19 9 z 52 30a 3 9 38 69.23& 4 97 9 93 2 98 29 56 39.34 3.97 14.76 29,77 19.69 19.86
...
...
0 $0 $0 $0
to
00 12 01 08 02 33 04 04 00 0.5 00
+O +0 -0 0 $0 0 -0 05 0 00 0 00 0.00 +0.05 +0.02 1-0.03
Alilliliters of cerium(1S') solution.
Table I the volume of h j drobroniic acid necessary to elute the lead can be determined. Csually 25 to 50 ml. more than the minimum is a safe amount with which to elute. In the second method, a 0.5-gram sample of the phosphor bronze sample was dissolved in a platinum crucible with approximately 3 ml. of concentrated nitric acid, 0.7 ml. of concentrated hydrofluoric acid, and a few milliliters of water. An insoluble residue, thought to be antimony, was filtered off and washed. The filtrate and washings were diluted to 200 ml. with water in a volumetric flask. Theii, 20-ml. aliquot portions were taken, diluted with water to 50 ml., and put on the plastic columns. The tin and the remaining antimony were eluted with 200 ml. of 0.5M hydrofluoric acid. The lead was then eluted with 225 ml. of 0.6Jf hydrobromic acid. Because of the larger diameter of the plastic columns, larger volumes of eluting solutions were used than would be needed with the glass columns. The lead base sample (78.87% Pb, 10.91% Sn, 10.09% Sb, 0.06% Bi, 0.05% Cu, 0.05% Fe) was analyzed in a similar manner to the phosphor bronze alloy. In the volatilization method, only about 0.2 gram of sample was used because of the difficulty in driving off the antimony. The rolatilization procedure had to be repeated about six times to obtain a fairly clear solution. The insoluble oxide of anti-
Table IV. Analysisof Lead in Standard Samples
Lead, W analyzed Present Found5 Foundb Phosphor bronze alloy, 9.35 9 31 9.32 NBS, #63b Allojr
Lead bas?
alltry, NBS, #53
Sjmthetic alloy"
78.87
78.92
50 00
50 00
78.62
a Tin and antimony volatilizcd off as bromide. b Tin and antimony srparated from lead by elution with 0.531 HF. c Alloy composition: 505%Pb, 25% Cd,
257b cu.
mony that couldn't be volatilized was separated by filtering and then washed thorough!y with water and dilute nitric acid. The filtrate was diluted and put on the column. Bismuth was eluted off with 130 ml. of 0.5N hydrochloric acid, then lead was eluted off with 170 ml. of 0.6X hydrobromic acid. In the second method for the analysis of lead base alloy, 0.5 gram of the sample was dissolved in a platinum crucible with about 5 ml. of concentrated nitric acid, 0.4 nil. of concentrated hydrofluoric acid, and a few milliliters of water. The antimony coming out of solution was filtered and thoroughly VOL. 35, NO. 7, JUNE 1963
813
washed. The filtrate and washings were combined and diluted in a volumetric flask. The same procedure was then followed as for the phosphor bronze alloy. The synthetic sample of lead, cadmium, and copper was dissolved with dilute nitric acid, and the resulting solution was diluted to 200 ml. in a volumetric flask. Aliquot portions of 20 ml. were taken, diluted with water to about 60 ml., and introduced onto the ion exchange column. The cadmium was eluted with 130 ml. of 0.5M hydrochloric acid and the lead with 145 ml. of 0.6M hydrobromic acid. The assays
of the alloys and the results of the analyses are given in Table IV. LITERATURE CITED
(1) Barnard, A. J., Broad, W. C., Fla-
schka, H., “The EDTA Titration; Nature and Methods of End Point Detection,” .J. T. Baker Chemical Co., Phillipsburg, N. J., 1957. (2) Erkelens, P. C. van. AnaZ. Chim. Acta 2 5 . 42 (1961). (3) Fritz,‘ J. S., Abbinli, J. E., Payne, 31. A , , ANAL.CHEX 33, 1381 (1961). (4) Fritz, J. S., Garralda, B. B., Ibid.,34, 102 (1962). ( 5 ) Fritz, J. S., Garralda, B. B., Karralicr, S. K., Ibid.,33, 882 (1961).
(6) Fritz, J. S.,Rettig, T., Ibid.,34, 1562 (1962).
(7) Klement, R., Sandmann, H., 2.Anal.
Chem. 145, 325 (1955). (8) Korbl, J., Pribil, R., Chemist-Analyst 45, 102 (1956). (9) Minami, E., Ishimoi, T., J . Chem. SOC. Japan 74, 378 (1953). (10) Nelson, F., Kraus, II. A., J . Am. Chem. Soc. 76, 5916 (1984). (11) Nelson, F., Rush, R. lM., Kraus, K. A., Ibid., 82, 339 (1960). (12) Sill, C. W., Peterson, H. E., ANAL. CHEY.24, 1175 (1952). ( 1 3 ) Tsintsevich, E. P., Nazarova, G . E., Zavodslz. Lab. 23, 1068 (1957). RECEIVED for rcview December 17, 196%. Accepted March 7, 1963.
Colorimetric Reagents for the Analysis of Aluminum Alkyls D. F.
HAGEN and W.
D. LESLIE
Research and Development Department/ Continental Oil Co., P onca City, Okla.
-
b Triphenylmethane type indicators can be used to determine the equivalence points in the titration of aluminum alkyls with electron-donor reagents such as aromatic arnines, ethers, and alcohols. Deactivated species, such as dialkylaluminum alkoxides, do not form stable complexes with these reagents, and the “activity” determination i s a direct measure of the purity of the alkyl sample. Small amounts of the dialkyl hydride destroy the triphenylmethane-type indicators, rendering them irreversible in the titration. W e have found a number of compounds which form highly colored reversible complexes with aluminum alkyls and can be used as indicators for the volumetric determination of “activity.” The intensity of the color i s greatly increased when two or more azo-methine linkages are present in the indicator molecule. Reversibility of the alkyl indicator complex i s dependent on the position of the coordination sites and the relative basicity of the indicator with respect to the titrant.
T
HE
ALUMINUM
ALKYL
MOLECULE
is electron-deficient and can be considered a Lewis acid in complex formation reactions with electron-donating reagents. Oxidation or hydrolysis of one of the aluminum-carbon bonds results in a reduction in the electronseeking character of the entire molecule. Dialkylaluminum hydride, dial kylaluminum halide, and trialkylaluminum are active species, while the dialkylaluminum alkoxide and tetraalkyl814
ANALYTICAL CHEMISTRY
Figure 1. Activity cell for Beckman Model DB
aluminum oxide are inactive. Dialkylaluminum hydrides are unique in that they form 2 :1 as well as 1: 1 basehydride complexes with certain nitrogen compounds. I n this study a large variety of aluminum alkyls have been analyzed and it has been found that volumetric methods using visual indicators can be employed for the majority of these samples with good accuracy and speed. Bonitz (1) has used the red-colored 2: 1 isoquinoline-dialkylaluminum hydride complex as the end-point indicator in a volumetric isoquinoline titration for alkyl activity. Dialkylaluminum hydride must be added to samples Yhich do not contain hydride, however, so that a n endpoint can be obtained. The red-colored 2 : 1 isoquinoline-dialkylaluminum hydride complex is less stable than the yellow 1: 1 complexes formed with the aluminum-trialkyls, dialkyl halides, or dialkyl hydrides. LIitchen ( 2 ) utilized these stability
relationships for the complete analysis of dialkyl hydride-trialkyl mixtures. However, this method is applicable to trialkyls only when dialkylaluminum hydride is added to the sample to function as an indicator. A modification of this method is used in our laboratory to determine the hydride content and we employ a photometric titration t o obtain the total hydride plus trialkyl content. Wadelin (4)has also reported a photometric titration utilizing the indicator properties of dialkylaluminum hydride. Razuvaev (3) reported that methyl violet, gentian violet, and crystal violet act as visual indicators in the volumetric determination of activity. Hydride-containing samples, however, destroy these indicators with evolution of hydrogen. Investigations of triphenylmethane-type indicators in our laboratory show that reversibility occurs only when the nitrogen atoms of the dye are not completely methylated. We have also found certain azinetype indicators to be more stable to hydride-containing samples. Difunctional reagents yield highly colored complexes with aluminum trialkyls and eliminate the necessity of adding hydride to obtain an end point. EXPERIMENTAL
Apparatus. absorbance measurements mere made using a borosilicate cell with a 0.1-cm. path length and a Beckman Model DB spectrophotometer. This cell, illustrated in Figure 1, has a total capacity of 35 cc. and is equipped with a small septum port for the addition of reagents and sample. Titrations are performed using
