776
ANALYTICAL CHEMISTRY
lead compounds in the atmosphere. Generally a saniple may be collected and analyzed in less than 10 minutes without a constant error. In the range corresponding to the human tolerance limits (0 to approximately 4 micrograms of lead per cubic foot of air) the precision of the method is 1 0 . 5 microgram; in the higher range up to 20 micrograms the precision is *lo% of the amount present. For greater precision, the lead djthizonate complex may be measured with a suitable photometer instead of with the Hellige comparator which is suitable for field use. I n either caw the only interfering metals in addition to other forms of lead are bismuth, monovalent thallium, and stannous tin. The sample size may be reduced to 1 or 2 cubic feet of air if the color of the lead dithizonate is viewed through a wider comparator tube or if the volume of dithizone solution is reduced to 5 ml. However, the collection of smaller samples will decrease the accwacy of the method because less homogeneous samples will be collected and the errors due to lead contamination will be correspondingly larger. The reason for specifying a definite sample size ( 4 cubic feet) is to reduce the probability of error whcn the method is used by nontechnical operators. The lead-in-air analyzer may also be used for the determination of lead in liquids such as water, gasoline, oils, etc.
To determine lead in liquids, place %bout0.2 to 0.3 gram of the sample in the color comparator tube and shake with 10 ml. of the iodine solution for about 1 minute. Add 30 ml. of Solution -4, and approximately 40 ml. of water, and shake about 2 seconds to destroy the iodine. Add 10 ml. of dithizone solution and shake vigorously for 30 seconds. Visually compare the color of the dithizone solution with the standards in the Hellige comparator. As the readings on the disk are given as micrograms of lead per cubic foot of air, they must be converted to micrograms of lead bv multiplying the reading by 4. r\ color matching 2 micrograms 01 lead per cubic foot of air is thus equivalent to 8 micrograms of lead. When the test is ap-
plied to the determination of lead in 50 ml. of water, the sensitivity of the method is +0.05 p.p.m. of lead in water. Enough dithizone is added to each ampoule to react nith a little more than 100 micrograms of lead. When a sample is collected which contains more than this amount of lead, the upper layer (aqueous solution) will appear almost colorless. Sormall>-,this layer appears yellow from an excess of the soluble ammonium salt of dithizone. If Solution ,4contains insufficient sulfite to reduce all t,he iodine present, the residual iodine will oxidize the dithizone solution, as evidenced by.a color change from green to yellowish orange. If this color change is observed, Solution A must he discarded and a fresh batch prepared. For consistently accurate work, a blank test should be run to check the equipment and reagents for lead contamination. The blank test is made by following all steps given under ‘‘Procedure’’ with the erception of collecting the air sample. ACKNOWLEDGMENT
The authors wish to thank the Hellige Company for cooperation in preparing a suitable color comparator disk for estimating lrad concentrations. LITERATURE CITED (1) Bykhovskaya, M. S., Gigiena i Sunit., 10, No. 9, 17-21 (1945). (2) Clark, W. M., Elvove, F., Remsburg, C. G., Hall, W., and Simkins, W. A,, U. S. Pub. Health Service, Bull. 163 (1926). (3) Fischer, H., Wiss. Verofent. Siemens-Konzern, 4, 158 (1925.); 2.angew. Chem., 42, 1025 (1929). (4) Greenburg and Smith, C. S. Bur. Mines, Repts. Znvestigatwns 2392 (1922). (5). Leake, J . P., and Bloomfield, I. J., U. S. Pub. Health Service, Bull. 163 (1926). (6) Eandell, E. B., IND. ESG.CHEM.,ANAL.ED., 9, 464 (1937). (7) Snyder. L. J., ANAL.CHEM.,19, 684 (1947). (8) Winters, 0. E., Robinson, H. M., Lamb, F. W.. and Miller, E. J.. IXD. ENQ.CHEM.,ANAL.ED., 7, 265 (1935). RECEIVED June 25, 1947
Ammonium Citrate in the Colorimetric Determination of Copper A. J. HALL AND R. S. YOUNG, Central Laboratory, Nkana, Northern Rhodesia RECENT work a t this laboratory i t was necessary to deI Stermine small quantities of copper in a strong solution of
mercuric chloride. It was found that large amounts of mercury, 3.5 grams of mercuric chloride in 50 ml. of water, interfered with the determination of copper with sodium diethyldithiocarbamate by giving a yellowish turbidity and no true formation of the copper color. The separation of small amounts of copper from mercury by precipitation is a tedious proceeding and therefore it was decided to determine the blue color of the copper ammonium citrate complex directly in the presence of mercury which was held in solution by ammonium citrate and excess ammonia APPARATUS AND PROCEDURE
h Spekker photoelectric absorptiometer was employed for the colorimetric observations, with Ilford 608 red color filters and a tungsten filament lamp. Comparisons of the absorptions were made with a cell containing water. Reagents for Absorptiometric Calibration. Copper sulfate solutions contained 1 mg. of copper per ml. (No. 1) and 6 mg. per ml (No. 2). Mercuric chloride solution, 70 grams per liter.
Ammonium citrate solution was made by dissolving 200 grams of citric acid in water, adding 270 ml. of ammonia, and making the whole up to 1 liter. Ammonia, C . P . grade, 0.88 specific gravity. Absorptiometric Calibration. 0 TO 50 U G . OF COPPER. Place varying amounts a t 5- or 10-mg. intervals, up to 50 mg., of copper sulfate solution 1 in 200-ml. standard flasks. Add 50 ml. of mercuric chloride solution, 50 ml. of ammonium citrate, and 50 ml. of ammonia. Shake well and make up to 200 ml. Measure the absorption of thesc solutions in 4-cm. (30-ml.) cells. 0 TO 200 MG. OF COPPER. Place varying amounts a t 10-mg. intervals, up to 200 mg., of copper sulfate soluticn 2 in 200-ml. standard flasks. Add mercuric chloride solution, ammonium citrate, and ammonia as above. Measure the absorption of these solutions in 1-cm. (8-ml.) cells. CONFORMITY TO BEBR-LAMBERT LAW
Plotting both sets of absorptiometer readings gave straight lines, which shows that the color reaction obeys the Beer-Lambert law over the range investigated. INTERFERING ELEMENTS
Because for this particular case in the test solutions the copper was to be determiwd in the presence of mercury, it was con-
m
V O L U M E 20, NO. 8, A U G U S T 1 9 4 8 sidt ied good absorptiometric practice to make up the oalibiatiiig d u t i o n s with mercuric chloride, but it was found experimentally that varying the amount of mercury present had no effect on the absorptiometer readings. Further experiments were carried out on the determination of copper in the presence of other metallic elements that give coloiless citrate complexes. It was found that copper could be determined in the presence of silver, zinc, cadmium, magnesium, aluminum, and lead. Varying the amounts of these metal ionb had no effect on the copper color. The presence of chloride, acetate, and nitrate ions also had no influence. Iron in amountq of 1 to 2 mg. has no effect on the copper color. Five to 10 mg. of iron will impart a greenish tinge but the correct absorptiometei wading is obtained. Fifty milligrams of iron will give a yelloii green color to a solution containing 20 mg. of copper and obviously interferes. Other colored ions such a t cobalt, chromium, and nickel will affect the copper color. DISCUSSION
The prcscrat work was carried out Kith the addition of 50 nil. (if ammonium citrate and 50 ml. of ammonia to neutral solutions. These amounts are in excess of those required for the immediate complexing of the quantity of mercury present (3.5 grams of mercuric chloride) but it was found that unless a large excess was present the mercury tended to precipitate on standing. The citrate complex of copper has been studied in detatl by Bobtelsky and Jordan ( I ) , who found in their photometric measurements that a tenfold excess of citrate did not further affect the oxidation. For many purposes it would be possible to employ ammonium citrate only, in order to retain other ion. in solution and
t o develop a copper color such the determination of copper in the presence of silver and zinc. It was necessary in the present instance to add a constant excess of ammonia in order t o a w s t in complexing the mercury present. This colorimetric method could be adapted for the rapid determination of copper in a number of light alloys, brasses, and solders and the actual amount of ammonium citrate and ammonia t o be added could best be determined for each particular case. Mehlig ( S , 4 ) found that the copper-ammonium color was always dependable, provided that a constant excess of ammonia was present, and was stable for 6 weeks. By the addition of ammonium citrate it is possible to determine copper in the prrscnce of other ions which would otherwise precipitate in a straight ammoniacal solution. The volatility of ammonia, to which some workers object (@, is not so great that there would be any serious diminution in the concentration during the length of time taken to carry out colorimetric observations on a number of samples, If the solutions are kept in stoppered flasks. By the use of 4-cni. (30-ml.) cells a calibration graph may be constructed which can be read to *0.5 mg. of copper and will give greater accura(v than smaller cells with low concentrations of copper. The I-tm (8-ml.) cells will, however, give a graph which enables the work to be carried over a wider range and can be read with an accuracy of + I mg. of copper. LITERATURE CITED (1)
Bobtelskj, M., and Jordan, J., J. Am. Chem. Soc., 61, 1824
(1945). (2) CrumDler. T. B.. ANAL.CHEM..19. 325 (1947). (3j Mehlig, J. P., IND. ENG.CHEM.,- ~ N A LED., . 13, 533 (1941 (4)Ibid., 14,903 (1942). RECEIVED October 23, 1947
Determination of Hydroxyl Groups in Organic Compounds
,
B. L. JOHNSON, Montuna Stote College, Bozernan, M o n t o m
\CETYLATIOS of alcoholic compounds, followed by isolation, purification, and saponification of the acetylation product, is time-consuming as a quantitative procedure. Simpler and more rapid methods for determining hydroxyl groups by hack-titration of the excess acetyl chloride or acetic anhydride aftcr acetylating a weighed sample of a hydroxylic h a w become well established in recent years (1, 6, 6, ?). The inethod here described oresents a modification of a oreviouslv published acetyl chloride procedure, which has been used succmsfully in this laboratory for a number of years on a variety of alcohols (Table I) and on fats and oils. I t is of doubtful value in the case of compounds insoluble in toluene. L
'Tahle I. Number of Hydroxyl Groups in C o m m o n Alcohols ri-Butyl alcohol (purified) Benzyl alcohol
8
Av. of Tests 1.04
6
1.12
No. of
Alcohol
Teste
1,3-Propanediol
5
2.11
Isobutyl alcohol
6
1.26
n-Propyl alcohol
5
1.15
1,2,3-PropanetrioI
3 5
2.96 3.06
12-Ethanediol monoethyl ether
5
0.99
CHEMISTRY OF PROPOSED METHOD
Scetylation in toluene as the solvent: R-OH Of
+ CHZCOC1
=
+ HC1
CH3-COOR
B)
the excess acetyl chloride:
CHSCOCI
+ HOH = CHjCOOH + HC1
21
Back-titration of the acetic acid and hydrochloric acid:
+ XaOH HC1 + YaOH
CH8COOH
= CH,COONa = Sac1
+ H20
+ HzO
31 4)
Reactioii 1 is carried out in an anhydrous system which is closed by a trap containing a measured volume of standardized sodium hydroxide. Loss of volatile hydrogen chloride or acetic acid 1s thus prevented.
__ Individual Results 1.04 1.06 1.15 1.16 2.26 2.06 1.44 1.34 1.22 1.17 2.84 3.11 3.04 0.94 1.11
.
1.18 0.94 1.06 1.06 2.13 2.05 1.27 1.23 1.07 1.19 3.09 3.09 3.01 0 94 0 92
0.98
EXPERIMENTAL REAGENTS
i:i6 1.15 2.07 1.25 1.18 1.19
.Icetyl chloride dissolved in toluene to make an approximately 1 S solution, accurately standardized. Approximately 1 K sodium hydroxide, accurately standardized. Phenolphthalein indicator, 1 gram in 100 ml. of alcohol. Recently cooled h i l e d distilled ivat,er, in large quantity.
2.91 2.94
SPECIAL EQUIP!$lE.%T
1.63
..
Kjeldahl flasks (500-ml.) fitted with a side arm as shown in Figure 1. Calcium chloride tubes partly filled with glass beaQ and indented close to stem outlet to prevent beads from 5ealing
