the cellulose nitrate used to prepare the propellant is available. There are three semicritical steps in the basic procedure. First, there must be sufficient potassium hydroxide to react with the acidic products of the hydrolysis. Each gram of propellant consumes about 1 gram of potassium hydroxide. Consequently, a t least 60 ml. of the hydrolysis solution should be used per gram of propellant. Whether sufficient alkaline solution n-as taken is checked when the solution is acidified after the 15-hour reaction period. The solution should turn red upon the addition of phenolphthalein. A large excess of hydrolysis solution should also be avoided. Two phases are present a t the end of the reaction. The lower phase, in contact with the cellulose, has a higher concentration of base than the upper. It may be possible, consequently, to raise the hydroxide concentration in the lower phase so as to attack the glucoside chain of the cellulose if a high total base content is present after hydrolysis.
The second semicritical step is in the washing procedure. Washing must be thorough if the solution being filtered comes in contact with the portion of the walls in contact with the Gooch rubber tubing. Because this portion is above the suction line, it is morc difficult to wash adequately. Obtaining and keeping dry cellulose is the third operation requiring care, Cellulose should be treated about the same as phosphorus pentoxide in regard to atmospheric exposure. The basic procedure required about 1 ‘/4 hours of operator time per sample when performed in groups of six. The total elapsed time for six samples is about 26 hours. This procedure minimizes equipment costs, safety hazards. working space, and operator time. Safety Certain precautions should be taken as to safety. Because diethyl ether forms explosive mixtures with air, all operations involving it, or the solvent containing it should be performed under a hood. This is partic-
ularly true when the solvent is acidified, because the gases evolved are reported to be poisonous ( 5 ) . Fifty per cent acetic acid will cause blisters if i t remains on the skin; hence, precautions should be taken when this reagent is used. The alkaline solvent, after filtration may still contain glyceryl trinitrate and should be disposed of properly. LITERATURE CITED
(1) Fletcher, A. N., ANAL. CHEM. 29,
1387-8 (1957). (2) Garetio, Giuseppi, Ruffoni, Alfredo, Ibid., 27, 400-1 (1955). (3) Joint Army-Navv Specification JANN-244 (July 31, 1925). (4) Kenyon, W. O., Gray, H. L., J. Am. Chem. SOC.58. 1422-7 11936). (5) Will, Ber. 24, 400 (1891). ’
.
RECEIVED for review October 27, 1958. A4cceptedFebruary 2, 1059. Division of Analytical Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958. Pacific Southvest Regional Meeting, XCS, Redlands, Calif., October 1958.
Photometric Determination of Zinc with Zincon Application to Water Containing Heavy Metals J. A. PLATTE and
V. M. MARCY
Hagan Chemicals & Confrols, Inc., Piffsburgh, Pa.
b The colorimetric test for zinc with Zincon (2-carboxy-2’-hydroxy-5’sulfoformazylbenzene) is r a p i d and fairly sensitive, but lacks specificity. Many heavy metallic ions, particularly copper, interfere. Zinc can b e separated from some interfering substances with an anion exchange resin and hydrochloric acid of various molarities, but the method i s somewhat cumbersome and time consuming. In this method, heavy metals including zinc are complexed by adding cyanide to the sample. Chloral hydrate is added to free the zinc without destroying the other metallic complexes. The blue zinc-Zincon complex, formed on addition of Zincon to the sample buffered to p H 9, is measured with a spectrophotometer.
A
accurate method is needed for the determination of minute amounts of zinc in potable and industrial waters. Zinc may be present as a n impurity or added to a water with polyphosphate to minimize corrosion by accelerating the formation of a protective film on metal surfaces ( 2 ) . RAPID
1226
ANALYTICAL CHEMISTRY
Although other chromogenic agents have been used, only methods employing dithizone (diphenylthiocarbazone) and Zincon (2-carboxy-2’-hydroxy-5’-sulfoforniazylbenzene) have been widely accepted (1, i’), and many heavy-metal ions interfere with both methods. The aim of this investigation TTas to devclop a rapid direct test with Zincon in which possible interfering substances would be niinimizcd or eliminated. Zincon forms a 1 to 1 blue complex nith both zinc and copper in the p H range 8.5 to 925. At a wave length of 620 mp, the absorbance of the zinc comples follows Beer’s l a r in the range of 0.1 to 2.4 p.p.ni. of zinc (5 to 120 y of zinc for 50 nil. of final test solution). According to Rush and Toe (T),aluminum, beryllium, bismuth, cadmium, cobslt, chroniiuni. iron, ninnganese, mol:-Ldenuni, titanium, and nickel interfere in addition to copper. Ton exchange resins and complexing agents have been tried to remove or mask 5. 7 ) . certain interfering metals (4, PRINCIPLE OF MODlFlCATlON
The
complexing
of
metals
cyanide, follon ed by selective demashing of zinc and cadmium cyanide coniplexes, has been used for the (ethylenedinitri1o)tetraacetic acid (EDTA) titrntion of zinc and cadmium in the presence of heavy metals ( 3 ) . Zinc can be determined photometrically with Zincon by the selective deniasking of the cyanide complex with chloral hydrate. Many metals form cyanide complexes. On the addition of chloral hydratc (6) or formaldehyde (3) to the solution, however, zinc and cadmium cyanide complexes are destroyed r i t h the excess cyanide and these metallic ions are liberated according to the reaction: [Zn(CN)d]-- t 4 CCll CH(OHh --c (chloral hydrate) Zn++ 4 CCla CH(0H)CN 4 OH(3-trichloro-2-hydrosypropionitrile) Although formaldehyde reacts more rapidly than chloral hydrate with the zinc complex, it liberates other metals from cyanide complexes fast enough to cause interferences.
+
+
APPARATUS AND REAGENTS
with
-4.11 absorbancc measurements ivere
I
I
1
I
Table 1. Change in Intensity of Zincon Color Complex with Time
(Solution contains 10 y zinc) Absorbance -Rending Cysnide-
Figure 1. Effect of varying volume of cyanide solution on test samples containing 10 y of zinc
Chloral
Usual Zincon Nethod
Hydrate Rfodificntion
All tests run with 3 rnl. of chloral hydrate solution
0.165 0.165 0.165 0.165 0.165 0.164
0.2 1 5
120 180
0.050
05
IO
15
20
25
30
300
1.80 0.155 0.145
1 . 8 0 0.155 0.154 1.80 0.155 0.155 1.80 0.155 0.156 1.80 0.153 0.360 1.80 0.151 1 . 7 0
CYANIDE SOLUTION, Ml
Table II.
made with either a Becknian Model DU spectrophotometer, using matched 1.00-cm. cells, or a Bausch &- Lomb Optical Co. Spectroiiic 20 colorimeter, using '/pinch tubes. Any photometer having a band pass of 20 mp or less a t a wave length of 620 nip may be used. d Becliman pH meter, Alodel G, was used for all pH measurements. Reagent grade chemicals were used. Cyanide Solution. Dissolve 1.00 gram of potassium cyanide in distilled n-ater and dilute to 100 ml. Buffer Solution, pH 9.0. Dilute 213 nil. of 1N sodium hydroxide to about 600 i d . with distilled water. Dissolve 37.3 grams of potassium chloride and 31.0 grams of boric acid in the solution and dilute to 1 liter. Zincon Solution. Prepare b y dissolving completely 0.130 gram of polvdered Zincon in 2 ml. of 114' sodium hydroxide and dilute t o 100 ml. T h e solution is deep red in color and is stable for about 1 week. Zincon may be obtained from T h e LnMotte Chemical Products Co., Chestertown, Md. Chloral Hydrate Solution. Dissolve 10 grams of chloral hydratc in distilled n-ater and dilute to 100 nil. Sodium Ascorbate. Standard Zinc Solution. Dissolve 0.2745 gram of zinc sulfatr monohydrate in distillrd water and dilute t o 1 liter. Dilute 10 ml. of stock solution to 100 ml. to prepare a standard containing 10 y of zinc per ml. Solutions of Diverse Ion,i. For the study of interferences, use solutions that contain either 10 or I00 y of metallic ion pcr mi PRELIMINARY STUDY
Sufficient cyanide solution must be added to complex all the metallic ions that form cyanide complexes. Also, sufficient chloral hydrate must be used to destroy the excess cyanide and to free the zinc from its cyanide complex. -4s seen in Figure 1, under conditions of the test 3 ml. of chloral hydrate solution is adequate for the reaction with somewhat more than 1 nil. of cyanide solution. It is dcsirahle to use only a minimum of chloral hydrate so as not to increase the color intensity of the treated blank.
Absorbance measurements were made at intervals on treated samples containing zinc, with and without copper, t o determine optimum test conditions and t o ascertain the stability of the zincZincon color complex. The results (Table I) show that test color in the absence of copper is completely developed in 12 seconds and is stable for at least 2 hours whether or not the cyanide-chloral hydrate modification of the Zincon test is employed. The intensity of the color with the cyanidechloral hydrate modification is slightly less than that with the usual Zincon method [Rush and Yoe's Procedure A (7) I. When the sample contains copper, the usual Zincon method measures all the copper in addition to zinc. The color reaction for both Zincon complexes is complete mithin 1 minute. However, with the cyanide-chloral hydrate modification the reaction M ith zinc is complete in 1 minute and the copper is effectively tied up as the nonreactive cyanide complex for 2 hours. l f t e r 2 hours a n increasing amount of copper is liberated by the chloral hydrate. A coloi development time of 2 to 5 minutes is suggested in this procedure to ensure that the zinc color is conipletely developed and that the other cyanide complexes are not affected. The use of formaldehyde in place of chloral hydrate as a demasking agent was unsatisfactory because it destroyed cyanide complexes much faster than chloral hydrate. PROCEDURE
Transfer to a 50-nil. Erlenmeyer flask a 10-ml. aliquot of the sample (approximately neutral) containing 0 to 50 y of zinc. Add reagents to the sample in the following order with mixing between additions: 1.0 nil. of cyanide solution, 5.0 of buffer, 3.0 of Zincon, and 3.0 of chloral hydrate. Measure absorbance 2 to 5 minutes after adding the last reagent. A.s reference solution, use a blank obtained by cftrryiiig distilled water through the complrte procedure. Estimate zinc from
Effect of Diverse Metallic Ions
(Solutions contain 0 and 10 y of zinc) Apparent Zinc Content, y Cvanideihloral Zinc hydrate Usual Metallic Present, modifi- Zincon cation method Ion, Y 7 10.0 cu++, 10 0 0.0 30.0 20 10.0 10 ... 10.0 300 10 10.0 c o + + , 30 0.0 0 19.9 9.9 30 10 ... 10.0 300 10 Ni++, 30 0.0 8.4 0 7.0 10.0 30 10 ... 200 10.1 10
M+++, 50
50 Fe+++, 10 20 70
Fe+T,
10 20
90 CP+++, 100 100 Cd++, 20 10 50
0
10 0
10 10 0
10 10 0 10 0
10 10
0.0
0.0
10.3
10.3
0.0
