Kimball, R. B., Rein, J. E., Ibicl., IDO-14380 (July 1956). LeStrange, R.. J.,- Lerner, M. W., Petretic, G. J., Ibid., NYO-2047 (February 1954). McKay, J. A. C., Proc. Intern. Conf. Peaceful Uses Atomic Energy, Geneva, 1955, 7,314 (1956). Moore, R. L., U. S. Atomic Energy
Comm. AECD-3196 (September 1949). (11) Paige, B. E., Elliot, M. C., Rein, J. E., Ibid., IDO-14349 (August 1955). (12) Pfibil, R., Jelinek, M., Chem. Listy 47, 1326 (1953). (13) Wright, W. B., Jr., U. S. Atomic
Energy Comm. Y-838 (January 1952). (14) Zbid., Y-884 (May 1952). (15) Yoe, J. H., Will, F., Black, R. A., ANAL.CHEM.25, 1200 (1953). RECEIVED for review December 3, 1956. Accepted September 9, 1957.
Spectrophotometric Determination of Nitrite and Thiourea KENNETH HUTCHINSON and
D. F. BOLTZ
Wuyne Sfute University, Detroit, Mich.
b The reaction between nitrous acid and thiourea to form thiocyanic acid can be used as the basis of a colorimetric method for the determination of nitrite. By measurement of the absorbance of the iron(ll1) thiocyanate complex, the method can be applied to the determination of 2 to 12 p.p.m. of nitrite and 4 to 32 p.p.m. of thiourea. Although this method is less sensitive than the classical Griess method for nitrite, the simplicity of the procedure and its applicability to the determination of thiourea are distinct advantages.
centration when a large excess of nitrite is used. APPARATUS
ilbsorbance measurements were made using matched 1.000-em. Corex cells and a Beckman Model DU spectrophotometer. The initial spectrophotometric measurements were made with a Warren Spectracord. The pH measurements were made with a Beckman Model H-2 pH meter. A water bath heated on an electric hot plate was also necessary. SOLUTIONS
T
Griess colorimetric method for the determination of nitrites, involving the diazotization of sulfanilic acid and subsequent coupling with 1-naphthylamine, has been studied critically by Rider and Mellon ( 7 ) . The Griess method is applicable to the determination of 0.05 to 1.2 p.p.m. of nitrite when 1-em. absorption cells are used. Ultraviolet spectrophotometric methods based upon the absorptivity of diazotized p-phenylenediamine and the diazotized 4-aminobenxenesulfonic acid have been developed by Kuemmel and hlellon (4) and Pappenhagen and hlellon (6). The approximate optimum concentration ranges for these two ultraviolet spectrophotometric methods are 0.2 to 1.2 and 0.2 to 3.3 p.p.m. of nitrite ion, respectively. The results of a spectrophotometric study of the reaction between nitrite and thiourea to form thiocyanic acid are reported here. The amount of thiocyanic acid formed is determined by measuring the absorptivity of the iron(II1) thiocyanate complex, FeSCN++ ( 2 ) . The amount of thiocyanic acid produced by this reaction is proportional either to the nitrite concentration when an excess of thiourea is used or to the thiourea conHE CLASSICAL
54
ANALYTICAL CHEMISTRY
Standard Nitrite Solution. Dissolve 1.850 grams of reagent grade potassium nitrite in distilled water, add one pellet of potassium hydroxide, and dilute to 1 liter. Standardize this stock solution using standard potassium permanganate as titrant. Transfer a 100-ml. aliquot and a pellet of potassium hydroxide t o a 1-liter volumetric flask and dilute to volume. This standard solution contained 0.0950 mg. of nitrite ion per ml. based upon standardization of the stock solution. Thiourea Reagent. Dissolve 10 grams of thiourea in 1 liter of distilled water. Filter through a sinteredglass Biichner funnel and transfer t o a 1-liter amber reagent bottle. This solution is stable for a t least 1 month. Butler Solution. Dissolve 3 ml. of glacial acetic acid and 4.90 grams of reagent grade potassium acetate in distilled water and dilute t o 1 liter. This solution is approximately 0.05M in acetic acid and 0.05M in acetate ion. Iron(II1) Perchlorate Reagent. Dissolve 80 grams of hydrated iron(111) perchlorate and 34 ml. of 72% perchloric acid (both from G. F. Smith Chemical Co.) in distilled water and dilute to 1 liter. Standardize this solution titrimetrically using stannous chloride as reductant and potassium dichromate as titrant. This solution contained 6.534 mg. of iron per ml. based upon standardization data.
Diverse Ion Solutions. Prepare from reagent grade chemicals and dilute t o a definite volume so that 1 ml. corresponds t o 5 mg. of diverse ion. Standard Thiourea Solution. Dissolve 0.1000 gram of thiourea in distilled nater and dilute to 1 liter. PROCEDURES
General. For the preliminary investigation, a definite volume of the Etandard nitrite solution was transferred to a 50-ml. volumetric flask. The desired volume of thiourea and either an acid or the acetic acidacetate buffer solution were added. I n studying the effect of diverse ions, the diverse ion solution was added prior t o addition of the buffer solution. The solution was heated for a definite period of time a t a specified temperature, then cooled. The desired volume of iron(II1) perchlorate reagent was added, and the solution was diluted t o the mark with distilled water and mixed thoroughly. The absorbance measurements were made using 1.000em. cells and a reagent blank solution in the reference cell. Recommended. The following procedure mas adopted as a result of the experimental work discussed below. Keigh or measure by volume a sample containing 0.1 to 0.7 mg. of nitrite ion and transfer to a 50-ml. volumetric flask. Solution volume should not exceed 25 ml. Add 5 ml. of the 1% thiourea solution and 10 ml. of the acetic acid-acetate buffer solution. After mixing, immerse the flask in a water bath at a temperature of 70" to 80' C. for 30 minutes. Cool; add 10 ml. of the iron(II1) perchlorate reagent and dilute to the mark with distilled nater. Measure the absorbance a t 455 nip using a reagent blank solution in the reference cell. MODIFICATION FOR DETERMIXATION OF THIOUREA.Small amounts of thiourea can be successfully determined by using the same general procedure, except that an excess ( 5 ml.) of a 1% potassium nitrite solution is used in-
SCALE
0 0.8I
1
2
A-ML.
3 I
I
OF THIOUREA
4
5
I
6 I
nese(II), potassium, sodium, strontium, uranyl, zinc, chloride, bromide, nitrate, sulfate, and tungstate. Those ions caused interference and their tolerances are listed in Table I.
REAGENT
7
8
1
I
9 I
1
0
1
I
Table I.
Q.2h 0
'
I
I
I
I
I
I
I
25 30 35 IO 15 20 SCALE B -REACTION TIME IN MINUTES 2 4 6 8 IO 12 14 SCALE C - M L . OF IRON (Ill) PERCHLORATE FtEAGENT 5
I
Ion
Tolerance, P.P.M.
Interfering Ions
Ion
Tolerance, P.P.M.
40 16
Figure 1. Effect of solution variables on absorbance of iron(ll1) thiocyanate complex
stead of the 1% thiourea solution. Beer's law applies, the optimum concentration range being 4 to 32 p.p.m. of thiourea. EXPERIMENTAL WORK
Conversion Reaction. I n slightly acidic solution nitrous acid is reduced by thiourea t o give thiocyanic acid, nitrogen, and water (3). This conversion reaction proceeds a t a slow rate a t room temperature and is not stoichiometric after several hours of reaction time. A study was made, using the general procedure, to determine the effect of elevating the reaction temperature. The reaction temperature was maintained fairly constant in a water bath, and the amount of thiocyanic acid produced was ascertained after definite reaction times. The rate of conversion increased markedly with increase in the reaction temperature until a temperature of 70" C. was reached. Figure 1,B, illustrates the effect of reaction time a t 70" C. Although the rate of conversion was higher at temperatures approaching 100' C., difficulty was encountered a t temperatures much above 70" C. because of the development of a turbidity. I n the subsequent examination of solution variables, it was decided to heat the reaction mixture for 30 minutes in a water bath maintained a t 70" C., although conversion is complete after 20 minutes. pH Study. The effect of p H was studied using 0.475 mg. of nitrite ion. Varied amounts of a dilute hydrochloric or acetic acid solution were added to the reaction mixture and the p H was measured with the p H meter. A slight turbidity developed when the p H was 1 t o 2, and increased greatly when the p H was less than 1. Clear solutions and reproducible results were obtained when the pH was between 3 and 5 . When 10 ml. of the acetic acid-acetate buffer solution was used, the pH of the solution after heating a t 70" C. for 30 minutes was 4.7.
Thiourea Concentration. The volume of the 1% thiourea solution added to 0.475 mg. of nitrite ion was varied from 1 t o 25 ml. Maximum absorbance was not obtained with this amount of nitrite until 5 ml. of thiourea reagent was used (Figure 1, curve A ) . A large excess of thiourea -Le., 25 m1.-is not recommended because of the development of a slight turbidity. The recommended concentration range is 5 to 10 ml. of the 1% thiourea solution. Iron(II1) Concentration. The effect of iron(II1) perchlorate-perchloric acid reagent was studied using 0.475 mg. of nitrite. The volume of the reagent was varied from 1 to 15 ml. The absorbance a t 455 mp increased until 8 ml. of the reagent had been added; it remained constant thereafter (Figure 1, curve C). A volume of 10 ml. of reagent is recommended as being sufficient for complete development of the color. The color is presumably due to the formation of the FeSCN++ ion because of the high ratio of iron(II1) to thiocyanate. Nitrite Concentration. Conformity to Beer's law was found a t 455 mp for concentrations from 1 to 15 p.p.m. of nitrite using 1-cm. cells. The optimum concentration range is 2 to 14 p.p.m. of nitrite on the basis of a Ringbom plot (1, 8). Time. The orange-red F e S C S + + complex was found t o be stable. A solution containing 9.55 p.p.m. of nitrite gave absorbance values of 0.720, 0.720, 0.718, 0.716, and 0.710 for elapsed times of 0, 10, 30, 50, and 80 minutes, respectively. Diverse Ions. The effect of diverse ions was studied using 9.55 p.p.m. of nitrite. Initially, 500 p.p.m. (25 mg.) of diverse ion was introduced; in the case of interference successively smaller amounts were added until an error of less than 2.5% was obtained. Of the following ions, 500 p.p.m. did not cause interference: aluminum, ammonium, barium, calcium, cobalt (11), lead(II), lithium, magnesium, manga-
A precipitate formed when 0.5 mg. of the following ions were present: copper(I1), tin(IV) , tin(II), mercury(II), bismuth(III), iron(III), silver(I), perchlorate, dichromate, thiosulfate, permanganate, and sulfide. The possibility of removing interfering multivalent cations such as iron(II1) and copper(I1) by means of a cation exchange resin (Amberlite IR-120) was investigated. If the resin is converted to its sodium salt form, it was possible to remove 25 mg. of these diverse ions from solutions containing 0.475 mg. of nitrite without the formation of the relatively instable nitrous acid which, if formed, gives low results. When the preliminary separation was performed, the results for the spectrophotometric determination of nitrite were 2 to 3% low. These slightly low results are attributed to incomplete column washing in an attempt to keep the volume of the solution a t a minimum. DISCUSSION
The reaction between nitrous acid and thiourea results in a quantitative yield of thiocyanic acid, if there is a large excess of one of the reactants and the mixture is heated to accelerate the reaction. The sensitivity of this new colorimetric method is less than that of the Griess method or the recently proposed ultraviolet methods. Increased sensitivity cannot be achieved through an extraction technique because the iron(111) thiocyanate complex is not soluble in organic solvents, an excess of thiocyanate being necessary for extraction. The shift of the absorbance maximum to a lower wave length is characteristic of the iron(II1) thiocyanate system when a high ratio of iron(II1) to thiocyanate is used ( 5 ) . Some of the interfering ions can be removed by preliminary treatment with a cation exchanger. The main interferences are caused by those ions which form precipitates during the heating process or within the pH range used. The reproducibility of the method VOL. 30, NO. 1, JANUARY 1958
55
for the determination of nitrites is satisfactory. Ten determinations using 9.80 p.p.m. of nitrite gave a mean absorbance value of 0.737 with a standard deviation of 0.004 or 0.53yG. A slight modification in procedurenamely, the use of excess nitrite instead of excess thiourea-enables small amounts of thiourea to be deterniined colorimetrically.
LITERATURE CITED
(1) .+-res, GI. H., .IY.~L. CHEM.21, 652 (1949). 1 2 ) Bent, H. E., French, C. L., J . A m . Cheni Soc. 63,568 (1941). ( 3 ) Coade. 11.E.. Weiner. E. I.. J . C'heiii. Soc.'103, 1221 (191i). ' (4) Kuemmel, D. F., Mellon. 11. G.. .\NAL. CHEY.28, 1674 (1956). (5) Ovenst,on, T. C. P., Parker, C. -I,, Anal. Chirn. Acta 3, 277 (1949).
( 6 ) Pappenhagen, J., liellon, 11. G., . i S A L . CHEM. 25, 341 (1953). (;j Rider, B. F., IIellon, 11. G., IND. E S G . CHEM., . ~ K A L . ED. 18, 96 (1946). (8) Ringbom, -\.,2 . anal. Chern. 115, 332 (1939). RECEIVEDfor revie\\- .April 1, 1957. -4ccepted August. 2, 1957. Pittsburgh
Conference on Analytical Chemistry and .Ipplied Spectroscopy, Pittsburgh, Pa., l l a r c h 1957.
Inexpensive Auto matic Recording The rmobaIa nce WESLEY W. WENDLANDT Department o f Chemistry and Chemical Engineering, Texas Technological College, lubbock, lex.
b The construction and operation of an inexpensive, automatic recording thermobalance are described. The instrument was built from a torsion balance, 0- to 102-mg. capacity. The null position of the balance was maintained b y a beam of light falling between two cadmium sulfide photocells. The apparent accuracy for a 1 00-mg. sample was approximately o.SQ/o. The reproducibility was 0.20y0. The advantage of this instrument over commercially available models was the low cost of construction. Excluding labor, the complete thermobalance cost about $400. The accuracy and reproducibility of the instrument agreed favorably with the more expensive models.
ated instrument (21). The null point of the balance is maintained by a beam of light falling between two sensitive photocells. The weight-temperature curve is pen recorded on a cylindrical drum. DESCRIPTION OF APPARATUS
Balance. A schematic diagram of the thermobalance is shoxw in Figure
1.
The balance was a torsion-wire type instrument, 0- to 102-mg. capacity, made by T'ereenigde Draadfabrieken, Nijmegen, Holland. The smallest scale dirision was 0.2 mg.; thus weighings could be read t o 0.1 mg. Recorder. The recording drum consisted of a n aluminum cylinder, 3
. -
I
x'rmm'r in the thermogravimetric of analytical precipitates has increased rapidly since the first tliermobalance was built by Hoiida ( 1 1 ) in 1915. The impetus given to this field by Duval and his coworkers ( 5 ) has resulted in a greater knowledge of the thermal stability of some 1000 analytical precipitates. The development of the therniobalance has been reviewed by Duval ( 6 ) . By far the most popular instrunient has been the Chevenard thermobalance (3, 5 ) . Gordon and Campbell (8) have described the conversion of the photographically recording model into a chart recording instrument. As interest in this field has increased, a iiumber of new thermobalances have appeared. 1IanunlIy operated ( 2 . 4, 9, 15, 17': 20, 21), as well a? recording instruments (1, 7 , 10, 12-14, 16, IS,19, 22), have been described. This paper describes an inerpeiisive, automatic recording therinohalance which has proved successful in studying the thermal decomposition of a number of analytical precipitat,es. The main components of the t'herniobalance are essentially those of the manually oper55
ANALYTICAL CHEMISTRY
inches in diameter and 10.5 inches in length, connected to the torsion wire shaft of the balance. The rotation of the drum was controlled by a 1 r.p.in reversible synchronous motor. How ever, as this speed caused too much overshoot, the shaft speed was reduced with a 4 to 1 reduction gear. The recording pen consisted of a size 000 Leroy lettering pen suitably mounted on a sliding carriage. This carriage was drann across the slide bar with a 1 revolution-per-liour synchronous motor. The motor contained a friction clutch so that the pen carriage could be manually reset to the starting position. Furnace. The furnace was t h e sanie as previously described ( 2 2 ) . It n as constructed by first winding 15 feet of S o . 22 gage, Xichrome alloy V,
4
0
I---.. c.
Figure 1.
Schematic diagram of the thermobalance
Torsion balance, 0- to 102-mg. capacity light source 6. D. C. Adjustable slit and focusing lens Beom mirror Recording drum E. Reflecting mirror F. G. Photocells H. Drum motor
A.
Combustion tube joint Pen carriage Furnace M . Pen drive motor N. Thermocouple 0. Plotinum sample p o n P. Exhaust gos connection
1. K. 1.
