Direct Volumetric Determination of Total Zinc in Mixed Paint Pigments with Ethylenediaminetetraacetic Acid M. H. SWANN and M. L. ADAMS f a i n t a n d Chemical Laboratory, A b e r d e e n Proving Ground,
The determination of total zinc in mixed paint pigments of unknown composition involves its separation from interfering cations with hydrogen sulfide, which is time-consiiming. A more rapid, direct, and specific method can be used to replace the variety of methods in current use. without knowledge of the nature of the pigment mixture.
T
H E determination of certain cations by complex formation with chelating agents ( I ) is particularly well adapted to the determination of zinc in mixed paint pigments. A limited number of the metal ions that can be determined in this manner occur in mixed paint pigments: calcium, magnesium, barium, zinc, iron, and chromium. Zinc pigments, with the exception of zinc sulfide, have the distinction of dissolving in ammoniacal ammonium chloride n-hich is used to buffer the solution a t p H 10 during the titration. KOother pigments are soluble in this solution, except certain chromium pigments which do not interfere n-ith the tit,ration of zinc. This provides a means for the rapid, specific, and direct determination of total zinc; a special modification can be made for pigments containing zinc sulfide, which is easily identified by the odor of hydrogen sulfide given off when acids are added.
Table I. Determination of Zinc Oxide and Zinc Chromate in Paint Pigments Specification TT-E-00485b enamel containing lead chromate, red lead, zinc oxide, a n d siliceous extender Zinc Oxide Zinc Oxide Found, % Present, 4rc 5.0 8.8 5.0 8.8 10.1 10 1 Specifications ~ I I L - P - 1 1 1 1 4 Aa n d TT-P-664 primers containing iron oxide, zinc chromate. and siliceous extender Zinc Chromate Zinc Chromate Found, % Present, So 10 1 10.4 10.1 10.5 10 2 10.2 Specification TT-P-bStib primer containing iron oxide, zinc chromate, zinc oxide, and d i c e o u s extender Total Zinc Zinc C h r o i n z ~ e Zinc Oxi% Calculated as Zinc Oxide Present, Gc Present, Zinc Oxide, Yo Found, Yo $1 . 1; 9.8 14.2 14.1 H i 9.9 14.3 14.2 sti 10.4 14.3 14.3
The method in current use for determining zinc in mixed pigments of unknown composition involves separation of the zinc with hydrogen sulfide, followed by ferrocyanide titration or ignition of the sulfide t o the oxide and weighing. Hydrogen sulfide separations are time-consuming. The accuracy of the method described here IS equal but not superior to existing methods, although the absence of excessive handling and transfer of sample contributes to a high degree of reproducibility of analytical results. In principle, the zinc pigment is dissolved in a buffered solution of ammonium chloride and ammonium hydroxide, and the insoluble pigments are filtered off. An indicator, Eriochrome Black T, is added which is a magnesium complev of a hydrovylated
Md.
azo dye, purple in color, The sample is then titrated n.ith a chelating reagent solution, ethylenediaminetetraacetic acid disodium salt [sodium(ethylenedinitrilo)tetraacetate], which complexes the zinc present. When all zinc has been complexed, the first excess of reagent breaks down the indicator by the formation of a stronger complex with the magnesium than the complex of the dye with magnesium, causing a color change to blue. REAGENTS
Buffer solution (pH 10). Fiftg-four grams of ammonium chloride and 350 ml. of concentrated ammonium hydroxide are diluted to 1 liter with water. Eriochrome Black T (0.5%). Eriochrome Black T, 0.25 gram, and 2.2 grams of hydroxylamine hydrochloride (as stabilizer) are dissolved in 50 ml. of methanol. Disodium ethylenediamine tetraacetate dihydrate (0.25); 37.2 grams of the reagent are dissolved in water and diluted t o 1 liter. Standard zinc oxide solution. Approximately 3 grams of ovendried zinc oxide of known purity are accurately weighed, dissolved in the buffer solution, then diluted to 500 ml. nith buffer. ANALYTICAL PROCEDURE
A small amount of pigment is tested for sulfides by the addition of dilute hydrochloric acid. If the odor of hydrogen sulfide is detected, the modified procedure is followed. Procedure in Absence of Zinc Sulfide. Approximately 1 gram of the isolated paint pigment is accurately weighed into a 250-ml. Erlenmeyer flask having a 24/40 ground joint. If the sample is believed to contain more than 1OY0 zinc oxide, proportionately smaller samples may be used. Twenty-five milliliters of the buffer solution are added, and the flask is stoppered and is shaken frequently and vigorously for 15 minutes. The solution is then filtered through paper of fine porosity into a 400-ml. beaker and diluted to 300 ml. with water. Exactly 25 ml. of the standard zinc oxide solution are transferred to a 400-ml. beaker and similarly diluted t o 300 ml. Ten drops of the Eriochrome Black T indicator are added and the sample and standard are titrated with disodium ethylenediaminetetraacetate reagent. As it becomes apparent that the end point is near, 5 additional drops of indicator are added. In the absence of chromium, the color change at the end point is from wine-red to blue; if much chromium is present, the color change is from orange-red to gteen. Procedure in Presence’of Zinc Sulfide. Approximately 1 gram of the isolated pigment is accurately weighed into a 250.ml. beaker, 25 ml. of G S hydrochloric acid are added, and the sample is heated for 30 minutes on a hot plate and finally boiled without cover for a few minutes t o expel the hydrogen sulfide. I t is then cooled and 2 ml. of 8570 phosphoric acid are added, followed by 35 ml. of concentrated ammonium hydroxide and 25 ml. of the buffer solution. The resulting mixture should be alkaline (approximately pH 10). It is then filtered through paper of fine porosity and the filtrate is diluted and titrated as above. The 25-ml. aliquot of standard zinc oxide Eolution should be treated in a similar manner-i.e., 2 ml. of phosphoric acid, 25 ml. of G.V hydrochloric acid, and the 35 ml. of concentrated ammonium hydroxide are added before diluting and titrating the standard.
% zinc (as ZnO)
=
ml. of reagent X normality of reagent X 4.069 weight of pigment sample
Some results of analysis are shown in Table I. LITERATURE CITED
(1) Walton, H. F., “Principles and Methods of Chemical Analysis,” p. 374, Pi-entice-Hall, New York, 1952.
RECEIVED for review July 18, 1955. Accepted August 25, 1555.
2005
