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Vol. 44, No. 1
dissolved copper down to 1.0 mg. or less in a 6-hour test. An accumulation of practical commercial experience indicates this test offers valuable guidance. SUMMARY
-
TIME-
HOURS
Figure 9. Rate of Solution of Copper by Sodium Hexametaphosphate and a Related Formulation 200 ml. of solution a t 80" C. I . 0 . 0 6 % (NaP0s)e 2. 0.06% (NaP0s)s
0.06% NaaSiOa.5HgO
0.023% NasPO~.lPHpO
of metals and tdloys, and electrical leakages from the power supply. Data on the effectiveness of agents added to reduce the corrosive action of polyphosphates on copper have been collected in Table VII. It is much more difficult to inhibit the solubilizing action of tetrasodium pyrophosphate than that of sodium tetraphosphate or sodium hexametaphosphate by the inclusion of sodium metasilicate in the formula. A metasilicate-polyphosphate ratio of 3 to 1 or 4 to 1 for pyrophosphate or nearly 1.5 t o 1 for tetraphosphate or hexametaphosphate appears to be needed to hold
Solutions of four polyphosphates have been shown to dissolve copper, zinc, or brass. Corrosion rate data are presented for several metal-polyphosphate combinations a t 80' C. and for several commercial and experimental dishwashing detergents. The inclusion of sodium metasilicate affords a practical means of retarding the corrosive action of polyphosphated dishwashing detergents on dishwashing machine parts. Of the several polyphosphates investigated, pyrophosphate is the most difficult to inhibit. Corrosion of copper, brass, or bronze pumps, valves, and spray nozzles by polyphosphated detergents may seriously reduce the efficiency of dishwashing machines. Many complete failures of machine parts within 6 months have occurred. A brassy tarnishing of silverware washed in certain dishwashing machines is attributed to copper solubilizedfrom machine parts by poorly inhibited polyphosphated detergents and then plated on the silverware by electrolytic action. LITERATURE CITED
(1) Bacon,
L. R.,and Nutting, E.
G., Jr., IND. ENQ.CEEM., 44, 150 (1952). (2) Gilmore. B. H.. Ibid.. 29. 584-90 (1937).
ENQ.CHEM.,26,998-1001 RECEIVED February 23, 1951. Presented before the Division of Water, SewCAEMage, a n d Sanitation Chemistry a t the 119th Meeting of the AMERICAN I C A L SOCIETY. Cleveland, Ohio.
(Polyphosphate Detergents in Mechanical Dishwashing) METALLIC STAINING OF SILVERWARE LESLIE R. BACON AND EUGENE G. NUTTING, JR.
observed after washing in conventional dishwashing machines. The stain was shown to be a surface deposit of copper, apparently derived from metal dissolved from machine parts by dishwashing solutions containing polyphosphates. Data were presented to show the solubilizing action of several polyphosphate and polyphosphated dishwashing compound solutions on copper, zinc, and brass. I n the present paper the conditions essential to the staining phenomenon and means for its avoidance will be reported more fully. Two typical examples of staining observed in the field may be described. Both followed the introduction of the detergent Formula A (tetrasodium pyrophosphate 40%) trisodium phosphate 30%, and sodium metasilicate pentahydrate 300j0)into service. After 10 days of use in a hotel dishwashing machine, the silverware suddenly began t o come through badly tarnished, appearing burned. I n short order 400 to 500 pieces were thus affected. Careful examination of the water supply and the machine did not disclose the cause of staining. In a large cafeteria, staining developed in a machine used for washing glasses and silverware. The stains were more prevalent on the lower grades of silverplate, especially a t spots where the silver was worn through. After a fresh detergent solution had been used for a short time, staining became evident and nearly
General experience indicates that exposure of the base metal through imperfect plate, whether porous, scratched, or worn through, accentuates staining, particularly in the case of cheaper ware which is lightly plated. Fanlike or circular colored spots at times may be seen radiating from imperfections in the plate. Colors vary, depending on thickness and character of the deposit. Although stains commonly appear metallic or brassy, shaded blues and browns suggestive of sulfide stains have also been seen. The base metal of better grades of silverware is usually nickel silver (German silver) the composition of which is approximately 65% copper, 5 t o 25% nickel, and 10 to 30% zinc. EXPERIMENTAL METHODS AND DATA
CONFIRMATION OF ELECTROCHEMICAL CHARACTER OF STA~N FORMATION. All solutions were made up in distilled water unless otherwise stated. After it was discovered that polyphosphates are capable of solubilizing copper, experiments were carried out to determine the conditions under which the copper would deposit on pure silver. Strips of pure silver 121 X 19 X 1.6 mm. with a 2.4-mm. hole drilled in one end were used. Two com-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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151
pounded detergents of the following cornpaitions were used in various oompositions: Formols A .
Foiiriuls 0,
9%
%
Sodium meteailiaate penfahydiste Trisodium phosphate dodecahydrslc
30
39.2
Soda ash Tetraodium pyrophosphste Rodium hexsmetnphosphate
40
30
24.7
4.5
31.2
Five ZWml. samples of b 0.15'% aolution of Fannula A were placed in 25Rml. jars and h a t e d to 80' C. A 76 X 19,X 0.84 mm. copper ntrip was inserted in each solution and heating w m continued for 3.5 hours. Analysis of one of the solutions then
showed that 2.6 mg. of cop er had dissolved. Tho copper strips tiwe removed from two o f t h e remaining four solutions and a silver strip was p l c e d in each. After 10 minutes at 80" C. the silver atrips were removed, dried, and examined. AR expected, none showed any trace of copper tamiah. To discover the aonditions neceanary to C B U S ~oopper to deposit on silver in a solution containing polyphos hate, strips of zinc, copper, and silver in a number oi electric& edupled and noncoupled combinations were placed in 250-ml. portion8 of a 0.15% solution of Formula A . Ohmrvations were made on t,hese cells after 1.5 hours at 80' C., then nfter 3 hours at 80" C., and lastly niter 3 days at room temperilture. Table I summarizes the results oi this test. Visible deposits of eopper formed on silver only when two conditions were EulfiIled--8 zinc rtrip in physical contact. with the silver, and B copper strip present hut not in eontact wit,h either of the other metals.
EFFECTOF Con~osrovR.
