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The Solubility of Metals in Milk

I. The Solubility of Copper under Various Conditions^*

ABSTRACT

1. In the review of previous work, the lack of concordant results was attributed to faulty procedure,—lack of agitation, and inadequate control of the temperature and dissolved gases. A procedure was developed to overcome these objections.
   
2. The effect of acidity, dissolved gases, and temperature on the solubility of copper in milk were studied. The effect of velocity was not studied, but it was controlled and kept uniform in all the experiments.
   
3. Higher acidities markedly decreased (rather than increased) copper solubility in milk, while slight increases in acidity had no effect.
   
4. The effect of dissolved gases was studied by bubbling them through the milk during the entire exposure. It was found that carbon dioxide markedly decreased, air slightly increased, and oxygen markedly increased the copper solubility in milk. Prolonged treatment of the milk with oxygen previous to exposure led to the formation of a protective oxide coating.
   
5. Temperature greatly affected copper solution in milk. The maximum solubility was always found at 70°C. whether the time of exposure was thirty minutes, one hour or two hours. In high acid milk the maximum was also at 70°C, but was appreciably lower than in sweet milk. At boiling temperature copper solubility was about the same as at room temperature.
   
6. Up to 50°C, the temperature-copper-solubility curve followed the temperature-reaction-velocity curve. At higher temperatures, the curves diverged, less than the expected amount of copper being dissolved.
   
7. Preheating of milk to various temperatures above 70°C. decreased copper solubility in such milk. The effect of preheating was greater as the temperature and time of preheating increased.
   
8. The oxidation-reduction potential of milk was found to be related to the solution rate of copper and the various factors that affect it.
   
9. Up to a temperature of about 65°C, the oxidation-reduction potential remained substantially constant; above 70°C. it decreased rapidly. With prolonged holding at 60 and 70°C, the potential remained constant. The presence of dissolved copper made the potential more positive. Increased acidity is known to make the potential more negative.
   
10. In the discussion it is shown how the observations on the oxidation-reduction potential may explain the mechanism through which acidity, dissolved gases, temperature and preheating affects copper solubility.
    
11. Copper solution in milk is a typical case of metal corrosion. The same factors in the same relative order of importance apply to copper corrosion by milk as apply in general to metal corrosion by weakly acid or alkaline solutions.
    

^* The work was supported in part by a grant from the Special Research Fund of the University of Wisconsin.

Published with the permission of the Director of the Wisconsin Agricultural Experiment Station.