NET DETERIORATION IN LAKE ERIE 



167 



contents diluted with water and neutralized with concentrated sodium 

 hydroxide solution. The point of neutralization can be determined 

 by noting the precipitation of copper hydroxide. 



The contents of the flask were cooled again, transferred to a beaker, 

 made acid to litmus with acetic acid, and solid potassium iodide 

 added. The liberated iodine was titrated with standard sodium 

 thiosuiphate solution, using starch as an indicator in the usual manner 

 and observing the usual precautions for this method. The sodium 

 thiosuiphate was standardized against metallic copper, repeating all 

 the steps of the usual procedure, and the factor calculated directly in 

 terms of copper per cubic centimeter of thiosuiphate solution. Care 

 should be taken to examine the sodium hydroxide used in this deter- 

 mination to make sure that it is free, from iron and copper. No 

 trouble was encountered on this score in this work, however. 



The analytical determinations were quite satisfactory as regards 

 accuracy and speed. In previous work the sample had been prepared 

 by burning the twine in a crucible. This procedure is sufficiently 

 satisfactory when only one or two determinations are necessary but 

 does not give good results when a large number of samples are to be 

 examined for the reason that the process of ashing is slow and each 

 sample has to be watched carefully during this process of ashing lest 

 material be lost. Pyrosulphate fusions and sodium peroxide fusions 

 were tried but did not give any better results than simple ashing with 

 regard to loss of material in the process of destroying the organic 

 matter. 



Table 4 — Tensile strengths and copper content of treated twine exposed at Erie, Pa., 



1928 



