n 2 na 2 o 8 2h 2 o the edta disodium can for example have the following composition 5 g titriplex iii 200 ml h 2 o 20 ml 30 h 2 o 2 solution and 20 ml 25 nh 3 solution as noted hereinabove titriplex iii is a trademark for the ethylene dinitrilo tetraacetic acid disodium salt dihydrate which has a formula of c 10 h 14 n 2 na 2 0 8 h 2 o after the titanium has been etched off the glass member 3 34 shown in fig3 will have two strip waveguides 31 and 32 on the undersurface 35 the undersurface 35 which is freed of the titanium i e the region of the undersurface 35 that was covered by the titanium acts as an ion barrier so that a second ion exchange including the whole surface cannot be carried out when etching the titanium off a thin ion blocking layer may remain this layer however can be polished off without problems and without deteriorating the strip waveguides 31 and 32 an etching treatment is also suitable to remove the layer and it can be either a wet etching or a dry etching method care must merely be exercised to see that the etching process is selected to be so short that the strip waveguides 31 and 32 are not deteriorated but long enough so that the blocking effect of the underside or surface 35 has been eliminated as in the case of polishing this can be easily determined with a few trials the second ion exchange is undertaken on the polished member for burying the.