gold or platinum gauze is used to maintain a reproducible electrode surface. Operational amplifiers provide rapid control of electrode potential. Dr. Scott and co-workers can keep the peak height within ±2% by holding the sparge rate to within ±25%, the volume of supporting electrolyte to within ±15%, and the temperature to within ± 2 ° C.
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Scan Rate Affects Sensitivity. For
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a cell of this type, a range of scan rates from 0.04 to 0.3 volt per minute gives similar calibration curves of peak height vs. ion concentration. A maximum slope occurs at about 0.2 volt per minute. Dr. Scott and associates, however, prefer to operate at a scan rate of 0.08 volt per minute. At lower rates, specificity of the method increases. And so does the size of sample that can be titrated. While voltage scanning coulometry now gives good detection limits—0.025 microgram of iron in a 5-ml. sample, for example—Dr. Scott thinks the detection limits can be lowered further Some of the ways to do this include expanding the vertical axis of the recorder chart with a larger readout resistor, reading difference between currents of a blank and titration cell, electronically subtracting a curve similar to the blank curve, concentrating the sample by mercury cathode electrolysis, or improving the cell design.
Smaller Lignins Are Best Dispersants
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40
C & E N J U N E 27, 1960
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6
While most lignin sulfonates are very good dispersing agents, the best ones seem to be those within a restricted molecular weight range, says Alec Rezanowich of the Pulp and Paper Research Institute of Canada. Watersoluble lignin sulfonates with molecular weights of 10,000 to 40,000, for example, are most successful in deflocculating aqueous TiOo dispersions, he told the Chemical Institute of Canada's 43rd Canadian Chemical Conference in Ottawa. Mr. Rezanowich and co-workers Dr. D. A. I. Goring and Joseph Jaworzyn note that, as a rule of thumb, the more effective lignin dispersants have lower intrinsic viscosities. To measure dispersing power, they observe viscosity reduction of a thick aqueous suspension of fine solid particles. A commercial rutile titanium dioxide (minimum 94% TiOo), with average particle size of 0.3 to 0.4 micron, is good enough
" * * * * .
These crystals were born lucky „
J the birth pangs of chemical synthesis!
Theirs was strictly a natural delivery f r o m FMC's pure trona ore resources found in a b u n d a n c e under t h e Wyoming earth crust. The crystals? Sodium sesquicarbonate. Use? They're calcined to form t h e purest SODA A S H . FMC soda ash has the inherent purity of a natural crystalline product. T h a t ' s why FMC, in a new expansion program, is adding 2 0 0 , 0 0 0 tons per year to present production of natural soda ash. FMC's Chlor-Alkali Division is one of t h e family of FMC Chemical D i v i s i o n s w h i c h i n c l u d e N i a g a r a C h e m i c a l , Becco C h e m i c a l , Chemicals & Plastics, and Mineral Products Divisions. With its diversity of products and services, FMC serves a broad base of customers in government, agriculture and industry.
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Coatings of polar organic compounds increase fatigue strength of some metals, National Bureau of Standards reports. NBS has tested C 8 and C 12 alcohols and C 1 2 and C 1 8 amines, finds the C 1 2 compounds best. Results show a 600% increase in the fatigue life of a magnesium alloy, and a 250% rise in that of a copper-beryllium alloy. The compounds apparently form tightly packed monolayers that keep out water and oxygen, NBS says.
WRITE for Bulletin 6-145. s* urree
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^-electrode. design increases pumping efficiency, keeps pressure stable, improves ionization of inert gases. 42
C & E N J U N E 27, 1960
for this purpose. It gives a viscous, smooth-flowing, rheologically reproducible suspension which thins markedly when they add lignin dispersants. Microgel Structure. Mr. Rezanowich and his colleagues propose that the lignin molecules form a microgel structure in solution. These macromolecules are made soluble by certain ionizing groups (such as sulfonate), which cause intermolecular repulsion as a result of double-layer interaction. Thus, when a layer of lignin microgel covers the T i 0 2 surfaces, the particles repel each other. This "protective colloid" action reduces the tendency of the titania to aggregate. Low molecular weight lignin compounds (the 10,000 to 40,000 fraction which provides the most effective deflocculation) are strongly and irreversibly adsorbed by titanium dioxide, Mr. Rezanowich reports. Below and above this range, dispersing power falls off. Drastically degraded lignin, though still polymeric, may no longer form a microgel. With samples of higher molecular weight, the microgels are comparatively large. With material having still higher molecular weight (150,000 and over), a second T i 0 2 particle attaches to one of the giant lignin molecules. This starts formation of agglomerates, and viscosity of the suspension rises. The effectiveness of a good lignin deflocculant increases as lignin is added to the system, the PPRI group finds. This holds true up to the limit of their measurements. Mr. Rezanowich considers it probable that lignins of all available molecular weights would act as dispersants if they could cover enough of the T i 0 2 surface. Thus, he concludes, a high molecular weight lignin sulfonate can act both as an aggregating agent or as a deflocculant, depending upon its concentration in the system.
Consolidated Vacuum ROCHESTER 3 , N E W YORK
