This paper is to report the results of a study which was made about the effect of magnesium ion and water content on caking of common salt. For this study were prepared sample salts by adding 0.002-1.0% magnesium ion and 0.005-2.0% water to refined salt, and the properties of these sample salts were determined in respect of critical humidities, free flow, caking tendencies by both the pressure and re-crystallization methods and absorption tendencies. Results of the study were as follows:
1. The critical humidities of these sample salts showed 40-70% R. H. at 25°C, when the liquid attached to the surface of these sample salts was saturated with both sodium chloride and magnesium chloride (magnesium ion concentration 107 mo1/1 000 mol H₂O). As the content of magnesium ion decreased, the critical humidities approached to 75% R. H. The critical humidities of those sample salts having impurities (MgCl₂·4H₂O) in the liquid + solid area, declined to less than 10% R. H. with an increase of solid phase. However, the critical humidities of those sample salts having impurities in the liquid area, showed the same values as those of the liquid attached to the surface of the sample salts.
2. Those sample salts having impurities in the solid + liquid area and possessing more than O.05% magnesium ion, kept a free-flow for 200 days in air-tight bottles.
3. In the re-crystallization method, caking strength of the sample salts reached to such a little amount as 1kg/cm², in general. The sample salt having either less than 0.007% magnesium ion or more than 0.1% magnesium ion and 1 % water caused a little caking.
4. In the pressure method, the sample salts decreased their caking strength in the liquid area in line with the increase of liquid. In the liquid + solid area for the sample of 0.1% water, the caking strength decreased until magnesium ion reached 0.05%, but increased considerably after magnesium ion increased over 0.05%.
5. In the liquid area, the sample salts showed a constant rate of absorption, while in the liquid + solid area they showed a large rate of absorption for an hour in the early stage but a constant rate afterwards.
6. The sample salts equilibrated with 30, 50, and 70% R. H. desiccator showed the water content according to the magnesium ion.

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For fear that conditions for an electrodialysis in seawater concentration will be worsened in future due to reduction in production cost, the author studied a method to determine the probability of scale deposition under anticipated worse conditions from scale deposition under present conditions. For this study, the amount of solute ions removed when they passed through a desalting compartment (the desalting range) was assumed to be a normal distribution, and the ratio of the standard deviation to the desalting range a, was used as an indicator of the distribution character in the unit.
The probability of scale deposition was determined by comparing the distribution with limited desalting ratio for scale deposition which the author had previously reported. Data of the scale deposition obtained with an experimental apparatus of 5 dm² in the effective area well agree with the author's calculated values, which proved to be applicable to practical use. The probability of the scale deposition under larger desalting ratio (i. e. utilization ratio of sea water) was calculated from observed values for a practical scale apparatus.
As a result, the probability of CaCO₃ scale deposition showed a rise from 0.4% to nearly 17%, when the desalting ratio increased from 23% to 40%. When the desalting was divided into two steps to avoid scale deposition, there were optimum values in the allotment of the desalting range to the first and second steps.

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As a measuring method of kinetic transport number of ion exchange membrane, the transport number of halide ions across anion exchange membrane was determined from transfer equivalent of potassium ion, that is co-ion, for a system having potassium chloride solution in anode side and sodium halide solution in cathode side to anion exchange membrane.
The transport number of halide ions decreased due to an increase in the concentration of potassium chloride solution in anode side and also of current density. These facts were due to the increase of Donnan absorbed ion with an increase of concentration of potassium halide in interfacial layer of anode side to anion exchange membrane. But the concentration of sodium halide solution in cathode side had no effect on the transport number of halide ions.
The transport number of halide ions determined by this method decreased inthe following order; F^->C1^->Br^->I^-

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For the purpose preventing alkaline scale from the desalination evaporator, the decomposition of carbonates in the brine was studied by adding the seed crystals to slaked lime under the following conditions: (1) apparatus; shaking bath,(2) brilie concentration; CF 2,(3) temperature; 40, 60, 80 and 100°C at the non boiling state,(4) slaked lime concenfration 1 eq. to Ca (HCO₃) ₂ (200ppm),(5) seed crystals; aragonite, calcite and brucite. The results were as follows:
1) Aragonite was the most effective on the decomposition of carbonates among these seed crystals.
2) Effect of aragonite on the decomposition of carbonates was increased due to the simultaneous use of slaked lime. In this case, the most effective temperature was at 40°C.
3) The values of K and A were calculated by applying Murotani's formula of velocity of crystallization ν=K(c/c₀-A) to these experimental results. The values of K in non-addition and addition of aragonite were 0.22 and 0.12, and the values of A in non-addition and addition of aragonite were 0.85 and -0.65 as the saturation index of CaCO3. The value of A was decreased as result of the addition of aragonite.

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For prevention of scale in the multi-stage flash evaporator, bench scale tests were made by useing 6-stage flash evaporator, Each stage had a heat transfer surface of 1m². The results were as follows:
1) A new method, in which equivalent amounts of CaCO₃ and Mg (OH) ₂ were added together with slaked lime, proved to be more effective on prevention of alkaline scale than any of the following three methods;“additive-treatment”,“addition of alkaline reagent” and “addition of seed mixture.” In the new method, the stable and high over-all heat transfer coefficient was obtained during the run of 314 hours.
2) The new method was applicable to either new or old multi-stage flash evaporator.
3) The experimental results from the addition of calcium sulfate anhydrate together with slaked lime showed that the new method was applicable to the range of deposition of calcium sulfate.

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