AGRICULTURAL CHEMISTRY — AGROTECHNY. 411 



a study of the Loieiiz method iiiid the Popi) iron citrate method (E. S. R., 29, 

 p. 410). 



The Lorenz metliotl was studied with chemically pure salts of known phos- 

 phoric acid content and later with additions of calcium and iron salts, and 

 silicates, and the results obtained corresponded well with the theoretical tigures. 

 With Thomas slag powders it gave figures which were always lower tlian those 

 obtained by the Official Method (removal of silica by hydrochloric acid). It 

 also presents many difliculties in the way of manipulation. 



The Popp method was studietl from the standpoint of time requiretl for 

 precipitation (determination made directly after shaking and after 48 hours), 

 the time for filtration (directly after stirring and 24 hours later), and the 

 effect of time of stirring (one-half hour and one-quarter hour). The results 

 obtained were practically the same in all instances. 



In a comparison with the Official Method, the Popp method with 23 samples 

 of slag powder gave slightly higher results, and in 17 cases somewhat lower 

 results. On the whole, the figures obtained by the two methods agreed very 

 well, and in only two cases was there a difference of over 0.2 per cent (0.25 to 

 0.22 per cent) noted. The Popp method always gave higher results than the 

 Lorenz method, the average results being 16.201 and 15.00.5 per cent of jihos- 

 phoric acid, re.s'TJectively. 



Winkler's method for the determination of oxygen in water; the effect 

 of nitrite and its prevention, F. E. Hale and T. W. Melia {Jour. Indus, and 

 Engin. C'hcm., 5 {1913), No. 12, pp. 976-980). — Considerable criticism has arisen 

 of late regarding the effect of nitrites ui)on the results obtained by the Winkler 

 method. The present experiments were undertaken for the purpose of ascer- 

 taining the extent of error with varying amounts of nitrite and to find, if 

 possible, a simple method for preventing the error. 



The results show that when nitrites are present in quantities below 0.2 part 

 per million in ordinary tap water, the figures obtained for dissolved oxygen are 

 fairly accurate. Winkler showed the danger point to be with 0.1 part per 

 million or over. 



" The effect of high nitrite present in any amount ever occurring in water 

 may be counteracted by the use of potassium acetate solution (or sodium acetate 

 crystals) to neutralize the hydrochloric acid before exposure to the air. The 

 procedure then is to add at the bottom of the bottle 2 cc. of manganous sulphate 

 solution (480 gm. per liter) followed by 2 cc. of potassium hydi'oxid-iodid solu- 

 tion (360 gm. hj^droxid and 100 gm. iodid per liter). Shake and settle. Add at 

 the bottom 2 cc. concentrated hydrochloric acid and shake until the precipitate 

 is entirely dissolved. Then add 2 cc. potassium acetate solution (1,000 gm. per 

 liter) at the bottom and mix. Withdraw by pipette 100 cc. into an Erlenmeyer 

 flask and titrate with 1/100-normal thiosulphate, adding a little starch solu- 

 tion at the end. Correction must be made for 6 cc. of water displaced by the 

 first two solutions and by the acetate solution. The acid needs no coi'rection, 

 since it displaces only water from which oxygen has been removed. 



" Precautions must be taken (1) to exclude contact with air as much as possi- 

 ble until the solution is ready to titrate, by replacing the stopper as quickly as 

 possible after each reagent is introduced; (2) to get complete solution of the 

 precipitate by the hydrochloric acid; (3) to give the thiosulphate a little more 

 time to react at the endpoint in the acetic acid condition; (4) to use small 

 amounts of starch solution. For introducing the solutions it is advisable to use 

 pipettes with two marks, measuring 2 cc. well up on the pipette so as to give 

 head and not contaminate the upper liquid in the bottle more than necessary 

 while actually displacing liquid from the bottle in proportion to the amount 

 introduced. 



