Feb.,
1920
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
sin A / 2 x IO0 sin A m a x . / 2 is not, however, serious for the work”required. Thus for A = s o , Amax. = 40°, we have C =
A/Amax.
X
IOO
instead of
Approximate C = 12.5 per cent
Accurate per cent
12.7
There appear t o be several advantages in replacing such expressions as “water-clear,” b y definite per cent clarity and definite colorimetric values, where color is a factor. Figs. I and 2 are reproduced by kind permission of Mr. H. E. Ives and the American Optical Society from t h e Journal of the Optical Society of America. M y thanks are also due t o Mr. Tompkins for assistance in making observations and computations.
169
The sample was placed in the cup of t h e decomposing bulb and drawn into t h e bulb. The cup was then washed with 2 one cc. portions of water, followed by 2 j cc. of 96 per cent sulfuric acid in three or four portions. The decomposition was started by gentle shaking of the bulb. When t h e gas was no longer rapidly liberated, t h e decomposing bulb was p u t under a vacuum of I O in. of mercury, t h e lower stopcock closed, and t h e bulb shaken violently for 4 min. The liberated gas was r u n h t o t h e measuring tube, allowed t o cool for 1 5 min., and the reading taken. A mirror was used in comparing t h e levels of mercury in the various tubes, thus avoiding any errors due t o parallax. The samples used were of such size t h a t t h e effect of slight errors in reading the volume of gas would be negligible.
THE EFFECT OF CHLORIDES ON THE NITROMETER DETERMINATION OF NITRATES’ FORCITE
By M. T. Sanders EXPERIMENTAL LABORATORY, ATLASPOWDER CO., LANDING, N. J.
T h e nitrometer furnishes a very rapid and accurate means of analyzing a substance which quantitatively liberates a gas when treated with liquid reagents, a n d is usually used t o determine nitrogen in nitrates. The nitrate solution is washed into t h e decomposing bulb and some sulfuric acid drawn in after it. I n t h e presence of t h e acid, t h e mercury reduces t h e nitrate, liberating nitric oxide. The gas is then run into t h e measuring tube, allowed t o cool, and t h e volume read. Nitrometer measuring tubes are of two kinds, one for guncotton, which is graduated t o read per cent nitrogen when a one-gram sample is used; t h e other, t h e so-called “universal tube,” is so graduated t h a t 0.01mole of a gas, a t 2 0 ’ C. and 760 m m . , will read 100. Crude sodium nitrate nearly always contains a few per cent sodium chloride. The purpose of this work was t o determine t h e maximum quantity of sodium chloride which can be present in t h e sample a n d yet permit accurate analyses t o be made on t h e nitrometer. As no reference t o t h e effect of sodium chloride on t h e nitrometer determination of nitrates could be found in such literature as was available at t h e laboratory, it was thought best t o determine t h e accuracy of t h e nitrometer method for samples of sodium nitrate containing varying quantities of sodium chloride. The samples used in t h e nitrometer are small, 0.85 g. for sodium nitrate. T o avoid t h e difficulty involved in mixing small quantities of t h e dry salts, nearly saturated solutions of sodium nitrate and sodium chloride were made up, and t h e samples prepared by mixing weighed portions of these solutions i n a bottle. The actual sample for analysis was weighed by means of a Lunge pipette. The procedure was as follows: The three-way stopcock and capillary tube of t h e decomposing bulb were filled with mercury. 1 Presented before the Division of Industrial and Engineering Chemistry a t the 58th Meeting of the American Chemical Society, Phdadelphia, Pa., September 5, 1919.
2
I
0
Mu/! NQC/
i
3
4
/Yo/s. NaNo,
The more chlorides in the sample, t h e more sludge was formed in t h e decomposing bulb, and t h e more difficult i t was t o clean t h e mercury in t h e decomposing bulb preparatory t o t h e succeeding determination. It will be noticed in Table I t h a t duplicate determinations on samples high in chlorides did not check well. This is probably due t o the fact t h a t t h e sludge in the decomposing bulb trapped some of t h e gas. TABLEI NaC1 Moles NaNOa 0.00
0.2918 0.5749 0.9776 1.950 2.728 3.129 3.782
Per cent NaNOs by Analysis
I
I1
42.21 31.87 25.63 20.22 13.57 11.00 11.82 14.96
42.20 31.90 25.64 22.13 13.31 10.63 11.99 14.68
Mean of Results Per cent NaNOa 42.21 31.89 25.64 22.18 13.44 10.82 11.90 14.82
Per cent NaNOa Actually Present 42.18 31.86 25.58 20.24 13.34 10.48 9.44 8.12
Error 4-0.03 f0.03 4-0.06 -0.06 f 0 . 10 4-0.34 4-2.46 f6.70
Per cent Error +0.07 +0.09 +0.23 -0.30 i-0.75 4-0.324 f26.1 +62.5
From t h e compositions and weights of t h e solutions of sodium nitrate and sodium chloride used, t h e per cent of sodium nitrate actually present in t h e sample was calculated. The results of the nitrometer determinations were also calculated as sodium nitrate. From these figures t h e percentage error of each determination was calculated. I n Table I are tabulated t h e ratio of moles of sodium chloride t o moles of sodium nitrate in t h e sample
I
70
T H E .JOURNAL OF I N D U S T R I A L A N D ENGTNEERING C H E M I S T R Y
used, t h e actual per cent of sodium nitrate present, t h e per cent of sodium nitrate as determined on t h e nitrometer, and t h e percentage error of each determination. This table shows t h a t if t h e percentage error in a determination is t o be kept below t h e usual value of 0 . I , t h e ratio of t h e moles of sodium chloride t o motes of sodium nitrate present in t h e sample must be less t h a n I : 3. This corresponds t o about 17 per cent sodium chloride in a mixture of t h e d r y salts. When t h e ratio of t h e moles of sodium chloride t o moles of sodium nitrate and t h e percentage error are plotted, it is seen t h a t t h e curve makes a sharp break at t h e point where Moles NaCl = 3.0. Moles NaN03 Moles HC1 in t h e equation This is also t h e ratio of Moles “ 0 3
It is probable t h a t thisreaction takes place in t h e decomposing bulb. The nitrosyl chloride attacks t h e mercury and liberates nitric oxide, which, recombining with t h e free chlorine, repeats t h e cycle until all t h e free chlorine is used up. If an excess of sodium chloride were present, hydrochloric acid would be formed, and t h e gas in t h e measuring tube would be a mixture of hydrochloric acid and nitric oxide. ’‘ Tests showed t h a t under these conditions hydrochloric acid and nitric oxide were actually present. No attempt was made t o prove t h e absence of chlorine in these gases, or t o show t h a t t h e sludge in t h e decomposing bulb contained chlorides of mercury. I n conclusion i t may be stated t h a t it was impossible t o obtain results accurate t o 0 . I per cent if t h e sample contained more t h a n I 5 t o I 7 per cent sodium chloride, on a dry basis A MODIFICATION OF THE THOMPSON METHOD THE DETERMINATION OF ACETIC ACID IN WHITE LEAD
FOR
By L. McMaster and A. E. Goldstein CESMICALLABORATORY, WASHINGTON UNIVERSITY, ST. LOUIS,Mo. Received August 18, 1919
Of t h e several methods described in t h e literature for determining acetic acid in white lead, t h a t of Thompson’ is t h e most reliable, both for white leads which have not been ground in oil and for those from which t h e oil has been extracted. In this method 18 g. of dry white lead are placed in a 5 0 0 cc. flask which is arranged for connection with a steam supply and also with a condenser. To the white lead are added 40 cc. of sirupy phosphoric acid, 18 g. of zinc dust, and about 50 cc. of water. The mixture is distilled down t o a small bulk and steam passed in until the flask is half full of water, when the steam is shut off and the mixture again distilled to the same bulk. This operation is conducted twice. The distillate is then transferred t o a special flask and one cc. of sirupy phosphoric acid added. This mixture is distilled until about 2 0 cc. remains in the flask, and steam is passed through the flask until it contains 1
J . SOC.Chem. I n d . , 24 (1905),487.
Vol.
12,
No.
2-
about 2 0 0 cc. of condensed water, when the steam is shut offand the liquid again distilled. These operations are repeated until I O cc. of the distillate require but one drop of N/ro alkali t o change the color of phenolphthalein. The bulk of the distillate is titrated with N / r o alkali, and the acetic acid calculated.
We have used this method t o determine t h e acetic acid in a number of samples of white lead, b u t t h e method is long and tedious, and i t is necessary t o use a special, rather fragile flask. Thompson states t h a t . “if t h e dry white lead under examination has been obtained by t h e extraction as a residue from white. lead paste, i t is well t h a t this extraction should be exceedingly thorough, as otherwise f a t t y acids may be held and distilled with t h e acetic acid. Even then they will not interfere with t h e final titration, as they may be filtered from t h e distillate before titration.” I n t h e analysis of a number of samples of extracted white lead by this method we were never able t o get a clear filtrate free from f a t t y acids, even after long extraction; nor were we able t o remove all of t h e f a t t y acids by filtration since a great part of them was present in colloidal suspension. The results, therefore, were always somewhat high. We have found t h a t t h e time of a n analysis can b e materially shortened, and t h e passing over of t h e f a t t y acids into t h e distillate obviated by performing one initial steam distillation followed by a second distillation under reduced pressure. The reduced-pressure method is as follows: 18 g. of extracted white lead are placed in an ordinary joo cc. flask, arranged for connection with a steam supply and also with a condenser, and 40 cc. of sirupy phosphoric acid (85 per cent) and 50 cc. of water are added. The flask is heated directly and t h e material distilled down t o a small bulk. Steam is next passed into t h e flask and t h e distillation continued until about a 600 cc. distillate is obtained. After adding about 0.5 cc. of t h e phosphoric acid, t h e distillate i: transferred t o a heavy-walled flask. I n t o t h e neck of the flask is inserted a two-hole stopper. Through one hole is passed a very small bore capillary tube through which a minute stream of air is allowed t o pass during t h e distillation. Through the other hole is passed a connecting bulb tube attached t o a condenser. The whole apparatus is so arranged t h a t t h e distillation may be conducted under a reduced pressure of about 150 mni., using a n Erlenmeyer filtering flask as a receiver. When I O cc. of t h e distillate require b u t one drop of N / I O alkali t o produce a color change in phenolphthalein, t h e distillation is stopped, t h e distillate titrated w;th N / I O alkali and t h e acetic acid calculated. It is seldom necessary t o distil over more t h a n joo cc. At no time should t h e liquid in t h e distilling flask be allowed t o go much below I O O cc. because of t h e fact t h a t phosphoric acid is often drawn over by t h e suction. We have used t h e same quantities of materials as were used by Thompson so t h a t t h e two methods could be compared. We found it unnecessary t o use t h e zinc dust. A series of determinations was carried out by t h e Thompson method and by t h e reduced-pressure method
