The Analyst : the journal of The Society of Public Analysts and other Analytical Chemists : a monthly journal devoted to the advancement of analytical chemistry. Vol. 69. No. 820

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K 2Cr20 7 Mol. W t. 294-21

A C T U A L B A T C H A N A L Y S I S (Not merely maximum Impurity values)

Batch N o. 18667 Patassium Dichrom ate... 99.9%

Chloride (C l)... 0.00025%

Sulphate ( S O

4

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JULY, 1944 Vol. 69, No. 820

T H E A N A L Y S T #

P R O C E E D I N G S O F T H E S O C I E T Y O F P U B L I C A N A L Y S T S A N D O T H E R A N A L Y T I C A L C H E M I S T S

T h e H y p o p h o s p h o r o u s A c i d R e d u c t i o n o f T i n . A n I m p r o v e d P r o c e d u r e a n d N o t e s o n i t s M e c h a n i s m *

B

y

B. S. E V A N S , M.C., M.B.E., D.Sc., F.R.I.C.,

a n d

D. G. H I G G S

Th e

research which forms the basis of the following paper w a s undertaken in the first instance to provide some definite data on the subject of titration of tin in presence of m u c h larger amounts of antimony. Statements ha d been m a d e that the end-point under these con­

ditions w a s so1 transitory as to be uncertain; in addition to this, the influence of a trace of copper,.with or without antimony, had always been suspect.' In the early stages, however, when endeavouring to carry out fairly large {e.g., 40 ml) titrations with 1V/100 iodme it became obvious to us that certain minor irregularities were liable to occur somewhat frequently, and it became necessary to investigate the causes of these before attempting further work. T h e process under investigation was first published in 1931.1

M

e t h o d

— A s carried out in this laboratory u p to the present, details of this process

are as follows. Place the soln. of tin in the flask (Fig. 1) and bring the acidity up to 1 :1 (HC1) and the vol. to 100 ml. A d d 1 m l of sat. mercuric chloride soln. and

4 g of sodium hypophosphite a n d place the stopper in position. Sweep out the apparatus with a rapid current of carbon dioxide for 10 min., then slightly reduce the rate of the current, place the flask on the hot plate and boil gently for 15 min. Close the tap of the funnel, open fully the tap of the carbon dioxide supply, and allow the flask to cool.

Meanwhile gently boil a mixture of 20 m l of 5 0 % citric acid soln., 10 m l of 4 % potassium iodide soln., a few m l of starch soln. and 250 m l of water for 10 min. a n d then cool. W i t h d r a w the glass plug from the stopper of the titration apparatus and run the other soln. (referred to below as the "diluting liquid”) in through the funnel, taking care to avoid admission of air. Finally insert the jet of the burette containing the titrating liquid into the hole which formerly contained the glass plug, and titrate.

A n illustration of the scope of the irregularities (which were mainly but not always negative) m a y be taken from a series carried out with only tin present, and under the exact conditions laid d o w n in the original process:— Taken: 0 0250 g. Found: 0-0253, 0-0248, 0-0247, 0-0243, 0 :0244, 0-0243, 0-0242 g.

Influence of Dissolved Oxygen— In view of the work of Okell and Lumsden2 it was obviously necessary to attempt to find out if dissolved oxygen was the cause of the low results. Whilst, as one of the authors pointed out during the discussion on that paper, Okell and L u m s d e n ’s results were obtained under conditions far more favourable to air oxida­

tion than those of the process under discussion, it was not to be hoped that this latter process would entirely escape the effect. N o r did it, figures such as the following being obtained:—

Added: 0-240 g. Found: b y N / 10 I titration 0-0251 g; by IV/1001 titration 0-0243 g, the lower figure being apparently due to oxygen dissolved in the larger volume of titrating liquid required. There is, however, no need to use such large vols. of titrating liquid, small vols.

of stronger solns., say, A r/10, serving equally well; the oxygen dissolved in the titrating liquid therefore, does not particularly concern us. T h e same cannot be said of the soln. used to dilute the reduced tin soln. after reduction: here the volume is large, approx. 300 ml, and

* C o m m u n ic a t io n f r o m t h e A r m a m e n t s R e s e a r c h D e p a r t m e n t { f o r m e r ly R e s e a r c h D e p a r t m e n t ,

W oolw ich). *

201

202 E V A N S A N D HIGGS: T H E H Y P O P H O S P H O R O U S ACID R E D U C T I O N O F TIN.

a small volume will not do. It is true the soln. is boiled before use and this seems to lower the ultimate content of oxygen, but Okell and L u m s d e n have shown2 that boiled water rapidly re-absorbs oxygen w h e n cooled in presence of air. It was necessary, therefore, to try to what extent this re-dissolved oxygen caused the low results referred to above and, if possible, devise some not too complicated mea n s of counteracting the interference. T h e method used throughout these experiments for ensuring absence of re-absorbed oxygen from the diluting liquid, i.e., cooling the latter under carbon dioxide, was adopted because it seemed to present no loopholes for contamination; it is not to be r e commended as a routine procedure on account of the extra complication. A very simple me t h o d of avoiding the difficulty will be described later (vid. inf.). Experiments carried out with the diluting liquid cooled in air, and others where it wa s cooled under carbon dioxide, immediately showed that re-absorbed oxygen is a major cause of these small errors. T w o titrations m a d e with a diluting liquid through which air had actually been bubbled gave a still lower result, providing still further confirma­

tion.

T i n t a k e n T i n f o u n d E r r o r

g g g

D i l u t i n g liq u id c o o le d in a ir 0 -0 1 0 0 0 0 0 0 5 2 - 0 - 0 0 0 4 8

0 -0 1 0 0 0 -0 0 9 4 2 - > 0 - 0 0 0 5 8 D i l u t i n g liq u id " b u b b l e d " w it h a ir 0 -0 2 6 0 0 -0 2 3 6 5 - 0 - 0 0 1 3 5

0 -0 2 5 0 0-0236 G - 0 - 0 0 1 3 5

D i l u t i n g liq u id c o o le d u n d e r C O s 0 -0 1 0 0 0 -0 1 0 0 0 n il

0 -0 1 0 0 0 - 0 1 0 0 0

.

^{n il}

Influence- of Acid Strength— A s it wa s found that the hydrochloric acid in use was con­

siderably weaker (sp.gr. 1-16 instead of T2) than that formerly used, it seemed worth while to ascertain what, if any, influence this had on the result. It was found b y a n u m b e r of titrations that the acid strength of the final liquid might vary between 1 an d 5 % of conc. hydro - chloric acid per 100 m l without any decided influence on the result; in fact, it appeared as if the weaker liquids were slightly the better (cf. R. Holtje,3 w h o states that H C 1 strength should lie between 0-2 and 1-0%). As, however, it wa s found in later work that antimony is partially titrated in 1 % acid, it is apparent that lowering of the acid concentration (e.g., b y addition of N a 2C 0 3) before titration introduces far more danger than advantage. T h e acid concn. of the » liquid in which the tin was being reduced was, however, another matter, and incomplete reduc­

tion of the tin might readily result from the hydrochloric acid strength of the reducing liquid being too low. T h e follow’ing errors in a series of titrations, in 3 of which the reducing liquid wa s 5 0 % of conc. hydrochloric acid (sp.gr. 1-20) while the other three were 5 0 % hydrochloric acid (sp.gr. 1-16), were obtained. Sp.gr. 1-2: +0-00004, — 0-00002, +0-00001; sp.gr. 1-16:

— 0-00013, — 0-00013, — 0-00033. As the escape of acid vapour from the flask during the.

15min. boiling for reduction is apt in the normal process to be extremely variable owing to superheating effects, reduction in these expts. wa s carried out with flasks immersed in boiling water for 20 min. and not boiled. Under these conditions reduction was complete and very little acid was evaporated. T h e procedure was therefore adopted as less liable to variation throughout this research; it has been discarded in the final process because it has proved to present no real advantage over the more direct boiling.

Influence of Mercury Catalyst— W h e n long series of titrations were attempted, using these refinements, it became evident that there were still unexplained factors; discrepancies still occurred occasionally and n o w slightly high results appeared from time to time. Examples of the latter are the following:— Taken: 0-0250 g. Found: 0-0253, 0-0254, 0-0256, 0-0254, 0-0254 g. L o w results are readily explainable, but occasional high results were very per­

plexing. T h e only factor which had been ignored hitherto would seem to be the mercury catalyst. There appears to be at least a possibility of the formation of mercurous chloride during the process of reduction by hypophosphite, and this mercurous chloride, once formed, might function conceivably in either or both of two ways: (i) It might so obscure the mercury surface as materially to reduce the rate of reduction of the tin, b y lessening the catalytic effect; (ii) It would probably be titratable with iodine, giving a tendency towards high results.

Experiments directed towards elucidating these points were as follows:

(a) O n e drop of metallic mercury was added in place of the usual 1 m l of mercuric chloride solution (sat.).

Taken: 0-0050 g of tin; Found: 0-0015 g of tin.

Here the greatly reduced surface of the mercury probably explains the low result.

A N I M P R O V E D P R O C E D U R E A N D N O T E S O N ITS M E C H A N I S M 203

(i>) Smaller quantities of mercuric chloride soln. were used.

0-5 m l Taken: 0-0100 g of tin; Found: 0-00995 g.

0-1 m l Taken: 0-0050g of tin; Found: 0-00236 g.

It is evident from this that the 1-0 m l prescribed is adequate but not greatly excessive, 0-1 m l yielding less than half reduction.

(c) In view of the fact that mercuric chloride in solution is largely un-ionised, it was thought that mercuric cyanide might be less prone to give mercurous chloride.

Results were better but still not sufficiently concordant.

( i d) A n attempt w a s m a d e to eliminate chlorides altogether b y reducing a sulphuric acid soln. of tin in 100 m l of 1 : 3 sulphuric acid.

(1) Mercuric chloride catalyst. 0-0250 g of tin required 4-5 m l of N/ 100 (should be 42-10).

(2) Mercuric cyanide catalyst. 0-0100 g of tin required 0-5 m l of N /100 (should be 16-89).

T h e mercuric chloride w a s used accidentally owing to our forgetting that it w a s introducing chlorides. T h e results, however, are very interesting, inasmuch as the 1 0 % reduction obtained with the chloride wa s lowered to 2 % w h e n chlorides were absent.

T h e results obtained so far seemed to indicate:— (i) That chlorides were necessary to the functioning of the catalyst; (ii) that the mercury was liable to be contaminated with a sub­

stance (Hg2Cl2?) which might produce results either slightly too high or slightly too low.

(e) O n the assumption that mercurous chloride wa s the interfering substance there seemed a likelihood that if the mercury were pptd. as metal in absence of chlorides and then added to the liquid to be reduced, the formation of mercurous chloride would be extremely small, although the metal would still be in a very finely divided state. Tests carried out on this assumption proved satisfactory; the catalyst used was a solution of mercuric cyanide reduced b y boiling in dii. (1 : 3) sulphuric acid with a little sodium hypophosphite; it was then added to the tin which was in solution in 1 : 1 hydrochloric acid and the process was carried out as usual. Subsequently it w a s found that so long as the mercury is reduced apart from the tin the inter­

ference does not seem to occur; consequently, to avoid, unnecessary complication, the mercuric cyanide in the process is reduced in 1 : 1 hydrochloric acid.

T h e process finally adopted was as follows:

A m e n d e d Process— Place the soln. containing the tin to be determined in the flask of the titrating apparatus, bring it to approx. 5 0 % strength (of the strong acid sp.gr. 1-2) with hydrochloric acid and add dii. (1 : 1) hydrochloric acid to give a vol. of ca. 90 ml. Place 10 m l of 1 : 1 hydrochloric acid in a small beaker, add I m l of 1 % mercuric cyanide soln.

and 0-5 g of sodium hypophosphite, boil for ca. 1 min. and ad d to the tin soln. in the flask.

Insert the stopper, sweep out the apparatus for 10 min. with a rapid current of carbon dioxide, leaving the tap of the funnel open and the glass plug in position. Boil gently for 15 min.

over a small flame, with the carbon dioxide still passing. Close the exit tąp, leaving the supply tap of the carbon dioxide open, remove from the burner, and allow to cool under pressure of the carbon dioxide.

M a k e u p a diluting liquid from 10 m l of 4 % potassium iodide soln., 20 m l of 5 0 % citric acid soln., 250 m l of water and a few m l of 1 % starch soln., boil gently for 10 min. and cool.

W h e n both flasks are cold add 3 g of sodium bicarbonate to the diluting liquid, remove the glass plug from the stopper of the titrating apparatus, and immediately run the diluting liquid through the funnel into the flask, taking care that no air is admitted. Insert the jet of the burette carrying the titrating liquid into the hole from which the glass plug was removed, and titrate in the ordinary manner.

This modification, which is put forward for ordinary use to replace the original method,1 was the process used in obtaining the results given in the remainder of the paper, except for two additional refinements, (i) In every expt. the diluting liquid wa s cooled under carbon dioxide and no bicarbonate w a s added; (ii) a slight modification is necessary in presence of antimony. This will be referred to later.

W i t h regard to (i) the cooling under carbon dioxide w a s adopted, in this instance, to

m a k e sure of the absence of disturbing factors; it is, however, too cumbersome for normal

204 E V A N S A N D HIGGS: T H E IIYPOP H O S P H O R O U S ACID R E D U C T I O N O F TIN

use, and the following trials showed that the bicarbonate addition is quite effective in eliminat­

i n g

the dissolved oxygen.

D i l u t i n g liq u id c o o le d in a ir

D i l u t i n g liq u id c o o le d u n d e r C O ,

D i l u t i n g liq u id co o le d in a ir ; 3 g o f N a H C O , a d d e d

D i l u t i n g liq u id n o t b o ile d ; 3 g o f N a H C O , a d d e d . .

T h e unboiled liquid is almost as good as that which had been boiled; boiling, however, gives a slight improvement and seems safer.

T h e process, as described, was tested against varying amounts of tin, other metals being

T i n t a k e n T i n f o u n d E r r o r

g g g

0 0 1 0 0 0 -0 0 9 5 2 - 0 - 0 0 0 4 8

0 -0 1 0 0 0 -0 0 9 4 2 - 0 - 0 0 0 5 8

0 -0 1 0 0 0 -0 1 0 0 n il

0 -0 1 0 0 0 -0 1 0 0 n il

0 -0 1 0 0 0 -0 1 0 0 n il

0 -0 1 0 0 0 -0 1 0 0 9 + 0 -0 0 0 0 9 0 -0 0 9 9 0 -0 0 9 8 9 - 0 - 0 0 0 0 1 0 -0 0 9 9 0 -0 0 9 8 3 - 0 - 0 0 0 0 7 0 -0 1 0 0 0 -0 0 9 8 9 - 0 - 0 0 0 1 1 0 -0 1 0 0 0 -0 0 9 9 4 - 0 - 0 0 0 0 6 0 -0 0 9 9 0 -0 0 9 8 6 - 0 - 0 0 0 0 4 0 -0 0 9 9 0 -0 0 9 8 6 - 0 - 0 0 0 0 4

absent, with the following results:

T i t r a t i o n

T i n t a k e n

r

^A T i n f o u n d E r r o r

g A c t u a l T h e o r e t ic a l g g

m l m l

0 -0 1 0 0 17-00 16-84 0 -0 1 0 0 8 + 0 -0 0 0 0 8

0 -0 0 9 0 14-95 15-16 0 -0 0 8 8 7 - 0 - 0 0 0 1 3

0 -0 0 8 0 1 3-30 13-4 7 0 -0 0 7 8 9 - 0 - 0 0 0 1 1

0 -0 0 7 0 11-94 11-79 0 -0 0 7 0 9 + 0 -0 0 0 0 9

0 -0 0 6 0 10-25 10-1 0 0 -0 0 6 0 8 + 0 -0 0 0 0 8

0 -0 0 5 0 8-25 8-42 0 -0 0 4 9 0 - 0 - 0 0 0 1 0

0 -0 0 4 0 6-80 6 -7 4 0 -0 0 4 0 3 + 0 -0 0 0 0 3

0 -0 0 3 0 5-05 5 -05 0 -0 0 3 0 0 n il

0 -0 0 2 0 3 -30 3 -37 0 -0 0 1 9 6 - 0 - 0 0 0 0 4

0 -0 0 1 0 1-75 1-68 0 - 0 0 1 0 4 + 0 -0 0 0 0 4

Titration of Tin in Presence of Antimony— T h e difficulty introduced b y antimony appears to be due to the extreme fugitiveness of the end-point at ordinary temperatures, which m a y lead to over-titration or even to no definite end-point at all. Cooling the liquid to as low a temperature as practicable both before and during the titration seems to give reasonably accurate results, thus:

Tin taken, g: 0-0099; antimony taken: 0-100; tin found, g: at 5° C. 0-0101; at 6° C., 0-0100; at 24° C „ 0-0110; -at 15° C., 0-0105.

Worse, however, than the results being generally high is their uncertainty at ordinary tem­

peratures. Cooling to a sufficient degree is, however, a somewhat laborious process, especially during a heat w a v e and it w a s our aim to find a reagent which would sufficiently stabilise the end-point at ordinary temperatures. M a n y reagents were tried, but only one— a m m o n i u m oxalate— gave the effect desired; if 5 g of a m m o n i u m oxalate are added to the diluting liquid before boiling, the end-point is quite sharp at such temperatures as 20° C. The following series w a s carried out b y the same process as for tin alone, with merely this modification:

T i t r a t i o n

T i n t a k e n A n t i m o n y t a k e n A c t u a l T h e o r e t ic a l T i n f o u n d E r r o r

g g m l m l g g

0 -0 1 0 0 0 -1 0 0 16-89 16-84 0 -0 1 0 0 3 + 0 -0 0 0 0 3

0 -0 0 9 0 0 -1 0 0 15-0 5 15-16 0 -0 0 8 9 4 - 0 - 0 0 0 0 6

0 -0 0 8 0 0 -1 0 0 1 3 -4 S 13-47 0 -0 0 8 0 0 n il

0 -0 0 7 0 0 -1 0 0 11-72 11-7 9 0 -0 0 6 9 6 - 0 - 0 0 0 0 4

0 -0 0 6 0 0 -1 0 0 10-07 10 -1 0 0 -0 0 5 9 7 - 0 - 0 0 0 0 3

0 -0 0 5 0 0 -1 0 0 8-47 8 -42 0 -0 0 5 0 2 + 0-0Q 002

0 -0 0 4 0 0 -1 0 0 6 -7 2 6 -74 0 -0 0 3 9 9 - 0 - 0 0 0 0 1

0 -0 0 3 0 0 -1 0 0 5 -0 5 5 -05 0 -0 0 3 0 0 n il

0 -0 0 2 0 0 -1 0 0 3 -3 5 3 -37 0 -0 0 1 9 9 - 0 - 0 0 0 0 1

0 -0 0 1 0 0 -1 0 0 1-65 1-68 0 -0 0 0 9 8 - 0 - 0 0 0 0 2

F r o m these figures it is obvious that, in this modified process, antimony has absolutely

no effect on a tin titration.

A N I M P R O V E D P R O C E D U R E A N D N O T E S O N ITS M E C H A N I S M 205

Blank— There is always a small blank to be deducted from a 1V/100 iodine titration due to the a m o u n t of iodine required to show a blue colour with the starch. This blank has been deducted from all the figures given here. O n e interesting fact, however, that emerged was that 0-1 g of antimony put through the complete original process, without modification, gave exactly the same blank as that done in acidified water alone and the end-point was stable. In other words, the high results and fading end-points associated with antimony seem only to be s h own in presence of tin.

End-Point— T h e end-point obtained in these hypophosphite reductions is never per­

manent in the sense of remaining unchanged for several minutes, and this is especially so where antimony is present. T h e end-points obtained in this process, however, are perfectly sharp; the blue colour spreads throughout the liquid and remains unchanged for at least several seconds; a trace of tin still unoxidised destroys it instantaneously.

Temperature— A s already stated, the unmodified procedure, in presence of antimony required the liquid to be somewhat drastically cooled in order to be manageable. Expts.

proved that this applies also to tin alone at a slightly higher temp., but the results are low instead of being high, thus: Tin taken, g: 0-0250; tin found, g: at 35° C., 0-0209, 0-0223, 0-0242; at 30° C., 0-0221, 0-0222. It is therefore obvious that the liquid must be quite cold before titration is performed.

Titration in Presence of Copper— In the main w e confirm Okell and L u m s d e n ’s finding2 that, with small amounts of copper, there is very little influence on the tin titration, but, in addition, it must be noted that, with these hypophosphite reductions, the presence of copper accelerates the fading of the end-point colour. U p to 0-003 g of copper, the end-point is still sharp, although very transient; above 0-003 g the fading is too rapid for the end-point to be reliable, and with, say, 0-01 g there is no end-point at all. It wa s of considerable interest to find out if an otherwise negligible a m o u n t of copper would upset the titration in presence of considerable amounts of antimony; the following figures were obtained:

T i t r a t i o n

T i n A n t im o n y - C o p p e r _{r '} ■££---

Í

^{T i n}

t a k e n a d d e d a d d e d A c t u a l T h e o r e t ic a l f o u n d E r r o r

g g g m l m l g

g

0 -0 0 5 0 — 0 -0 0 3 0 8 -74 8-42 0 -0 0 5 1 9 + 0 - 0 0 0 1 9

0 -0 0 5 0 — 0 -0 0 2 7 8-65 8-42 0 -0 0 5 0 7 + 0 -0 0 0 0 7

0 -0 0 5 0 — _{0 -0 0 0 9 8-06} 8-42 0 -0 0 4 7 8 — 0 - 0 0 0 2 2

0 -0 0 5 0 0 -1 0 0 0 -0 0 3 0 8-46 8-42 0 - 0 0 5 0 2 + 0 -0 0 0 0 2

0 -0 0 5 0 0 -1 0 0 0 -0 0 2 7 8 -39 8-42 0 -0 0 4 9 8 - 0 - 0 0 0 0 2

0 -0 0 5 0 0 -1 0 0 0 -0 0 0 9 8 -6 4 8-42 0 -0 0 5 1 3 + 0 - 0 0 0 1 3

0 -0 0 5 0 0 -1 0 0 0 -0 0 0 3 8 -45 8-42 0 -0 0 5 0 2 + 0 -0 0 0 0 2

It is evident from the above that copper below 0-003 g causes but little disturbance either in presence or in absence of antimony; the error is slightly greater than usual in three instances, but this is probably attributable to the rapid fading of the end-point. This trial has its value because it proves that the presence of accidental traces of copper (e.g., after a separation) has no measurable influence on the tin titration ; this is a point which, so far as w e are aware, has not been established before.

Catalytic Effect of Mercury— This effect has, as far as w e know, never been explained.

The effect itself has been adopted b y Feigl4 to provide a delicate test for mercury. W e would suggest that the real catalyst is a minute trace of mercurous chloride formed on the surface of the mercury; at any rate, the presence of chlorides appears to be necessary to its adequate functioning.

Reduction of Tin in Presence of Antimony by Metallic Reductants— Tin seems generally to be brought to the stannous condition prior to titration b y reduction with a variety of metals. Of these, lead seems to be far superior to the others, but quite a n u m b e r are e m ­ ployed, and each has its advocates. T h e y have, however, one bad feature in c o m m o n ; they all reduce antimony to the metallic form. It has been shown:

(i) that pptd. antimony co-precipitates appreciable amounts of tin 5*8;

(ii) that finely divided antimony will, in the cold, re-reduce tin which has been titrated6*7;

(iii) that where antimony itself is used as the reductant, results either high or low can be obtained at will, according to the fineness of the grinding6;

(iv) that where copper is present, also in solution, copper and tin are pptd. together,

presumably as a compound.6

206 c r o s s l e y

:

t h e r a p i d p h o t o m e t r i c d e t e r m i n a t i o n o f t e l l u r i u m

In view of the above findings, it would appear that where correct results are obtained, in presence of antimony or copper, with a metallic reductant, they cannot be true results at all but are due to a balance of errors.

T h e principal advantage claimed for the me t h o d of reduction which is the subject of this paper is that, except for a trace of mercury, which is inert, there is no metallic surface present in the liquid at all either before or after reduction.

Determination of Tin in Lead Alloys— F r o m the foregoing it is evident that whilst the m e t h o d already published,8-9 for the solution of lead alloys in perchloric acid and subsequent titration of tin, remains valid: (a) it is essential that care should be taken that the hydro­

chloric acid concentration in the reduction liquid is u p to the prescribed strength; (6) it is desirable that the mercury catalyst should be prepared in the w a y given in this paper and not as in the former ones8-9; (c) sodium bicarbonate should be added to the cooled diluting liquid before it is run into the titrating flask; (d) in presence of antimony, a m m o n i u m oxalate should be added to the diluting liquid before boiling.

S u m m a r y — (a) T h e causes of minor errors in the hypophosphite reduction m e t h o d for tin have been investigated and simple modifications for their elimination have been tested.

These modifications, which appear to be entirely effective, are:

(i) Ensuring that the acid concentration of the liquid during reduction is actually 1 :1 of the strong acid (sp.gr. 1-2); (ii) the use of mercuric cyanide instead of mercuric chloride and its separate reduction to mercury before its addition to the tin solution;

(iii) the addition to the diluting liquid, after cooling and before running into the tin solution, of 3 g of sodium bicarbonate; (iv) w h e n antimony is present, the addition of 5 g of a m m o n i u m oxalate to the diluting liquid before boiling.

(b) It has been shown that, using these modifications, tin m a y be accurately titrated in presence of m u c h larger amounts of antimony or of not m o r e than 0-003 g of copper or of both together.

It is not practicable to publish all the results of titrations, m a d e in the course of this research, which ran into hundreds; but care has been taken that those given are represen­

tative.

This paper is published b y permission of the Director-General of Scientific Research an d Development, Ministry of Supply.

R

e f e r e n c e s

1.

E v a n s , B .

S., Analyst, 1931,

5 6 ,

171.

2.

O k e ll, F .

L„

a n d L u m s d e n ,

J.,

Id..,

1935, 60, 803.

3.

H O ltje , R ., Z . a n o r g . C h e m . ,

1931,

1 98 , p .

287.

4. F e ig l, F ., "S p e c i f i c a n d S p e c i a l R e a c t i o n s , " t ra n s . b y R . F . O e sp e r, E l s e v i e r P u b . C o y ., N e w Y o r k ,

1940, p. 77.

5.

j a r v in e n ,

K.,

_{Z .} a n a l . C h e m . ,

1923,

6 2 ,

184.

6. C la r k e , S .

G., Analyst, 1931,

5 6 ,

82.

7. H o u r ig a n , H . F ., Id.,

1936,

6 1 ,

329.

8.

E v a n s ,

B.

S., I d . ,

1932,

5 7 ,

555.

9.

B r i t i s h S t a n d a r d S p e c if ic a t io n f o r L e a d a n d L e a d A l l o y s f o r C a b le S h e a t h in g ,

801-1938.

A p ­ p e n d ix C.

M a r c h ,

1944

T h e R a p i d P h o t o m e t r i c D e t e r m i n a t i o n o f T e l l u r i u m i n T e l l u r i u m C o p p e r A l l o y s

B

y

P. B. C R O S S L E Y , F.R.I.C.

(Read at the Meeting, M a y 3, 1944)

W

i t h

the increasing production of high speed machining copper it became evident that a

m e t h o d for the determination of tellurium more rapidly than b y the usual gravimetric procedure would be desirable. T h e tellurium is present in quantities u p to 1 % , and a deter­

mination w a s required, within the day, on large batches for control purposes. For this

reason an investigation wa s instituted an d the m e t h o d detailed below wa s evolved. It will

be seen that considerable e c onomy wa s effected in reagents and time, the min. time

previously required being 6 hours,1 whilst this m e t h o d takes only 2 hours.

IN T E L L U R I U M C O P P E R A L L O Y S 207

Solutions R e q u i r e d — (1) Nitric acid— 5 0 % v/v. (2) Stannous chloride— Dissolve 30 g of stannous chloride in 350 m l of conc. hydrochloric acid, w a rming slightly, and add 100 m l of water. (3) Thiourea (thiocarhamide)— Dissolve 100 g in 1000 m l of water and, after complete solution, add 60 m l of conc. nitric acid.

P

r i n c i p l e

— R eduction is effected b y the use of stannous chloride soln. Copper is not removed, but all interference therefrom is suppressed b y using thiourea in acid solution.

Me t h o d

— Accurately weigh 1 g of the alloy drillings into a 250 m l conical flask. A d d

20 m l of 5 0 % nitric acid and close the m o u t h of the flask with a small funnel to prevent losses due to spray. Allow 10 to 15 min. for solution to become complete, and place on the hot plate for a further 15 min. so that the liquid just boils.

Cool, dilute to 200 m l in a graduated flask, m i x thoroughly, and pipette 10 m l of this soln. into a 200 m l beaker. F r o m burettes add exactly 15 m l of thiourea soln., 60 m l of water and 15 m l of stannous chloride soln. M i x well and transfer a portion of this soln. to the 4-cm cell of Hilger’s “ Spekker” Absorptiometer; then, using Spectrum violet filters (No. 601) and water: water: 1 setting, measure the absorption a n d read the % of tellurium from a calibration graph.

Note I— Selenium interferes if present in quantities greater than 0-01%. T h e effect of smaller amounts is ignored for control purposes.

Note II— T h e calibration graph m a y be constructed in either of two ways:— (a) by testing on the Spekker Absorptiometer a n u m b e r of samples previously analysed b y the gravimetric procedure and plotting the d r u m differences obtained against the tellurium percentages: (6) b y taking one sample containing a k n o w n a m o u n t of tellurium (say 0-7%), and weighing, instead of 1 g, a series of 0-7, 0-8, 0-9, TO, I T g; then, calculating on the basis of an assumed in-weight of 1 g, percentages of tellurium over the range 0-49%, 0-56%, 0-63%, 0-70% and 0-7 7 % are obtained, which are plotted against the d r u m differences.

Note III— T h e photo-electric absorptiometer naturally suggested itself as the best m e a n s of measuring the turbidities produced, but comparisons m a y be m a d e visually in the usual manner. In that event the 10-ml fraction will be run from the pipette into a Nessler tube, the same additions made, and the colour of the resulting soln. matched against that of stan­

dards prepared b y adding appropriate amounts of a standard tellurium soln. to Nessler tubes each containing 10 m l of 2 % cupric nitrate soln., 15 m l of thiourea soln., 55 m l of water a n d 15 m l of stannous chloride soln.

Note I V — T h e me t h o d can be used to determine tellurium in amounts ranging from 0 T to 1 % . It is possible to extend the effective range to amounts outside those limits by taking suitable quantities of the sample.

Note V — Close consideration wa s given to the use of the following stabilisers— (1) starch, (2) g u m arabic, and (3) g u m tragacanth, but no special advantages were observed. T o obtain results within the degree of accuracy mentioned above their use does not appear essential.

In conclusion it m a y be stated that this turbidimetric me t h o d has been used for m a n y hundreds of determinations, and has given consistently good results against tellurium deter­

minations b y the standard normal gravimetric method. A n accuracy of 0-02 to 0 - 0 3 % of tellurium on the sample wa s obtained.

This me t h o d has been developed in the laboratories of Messrs. Enfield Rolling Mills, Ltd.

I wish to thank the Directors for permission to publish this paper, an d also Messrs. N. J. Stead and M. S. Naik, w h o have carried out the experiments.

R

e f e r e n c e

1. Willis, U. F., A

n a l y s t

, 1941, 66, 414.

i 43, S

l a d e s

H

ill

,

E

n f i e l d

, M

i d d l e s e x

M a n g a n o u s S u l p h a t e a s C a t a l y s t i n t h e C e r i m e t r i c D e t e r m i n a t i o n o f S e r u m C a l c i u m

B

y

V. R. W H E A T L E Y

. A

t t e n t i o n

has been recently dra w n by.W. R. Smith1 to the lack of a convenient cerimetric

method for the determination of serum calcium. Smith proposed heating the solution of

the oxalate to 70° C. and titrating with eerie sulphate until a slight excess imparted a definite

208

W H E A T L E Y ! M A N G A N O U S SULPHATE AS CATALYST IN T H E CERIMETRIC

yellow tint to the soln., a blank correction then being made. This procedure is hardly superior to the standard permanganate me t h o d and at 70° C. the use of a redox indicator would be impossible, since it would be preferentially oxidised. K a t z m a n an d Jacob2 have used iodine monochloride as catalyst and ferroin as indicator, but the titration must be performed at 50° C. and the temp, carefully controlled; moreover, the end-point is poor/ T h e discovery b y G. F. Smith and Getz3 that eerie perchlorate oxidises oxalic acid quantitatively at room temp, has been applied b y Reitemeieri to the micro-determination of calcium. T h e method can be applied to blood analysis but the expense of perchloric acid and the need for frequent standardisation of the eerie perchlorate soln. prohibit its general use in routine work. Several writers have preferred to oxidise the oxalate with an excess of eerie sulphate and to determine the excess iodimetrically,5 with standard ferrous soln., using ferroin as indicator,6-7 or photo­

metrically8; but such indirect procedures m u s t always be a second choice.

In view of the defects of these current procedures an attempt was m a d e to find a suitable catalyst and the possible use of m a n ganous salts, proposed b y Szebelledy and Tanay,9 was investigated. For the micro-titration of oxalate these writers use a solution of oxalate in N sulphuric acid containing 2 % of man g a n o u s sulphate or chloride, the titration being performed at 50° C., with the use of ferroin as indicator. If the concn. of sulphuric acid is increased the catalytic effect is inhibited, but it w a s found that b y increasing the concn.

of the m a nganous salt to 5 % the titration could then be performed rapidly at r o o m temp.

T h e m e t h o d can then be used for both the micro-determination of calcium and the standardi­

sation of 0-01 N eerie sulphate. For the former the oxalate ppt. can conveniently be dissolved in N sulphuric acid containing the m a n ganous sulphate catalyst, ferroin is added, and the soln. then titrated with 0-01 N eerie sulphate; this reagent is stable for several months and is a convenient stock soln.

Re a g e n t s

— (1) 0-01 N Ceric Sulphate— Digest

4

to 5 g of eerie sulphate (B.D.H.,

low in other rare earths) with 30 m l of conc. sulphuric acid and dilute to a litre with water.

Standardise as follows: Pipette 25-00 m l of 0-01 N sodium oxalate into a conical flask, add 25 m l of 1 0 % manganous sulphate soln., 2-5 m l of 6 0 % v/ v sulphuric acid and 3 drops of 0-0025 M ferroin, and titrate with the ceric sulphate soln. After the first addition the indicator changes to blue, but in a few seconds the red colour returns and the titration can then be continued rapidly to the blue end-point. M a k e a blank titration with water in place of the oxalate soln.; this should be 0-2 m l or less. (2) Acid M a n g a n o u s Sulphate Solution—

Dissolve 5 g of ma n g a n o u s sulphate (Analar) in 100 m l of N sulphuric acid. (3) 0-0025 M Ferroin— This is stable for several months and can conveniently be prepared, in amounts sufficient to fill a dropping bottle, b y dissolving 35 m g of ferrous sulphate a n d

7 4

m g of o-phenanthroline in 50 m l of water.

P r o c e d u r e — Ppt. the calcium and w a s h centrifugally b y the usual procedure of Clark and Collip.10 Dissolve the ppt. in 2 m l of the acid manganous sulphate soln. b y wanning in a water-bath. Cool the solution (thorough cooling is unnecessary), add 1 drop of the ferroin indicator, and titrate with 0-01 N ceric sulphate. A d d 1 drop of the ceric sulphate soln.

first, allow the red colour to return, and continue the titration rapidly. Towards the end of the titration the red colour of the ferroin fades slightly and finally turns blue w h e n the end­

point is reached. M a k e a blank titration on 2 m l of the acid man g a n o u s sulphate soln.

and 1 drop of ferroin soln.; this should be 0-04 m l or less.

Accuracy of the Method— Eight determinations on the same specimen of h u m a n serum gave the following results:

10-04, 10-02, 9-96, 10-04, 10-14, 10*08, 10-10, 10-12; m e a n 10-06 mg/100 ml. Result b y permanganate 10-06 mg/100 ml.

Comparative results b y this me t h o d and the permanganate m e t h o d on a n u m b e r of specimens of serum differed b y not more than 0-1 mg/100 ml.

A 0-01 N soln. of ceric sulphate was standardised b y the above procedure, against sodium oxalate with iodine monochloride as catalyst, using the procedure of Willard and Y o u n g / an d against M o h r ’s salt, using ferroin as indicator. T h e following are the average results of several titrations b y each method, (a) against oxalate ( M n S 0 4 as catalyst), 0-00994A;

(6) against oxalate (ICl as catalyst), 0-00993 N \ (c) against M o h r ’s salt, 0-00993 N.

< S u m m a r y — Manganous sulphate has been used as catalyst in the determination of blood

serum calcium with ceric sulphate, with the use of ferroin as indicator. T h e titration can be

done at r o o m temp, and is rapid and accurate. A me t h o d is given for standardising 0-01 A

D E T E R M I N A T I O N O F S E R U M C A L C I U M 209

eerie sulphate against sodium oxalate which is more convenient than other published pro­

cedures.

R

e f e r e n c e s 1. S m it h , W . R .,

Analyst,

1944,

69,

15.

2. K a t z m a n , E ., a n d J a c o b i, M . , J . B i o l . C h e m . , 1937, 118, 539.

3. S m it h , G . F., a n d G e tz, C. A ., I n d . E n g . C h e m . , A n a l . E d . , 1938,

10,

304.

4. R e ite m e ie r, R . F., I d . , 1943, 15, 395.

6. R a p p a p o r t , F ., K l i n . I V o c h . , 1933, 12, 1774.

6. L in d n e r , R ., a n d K i r k , P . L . , M i k r o c h e m . , 1937, 2 2 , 291.

7. L a r s o n , C.

E.,

a n d G re e n b e rg , D .

M.,

J . B i o l . C h e m . , 123, 199.

8. S e n d r o y , J. (Jr.), P r o c . S o c . E x p . B i o l . M e d . , 1941,

47,

136.

9. S z e b e lld d y , L ., a n d T a n a y , S., P h a r m . Z e n t r . , 1938,

79,

441.

10. C la r k , F . P i, a n d C o llip , J. B ., J . B i o l . C h e m . , 1925,

63,

461.

11. W ill a r d , H . H . , a n d Y o u n g , P., J . A m e r . C h e m . S o c . , 1933, 5 5 , 3260.

S

h r u b s a l l

B

i o ch e m i c a l

L

a b o r a t o r i e s

Westminster Hospital Medical School

M a y ,

1944

D a i l y V a r i a t i o n s i n t h e F r e e z i n g - p o i n t D e p r e s s i o n s o f C o w s ’ M i l k

By

A R N O L D R. T A N K A R D , F.R.I.C.,

a n d

D. J. T. B A G N A L L , A.C.G.F.C., F.R.I.C.

T

h e

variations in the freezing-point depressions given b y cows’ milk over a period of a few days are not only of theoretical interest but also of practical importance, since the analyst frequently calculates the extraneous water in an adulterated sample from a comparison of its f.pt. depression with that, of the "appeal-to-cow” sample. So far as w e are aware, very few figures of the daily variations in the f.pt. depressions of cows’ milk are available, and w e therefore thought that it would be of value to record the results of an investigation, m a d e in the winter of 1935, on the milk from 6 herds of cows and from the 28 individual cows comprising those herds. A s the n u m b e r of cows in each herd was small (3 to 7), there was an absence of any balancing factors that might be expected to stabilise the f.pt. depressions of the milk from larger herds.

A F o o d and Drugs sampling officer supervised the milkings and took the samples, all of which were evening milk. T h e samples, numbering 24 from the herds and 112 from the individual cows, were taken daily for four days, a n d the results obtained are given in the following Tables.

T

a b l e

I— D

a i l y

V

a r i a t i o n s i n

F

r e e z i n g

-

p o i n t

D

e p r e s s i o n s o f

H

e r d

S

a m p l e s F r e e z in g - p o in t d e p r e s s io n s °C. ( H o r t v e t )

t ~ ‘ t

N o . M a x i m u m v a r i a t io n s

o f F i r s t S e c o n d T h i r d F o u r t h _r A ₁

3 d a y s

H e r d c o w s d a y d a y d a y d a y 1 d a y 2 d a y s

A 3 0 -5 5 5 0 -5 5 7 0 -5 5 9 0-5 5 9 0-0 0 2 0 -0 0 4 0 -0 0 4

B 7 0 -5 5 3 0-5 4 8 0 -5 5 6 0 -5 5 4 0 -0 0 8 0 -0 0 6 0-0 0 1

C 5 0 -6 5 2 0-5 4 6 0 -5 5 3 0-5 4 3 0 -0 1 0 0 -0 0 3 0 -0 0 9

D 5 0 -5 4 8 0 -5 5 4 0*5 4 5 0 -5 5 7 0-0 1 2 0-0 0 3 0 -0 0 9

E 4 0 -5 4 6 0-5 4 2 0 -5 4 7 0-641 0 -0 0 6 0-001 0 -0 0 5

F 4 0 -5 5 5 0 -5 4 6 0-5 5 5 0-5 4 8 0 -0 0 9 0 -0 0 2 0-0 0 7

A v e r a g e : 0 -0 0 8 0 -0 0 3 0-0 0 6

Th e m i n i m u m an d m a x i m u m f.pt. depressions observed were 0-541° and 0-559° C. in the herd samples, and 0-536° a n d 0-564° C. in the individual c o w samples, whilst the m a x i m u m variations were 0-012° C. (herds) and 0-026° C. (individual cows), and were sho w n b y samples taken on consecutive days. #

F r o m a consideration of the whole of these results, it will be seen that the f.pt. depressions given by the herd samples, with one exception, did not vary m o r e than 0-010° C., whilst on 8 0 % of occasions the variations given b y the individual c o w samples did not exceed this figure.

During this investigation, the opportunity was taken to determine the daily variations

in the fat and solids-not-fat °/n of the milk samples collected, and the results are set out in

Table IV.

210 T A N K A R D A N D B A G N A L L : D A I L Y VARIATIONS IN T H E FREEZING-POINT.

Ta b l e

II

— D a i l y Va r i a t i o n s i n Fr e e z i n g-p o i n t D e p r e s s i o n s o f In d i v i d u a l

C o w ’s

M i l k Sa m p l e s

Freezing-point depressions °C. (Hortvet) N o .

o f F i r s t S e c o n d T h ir d F o u r t h

M a x i m u m v a r i a t io n s --- :--- »---

H e r d c o w d a y d a y d a y d a y 1 d a y 2 d a y s 3 d a y s

A 1 0-5 5 6 0-5 5 6 0-5 5 6 0 -6 5 9 0 -0 0 3 0-0 0 3 0 -0 0 3

A 2 0 -5 6 3 0-5 6 3 0 -5 6 8 0 -5 5 7 0-0 0 5 0-0 0 6 0 -0 0 6

A 3 0 -5 5 0 0 -5 5 7 ■ 0-5 5 8 0 -5 5 8 0 -0 0 7 0 -0 0 8 0 -0 0 8

B 4 0 -5 4 4 0-5 4 3 0 -5 3 6 0-5 3 6 0 -0 0 7 0-0 0 8 0-0 0 8

B 5 0-6 5 0 0-5 4 3 0-5 4 8 0 -5 4 9 0-0 0 7 0 -0 0 6 0-001

B 6 0-5 4 0 0-541 0-5 4 3 0 -5 4 4 0 -0 0 2 0-0 0 3 0 -0 0 4

B 7 0-5 4 5 0-5 4 3 0-5 4 3 0 -5 4 4 0 -0 0 2 0 -0 0 2 0-001

B 8 0-5 4 3 0 -5 5 2 0 -5 5 S • 0-5 4 3 0-0 1 5 0 -0 1 5 0 -0 0 0

B 9 0 -5 5 3 0 -5 5 2 0-5 5 6 0 -5 5 4 0 -0 0 4 0-0 0 3 0-001

B 10 0 -5 5 6 0 -5 5 6 0-5 6 2 0-5 5 6 0-0 0 6 0-0 0 6 0 -0 0 0

C 11 0-5 5 5 0 -5 5 4 0 -5 5 4 0-5 5 3 0-001 0-001 ' 0 -0 0 2

C 12 0 -5 5 3 0 -5 5 4 0-5 5 4 0-5 4 3 0-011 0-001 0 -0 1 0

C 13 0 -5 5 2 0 -5 5 8 0 -5 4 9 0-5 5 2 0-0 0 9 0 -0 0 6 0 -0 0 0

C 14 0-5 5 2 0 -5 4 4 0 -5 5 3 0-5 4 3 0 -0 1 0 0-001 0-0 0 9

C 15 0 -5 5 4 0-5 4 5 0 -5 5 4 0-5 4 6 0-0 0 9 0-001 0-001

D 16 0-553 0-5 5 2 0-5 4 6 0 -5 5 7 0-011 0 -0 0 7 0 -0 0 4

D 17 0 -6 5 2 0-561 0 -5 4 4 0 -5 5 6 0-0 1 7 0 -0 0 8 0 -0 0 4

D 18 0-551 0 -6 5 4 0 -5 4 7 0-5 6 6 0-0 1 9 0-0 1 2 0-0 1 5

D 19 0 -5 5 4 0-5 5 3 0 -5 4 7 0 -5 4 8 0-0 0 6 0-0 0 7 0-0 0 6

D 2 0 0 -5 6 3 0 -5 5 4 0-5 5 5 0 -5 6 4 0-009 0-0 1 0 0-001

E 21 0 -6 6 4 0 -5 4 4 0-5 4 3 0-5 4 2 0 -0 2 0 0-021 0-0 2 2

E 2 2 0 -5 5 4 0 -5 4 5 0-5 4 6 0 -5 4 4 0-0 0 9 o -o o s 0 -0 1 0

E 23 0 -5 4 2 0-541 0 -5 4 4 0 -5 4 4 0-0 0 3 0-0 0 3 0-0 0 2

E 2 4 0 -5 5 S 0-5 4 7 0 -5 5 6 0-5 3 5 0-021 0 -0 1 2 0-0 2 3

F 2 5 0 -5 6 3 0 -5 3 7 0 -5 5 5 0 -5 4 6 0-0 2 6 0 -0 0 9 0-0 1 7

F 26 0 -5 4 8 0 -5 4 6 0-5 5 5 0 -5 5 4 0 -0 0 9 0 -0 0 8 0-0 0 6

F 2 7 0 -5 4 5 0 -5 4 4 0-5 4 4 0 -5 4 6 0-0 0 2 0 -0 0 2 0-001

F 28 0-5 5 6 0 -5 5 6 0 -5 5 7 0-5 5 6 0-001 0-001 0-0 0 0

A v e r a g e : 0-0 0 9 0-0 0 6 0-0 0 6

Ta b l e

III

— Sh o w i n g t h e N u m b e r o f Ti m e s t h a t t h e Fr e e z i n g-p o i n t De p r e s s i o n s Va r i e d b e t w e e n Ce r t a i n L i m i t s

V a r i a t i o n s in fr e e z in g -p o in t d e p re s s io n s °C.

( H o r t v e t )

H e r d s

A I n d i v i d u a l c o w s

.... . A

f

1 d a y 2 d a y s " ^-\

3 d a y s

(

1 d a y 2 d a y s 3 d a y

0 - 0 0 0 - 0 -0 0 5 7 10 3 27 33 15

0 -0 0 6 - 0 - 0 1 0 10 2 3 33 18 9

0 - 0 1 1 - 0 -0 1 5 1 — __ 9 4 1

0 - 0 1 6 - 0 - 0 2 0 — — _{__ 9} __ ₁

0 -0 2 1 -0 - 0 2 5 _ _ . _{3 1 2}

0 - 0 2 6 -0 - 0 3 0 - — — 3

P e r c e n t a g e o f v a r i a t io n s

< 0-0 1 1 9 4 100 1 0 0 71 91 85

Ta b l e

I V

— Sh o w i n g t h e N u m b e r o f Ti m e s t h a t t h e Fa t a n d

S.N.F.

o f t h e Sa m p l e s Va r i e d b e t w e e n Qe r t a i n Li m i t s

H e r d s I n d i v i d u a l c o w s

V a r i a t i o n

F a t

A S . N . F . F a t S . N . F .

1 2 3

r

l 2 3

f

1 2 1

3

f

1 2 3

% d a y d a y 's d a y s day- d a y s d a y s d a y d a y s d a y s d a y d a y s d a y s

0 -0 0 - 0 - 2 0 9 7 6 16 9 4 41 2 5 13 6 9 4 5 24

0 -2 1 - 0 - 4 0 4 3 0 2 3 2 18 11 9 10 9 3

0 -4 1 - 0 - 6 0 1 — — — — — 11 11 5 4 2 T

0 -6 1 - 0 - 8 0 — — — __ __ __ * 6 4 __ 1 __ —

0 - 8 1 - 1 -0 0 1 * 1 * __ __ __ __ 2 1 1 __ —

1 -0 1 -1 -5 0 3 * 1 * __ __ __ ■ __ _{1 3} ___ r- - —

1 -6 1 - 2 -1 0 5 1 — --- N _____ _—

n t a g e o f v a r i a t io n s

< 0 -41 7 2 8 3 100 1 0 0 100 1 0 0 7 0 64 79 9 4 9 8 96

* D u e t o s a m p le s f r o m 2 h e r d s o f 3 a n d 4 cow s.