Sulfur Determinations from Bomb-Washings Titrations

RAYMOTSD E. x i L - n r m \ala1 Boiler Laboratory, Philadelphia ";a\! T a d , Philadplphia. Pa.

S THE Parr procedure foi B. t . u. determinations (1 I , I carried out in an Emerson or Parr oxygen bomb calorime- ter or siniilar apparatus, the total heat n-hich is the product of the temperature rise from the combustion of a sample and the water equivalent of the calorimeter is too high. The proper value is obtained by applying the wire correction, the acid correction, and the sulfur correction. These are collectively termed the total correction.

The wire correction is the heat giyen off by the fuse wiie tha t is burned when i t ignites the fuel in the bomb. The fuse wire is so drawn tha t 1 em. when burned will give off 2.8 calories of heat; 2.8 times the length of wire burned is the wire correction.

The acid correction is due to the heat of formatioil of the nitric acid formed in the combustion. When neutialized with sodium carbonate solution (1 ml. equals 1 calorie). the number of milliliters required for a bomb-washings titration is the acid correction. However, some of the acid neutralized is sulfuric acid, whose heat of conversion from sulfur dioxide t o sulfur trioxide is greater than the heat of formation of m i equivalent amount of nitric acid. The heat of conversion is the difference between the heat of formation of sulfur dioxide, which is formed in ordinary combustion, and the Iicat of formation of sulfur trioxide, which is formed in the bomb. The difference between the heat of conversion of sulfur dioxide to sulfur tri- oxideand the heat of formation of an equiva- lent amount of nitric acid amounts to 1300 calories per gram of sulfur. Then 13 X per cent of sulfur X weight of fuel sample equal< the sulfur correction.

Obviously, a B. t. u. determination of most fuelsrequires a sulfur determination. The gravi- metricmethod used in the routine B. t. u. de- terminations referred to below is briefly: Precipi- ta te the sulfuric acid in the bomb washings by excess barium chloride solution, allow to stand. filter, ignite, and weigh the residue as barium sulfate. Duplicate determinations can easily be checked within 5 in the second decimal place of the percentage of sulfur-i. e., 1.307, - 1 . 3 5 5 . This is also its approximate accuracy. This ac- curacy is more than is necessary for a B. t . 11. determination, but i t is useful in determining a very desirable correlation between the percent- age of sulfur and the bomb-washings titration.

Correlations The sulfur percentages and average inillilitei -

of bomb-washings titrations from the routine B. t. u. determinations of 54 oils were plotted and by the method of least squares the most suit- able straight line was determined (Figure 1). Theexpressionfoundwas: -11 - 11.46 = 6 . 3 6 s

Since the assumption was that the amount of nitric acid formed is a constant, the question ih raised: How much is this constant affected by the amount of nitrogen in the fuel and also by the heat of combustion of the oil? Figure 2

shows the variatloii of the calculated total corrections from the standard total corrections (where sulfur is actually de- termined), plotted against the nitrogen percentage of the fuel. The oil nitrogen was determined by the Kjeldahl method, and, although of uncertain accuracy, Figure 2 s h o w that the correction appears reasonably constant up to 1 per cent of nitrogen. Figure 3 shows the variation of the cal- culated corrections from the standard total corrections plotted against B. t. u. per pound of the oil. The variation was not affected between 18,000 and 19,000 B. t. u. per pound. These two figures show that the nitric acid constant was not affected appreciably within the prescribed limits, about 1 per cent of fuel nitrogen and B. t. u. per pound range from 18,000 to 19,000 (10,000 to 10.556 calories per gram).

Application

I n the above the coefficient of correlation was 0.92, showing good agreement, Figure 3 shows that in about 96 per cent of the case5 plotted the total correction calculated from the developed equation had an adverse effect of only 6 B. t . u.'s. The accuracy of the correlation can also be seen from the coniparison of the calculated slope v i t h the theoretical slope.

Calculated slope tan 0: = 6.36 Theoretical slope tan 01 = 6.33

CALCULATIOS O F THEORETICAL SLOPE. The equation for re- lating the milliliters of bomb-washings titration and the percentage of sulfur in a 0.7-gram sample of oil is:

M - a = mS nhere -If = nil. of titration

a = nitric acid constant nz = slope of (Y

$5' = percentage of sulfur in a O.i-gram sample

!wight of sulfur - - S -~ Xa2C03 weight of K:a?CO:

If the weight of sodium carbonate is 0.003658 gram (the m i g h t of sodium carbonate in 1 ml. of solution used in titrat- ing bomb washings), the corresponding weight of sulfur is 0.0011064 gram. Then 0.007 gram of sulfur (weight of sulfur in a 0.7-gram sample, 1 per cent sulfur) divided by 0.0011OG1 gives the milliliters of sodium carbonate solution equivalent to 1 per cent of sulfur.

6.326 0.001 1061/-

The chemical reaction between sodium carbonate and sul-

the weight of sulfur is proportional t o the weight of sodium carbonate with which it reacts, the following proportion readily obtains:

M - n __ -- - 6 33 furic acid is H2S0d + Sa2COs -+ H?C03 + XalSO4. Since Then S

All calculations given thus far are based on a 0.T-gram sample of fuel, the actual size of sample burned. HoTved,

FIGURE 2 . 17.4RI.%T10S O F C.4LCULATED TOTAL CORRECTIOSS FROM ST-4SDARD TOTAL COR- RECTIOSS AGAISST XITROGES PERCEXTAGE OF OIL

8

6

z ; 4

; + 2

0

y o i- - 2 6 2 4 + " 6

8

10

1

I

I

i8 ,OOO 18$00 ~9,000 8.T U. OF THE OIL

FIGVRE 3. 17.4RIhT10X O F C.4LCULATED TOTAL CORRECTIONS FROM SThND.4RD To.r . iL COR- RECTIONS AGAISST B. T. U.'S O F OIL

540 INDUSTRI 41, 4UI) EUGINEERIIG CHEIiIISTlIY

if the B. t. ii. correct,ioii only is requireti, it' is much simpler to avoid the intermediate steps involving sulfur percentages or weight, aiitl t o coiir-ert the sulfur correction directly to calo- ries anti combine n itli the corrections due to acid and wire.

Original equation .zr - i i . ~ = 0.30 s (1)

Since S is iii per cent

( 2 ) S X 0.i 100

=

G = grnni? of sulfur Substituting 2 in 1

( 3 ) 6.30 0.007

-11 - 11.40 = - G Since wlfur correction is 1300 calories per gram

1300 X G = CI (4)

CI = calories due to sulfur Substituting 4 in 3

: 5 ) 6.36 9.10

.w - 11.46 = - e, Transposing

'& (-If - 11.46) = CI I A )

Simplifying

The espre>+iori for the n-ire correction i- 7 S L -. = C7

Ca = calories due to rvire The expression for the total correction i+ theti

I': + c2 -c r< = c'

C = total correctiori

c = i . a i .II - 10.4 c .ir + 2.8 I,

Simplifying

(I, + C? 4- C., = 2,431 f 2.8 I, - I(i.4

The expression found is

2.131 -11 + 2.8 L - 16.4 = e

, 1 0 1

11 8

Figure 4 s h o w a Leries of curves constructed froin the above expression for various values of L.

DEI-ELOP~IEST OF C T ~ ~ ~ ~ i l ~ , EQUATIOS;. For application in another calorimeter oet,up, since the calculated slope vas so close to the theoretical dope, the theoretical slope can he used to develop a general expiwsion for total corrections.

General equation for iulfur percentage and milliliters of tit ra- tion relation

.lf - a = 6.33 S (121

G

G 33 0 007

0 . i S 100 _ =

1.3) . l I - u = - G

1300 G = CI

1.431 If - 16 4 = CI

The expreqsion for acid correction is M = C2

Cy = calories due to acid

6.33 9.10

1.i - a = - CI

Transposing and simplifying

1.438 .If - 1.438 a = L'I

Total correction is

CI + ci + c * 3 = c C = 1.438 .If - 1.438 n + 3f + 2.8 L t I t !

Simplifying C' = 2.438 .If + 2.8 L - 1.4385 '171

To find a, run a number of sulfur determinations a i d substitute milliliters of titration (1 nil. contains 0.00365s gram of sodium carbonate) and grams of sulfur in Equa- tion 13. Take an average of at leavt one dozen clrter- niinations.

Conclusion

From the foregoing it is concluded tha t the general expression for t,otal correction

2.138 X -11 + 2.8 X L - 1.138 X C

has application in any calorimeter setup d i e r e the fuel nitrogen and B. t. u. content, are n-ithin limits that har-e no appreciable effect on tlie variation of iiitric acid formed, vhere tlie sulfur percentage is not too high to be subst,antially converted to sulfuric acid. and where the same oxygen pressure is used for every determination,

FIGURE 4. CURVES FOR TOT.~L CORRECTIOSS

Literature Cited (1) Parr Instrument Co., Booklet 114.

RECEI%FD i I a r c h 28, 1938