Industrial Hygiene Calculations

Complete the following: 

CHEMICAL	MW	PPM	MG/M 3
Toluene	92	100	376.28
MEK	72	28.86	85
Methanol	32	750	981.6
Trichloroethylene	131	25	134.36
Dichlorodifluoromethane	121	202.23	1000
Ethylbenzene	106	120	520.25

Toluene 

The molecular weight of toluene (C 7 H 8 ) is 92 g/mol, while the parts per million (ppm) value for toluene = 100

Ppm, molecular weight and concentrations in mg/m 3 are related at standard temperature and pressure (Neufeld, & Stalke, 2015), as follows; taking a as an algebraic value.

a ppm = (y mg/m 3 ) * (24.45) / (molecular weight), Also:

y mg/m 3 = (a ppm) * (molecular weight)/24.45

Letting y be the concentration; the formula implies that

y = (100 ppm) * (92gmol -1 ) / 24.45 = 376.28 mg/m 3 

MEK 

Given the molecular weight of methyl ethyl ketone shortened as MEK, (C 4 H 8 O), as 72 g/mol

Given the concentration in mg/m 3 of MEK as 85 mg/m3

Similarly, at standard temperature and pressure, the ppm, molecular weight and concentration in mg/m 3 are related through the following formula

a ppm = (y mg/m3) * (24.45)/(molecular weight)

=> a = (85 mg/m3) * 24.45 / (72gmol -1 ) = 28.86 ppm

Methanol 

The molecular weight of Methanol (CH 4 O) is 32 g/mol

Ppm value for methanol = 750 PPM

The below formula applies for ppm, molecular weight and concentrations in mg/m 3 at standard temperature and pressure:

y mg/m3 = (a ppm) * (molecular weight)/24.45

=> y = (750 ppm) * (32gmol-1) / 24.45 = 981.6 mg/m 3 

Trichloroethylene 

Molecular weight (MW) of Trichloroethylene (C 2 Cl 3 H) = 131.4 g/mol

PPM of Trichloroethylene = 25 PPM

PPM, molecular weight and mg/m 3 are related to each other at 298K and 1 atm pressure as

y mg/m3 = (a ppm) * (molecular weight)/24.45

=> y = (25 ppm) * (131.4 gmol-1) / 24.45 = 134.36 mg/m 3 

Dichlorodifluoromethane 

The molecular weight of Dichlorodifluoromethane (CF 2 Cl 2 ) = 120.9 g/mol

Given the concentration in mg/m 3 of Dichlorodifluoromethane (CF 2 Cl 2 ) = 1000 mg/m 3 

The ppm, molecular weight and mg/m3 are at 298K and 1 atm pressure, calculated as

a ppm = (y mg/m3) * (24.45)/(molecular weight)

=> a = (1000 mg/m3) * 24.45 / (120.9 gmol-1) = 202.23 ppm

Ethylbenzene 

molecular weight (MW) of Ethylbenzene (C8H10) = 106 g/mol

PPM of Ethylbenzene (C 8 H 10 ) = 120 PPM

PPM, molecular weight and mg/m3 are calculated as below at 298K and 1 atm pressure as

y mg/m3 = (a ppm) * (molecular weight)/24.45

=> y = (120 ppm) * (106 gmol -1 ) / 24.45 = 520.25 mg/m3

2. Equilibrium concentration for toluene 

Equilibrium concentration for toluene = (21) * (10^6)/760 = 27632 ppm

3. TWA concentration 

To obtain TWA concentrations in mg/m 3 

Sample 1: 110/ (Flowrate * duration = 120) = 0.916µg, which is equivalent to 0.916 mg/m 3 

Sample 2: 745 / 240 = 3.104(µg) or 3.104(mg/m3)

Sample 3: 350 / 180 = 1.458(µg) or 1.458(mg/m3)

Sample 4: 700 / 240 = 2.916(µg) or 2.916(mg/m3)

Sample 5: 255 / 180 = 1.416(µg) or 1.416(mg/m3)

Now, sums = (0.196 * 60) + (3.104 * 120) + (1.458 * 90) + (2.916 * 120) + (1.416 * 90) = 1079.76

Total time = 60 +120 + 90 + 120 + 90 = 480

TWA = 1079.76 / 480 = 2.2495mg/m 3 

Reference

Neufeld, R., & Stalke, D. (2015). Accurate molecular weight determination of small molecules via DOSY-NMR by using external calibration curves with normalized diffusion coefficients. Chemical science, 6(6), 3354-3364.