def TOC_rho_vernik(rho, phi_k=0.1, phi_nk=0.038, rho_k=1.3, rho_nk=2.765, 
                   rho_hc=0.25, rho_w=1.05, Ck=80.0):
    """
    Vernik (2017) method of calculating TOC from bulk density. Solved Ch.6 Eq.10
    for TOC to get the equation for TOC = ... below.
    
    density units: g/cc
    porosity units: fraction
    Ck unit: percentage
    """
    
    rho_bk = rho_k - phi_k*(rho_k-rho_hc)
    rho_bnk = rho_nk - phi_nk*(rho_nk-rho_w)
    
    A = Ck*rho_k*(phi_k*rho - phi_k*rho_bnk - rho + rho_bnk)
    B = phi_k*rho*rho_k - phi_k*rho_bnk*rho_k - phi_nk*rho*rho_nk + \
        phi_nk*rho_bk*rho_nk - rho*rho_k + rho*rho_nk - rho_bk*rho_nk + \
        rho_bnk*rho_k
        
    TOC = A/B
    
    return TOC